MySQL Reference Manual for version 3.22.10-beta.

1 General Information about MySQL

This is the MySQL reference manual; it documents MySQL version 3.22.10-beta.

MySQL is a basically free, very fast SQL database server. See section 3 Licensing or When do I have/want to pay for MySQL?.

The MySQL home page provides the latest information about MySQL.

For a discussion of MySQL's capabilities, see section 1.4 The main features of MySQL.

For installation instructions, see section 4 Installing MySQL. For tips on porting MySQL to new machines or operating systems, see section G Comments on porting to other systems.

See section 4.15.1 Upgrading from a 3.21 version to 3.22, for information about upgrading from a 3.21 release.

For examples of SQL and benchmarking information, see the `bench' directory.

For a history of new features and bug fixes, see section D MySQL change history.

For a list of currently known bugs and misfeatures, see section E Known errors and design deficiencies in MySQL.

For future plans, see section F List of things we want to add to MySQL in the future (The TODO).

For a list of all the contributors to this product, see section C Contributors to MySQL.

IMPORTANT:

Send bug (error) reports, questions and comments to the mailing list at

Please use the mysqlbug script when posting bug reports or questions about MySQL. mysqlbug will gather some information about your system and start your editor with a form that you can use to describe your problem. Bug reports not generated by mysqlbug that include no good reason why not might be ignored by the MySQL maintainers. A report that says "MySQL does not work for me. Why?" is not considered a valid bug report.

For source distributions, the mysqlbug script can be found in the `scripts' directory. For binary distributions, mysqlbug can be found in the `bin' directory.

If you have any suggestions concerning additions or corrections to this manual, please send them to the MySQL mailing list (mysql@tcx.se) with the following subject line: documentation suggestion: [Insert Topic Here]. See section 2.1 The MySQL mailing lists.

1.1 What is MySQL?

MySQL is a true multi-user, multi-threaded SQL (Structured Query Language) database server. SQL is the most popular database language in the world. MySQL is a client/server implementation that consists of a server daemon mysqld and many different client programs and libraries.

The main goals of MySQL are speed, robustness and ease of use. MySQL was originally developed because we at TcX needed a SQL server that could handle very large databases an order of magnitude faster than what any database vendor could offer to us. We have now been using MySQL since 1996 in an environment with more than 40 databases containing 10,000 tables, of which more than 500 have more than 7 million rows. This is about 100 gigabytes of mission-critical data.

The base upon which MySQL is built is a set of routines that have been used in a highly demanding production environment for many years. While MySQL is still in development, it already offers a rich and highly useful function set.

The official way to pronounce MySQL is "My Ess Que Ell" (Not MY-SEQUEL).

1.2 About this manual

This manual is currently available in TeXInfo, plain text, Info and HTML versions. Because of their size, PostScript and PDF versions are available for separate download.

The primary document is the TeXInfo file. The HTML version is produced automatically with a modified version of texi2html. The plain text and Info versions are produced with makeinfo. The Postscript version is produced using texi2dvi and dvips. The PDF version is produced with the Ghostscript utility ps2pdf.

This manual is written and maintained by David Axmark, Michael (Monty) Widenius, Paul DuBois and Kim Aldale. For other contributors, see section C Contributors to MySQL.

1.2.1 Conventions used in this manual

This manual uses certain typographical conventions:

constant: Constant-width font is used for command names and options; SQL statements; database, table and column names; C and Perl code; and environment variables. Example: "to see how mysqladmin works, invoke it with the --help option."
`filename': Constant-width font with surrounding quotes is used for filenames and pathnames. Example: "the distribution might be installed under the `/usr/local/' directory."
`c': Constant-width font with surrounding quotes is also used to indicate character sequences. Example: "to specify a wildcard, use the `%' character."
italic: Italic font is used for emphasis, like this.
boldface: Boldface font is used for access privilege names (e.g., "do not grant the process privilege lightly") and occasionally to convey especially strong emphasis.

When commands are shown that are meant to be executed by a particular program, the prompt for the command indicates the program. For example, shell> indicates a command that you execute from your login shell, and mysql> indicates a command that you execute from the mysql client:

shell> type a shell command here
mysql> type a mysql command here

Shell commands are given using Bourne shell syntax. If you are using a csh-style shell, you may need to issue commands slightly differently. For example, the sequence to set an environment variable and run a command looks like this in Bourne shell syntax:

shell> VARNAME=value some_command

For csh, you would execute the sequence like this:

shell> setenv VARNAME value
shell> some_command

Database, table and column names often must be substituted into commands. To indicate that such substitution is necessary, this manual uses db_name, tbl_name and col_name. For example, if you see this:

mysql> SELECT col_name FROM db_name.tbl_name;

It means that were you to enter a similar statement, you would supply your own database, table and column names, perhaps like this:

mysql> SELECT author_name FROM biblio_db.authorlist;

SQL statements may be entered in uppercase or lowercase. When this manual shows a SQL statement, uppercase is used for particular keywords if those keywords are under discussion (to emphasize them) and lowercase for the rest of the statement. So you might see the following in a discussion of the SELECT statement:

mysql> SELECT count(*) FROM tbl_name;

On the other hand, in a discussion of the COUNT() function, the statement would be written like this:

mysql> select COUNT(*) from tbl_name;

If no particular emphasis is intended, all keywords are written in uppercase.

In syntax descriptions, square brackets (`[' and `]') are used to indicate optional words or clauses:

DROP TABLE [IF EXISTS] tbl_name

When a syntax element consists of a number of alternatives, the alternatives are separated by vertical bars (`|'). When one member from a set of choices may be chosen, the alternatives are listed within square brackets. When one member from a set of choices must be chosen, the alternatives are listed within braces (`{' and `}'):

TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)
{DESCRIBE | DESC} tbl_name {col_name | wild}

1.3 History of MySQL

We once started off with the intention to use mSQL to connect to our own fast low-level (ISAM) tables. However, after some testing we came to the conclusion that mSQL was not fast enough or flexible enough for our needs. This resulted in a new SQL interface to our database but with almost the same API interface as mSQL. This API was chosen to ease porting of third-party code.

The derivation of the name MySQL is not perfectly clear. Our base directory and a large number of our libraries and tools have had the prefix "my" for well over 10 years. However, Monty's daughter (some years younger) is also named My. So which of the two gave its name to MySQL is still a mystery, even for us.

1.4 The main features of MySQL

Fully multi-threaded using kernel threads. That means it easily can use multiple CPUs if available.
C, C++, Java, Perl, Python and TCL API's. See section 18 MySQL client tools and API's.
Works on many different platforms. See section 4.2 Operating systems supported by MySQL.
Many column types: signed/unsigned integers 1, 2, 3, 4 and 8 bytes long, FLOAT, DOUBLE, CHAR, VARCHAR, TEXT, BLOB, DATE, DATETIME, TIMESTAMP, YEAR, SET and ENUM types. See section 7.2 Column types.
Very fast joins using an optimized one-sweep multi-join.

Full operator and function support in the SELECT and WHERE parts of queries. Example:

mysql> SELECT CONCAT (first_name, " ", last_name) from tbl_name
           WHERE income/dependents > 10000 AND age > 30;

SQL functions are implemented through a highly-optimized class library and should be as fast as they can get! Usually there shouldn't be any memory allocation at all after the query initialization.
Full support for SQL GROUP BY and ORDER BY clauses. Support for group functions (COUNT(), AVG(), STD(), SUM(), MAX() and MIN()).
Support for LEFT OUTER JOIN with ANSI SQL and ODBC syntax.
You can mix tables from different databases in the same query (as of version 3.22).
A privilege and password system which is very flexible and secure, and which allows host-based verification. Passwords are secure since all password traffic when connecting to a server is encrypted.
ODBC (Open-DataBase-Connectivity) for Windows95 (with source). All ODBC 2.5 functions and many others. You can, for example, use Access to connect to your MySQL server. See section 15 MySQL ODBC Support.
Very fast B-tree disk tables with index compression.
16 indexes per table are allowed. Each index may consist of 1 to 15 columns or parts of columns. The maximum index length is 256 bytes (this may be changed when compiling MySQL). An index may use a prefix of a CHAR or VARCHAR field.
Fixed-length and variable-length records.
In-memory hash tables which are used as temporary tables.
Handles large databases. We are using MySQL with some databases that contain 50,000,000 records.
All columns have default values; you can use INSERT to insert a subset of a table's columns and columns that were not explicitly given values will be set to their default values.
Uses GNU Automake and Autoconf for portability.
Written in C and C++. Tested with a broad range of different compilers.
A very fast thread-based memory allocation system.
No memory leaks. Tested with a commercial memory leakage detector (purify).
Includes isamchk, a very fast table check, optimize and repair utility. See section 13 Using isamchk for table maintenance and crash recovery.
All data are saved in ISO-8859-1 Latin1 format. All comparisons for normal string columns are case insensitive.
Full support for the ISO-8859-1 Latin1 character set. For example, the Scandinavian characters @ringaccent{a}, @"a and @"o are allowed in table and column names.
Sorts according to the ISO-8859-1 Latin1 character set (the Swedish way at the moment). It is possible to change this in the source by adding new sort order arrays. To see an example of very advanced sorting, look at the Czech sorting code. MySQL supports many different character sets that can be specified at compile time.
Aliases on tables and columns as in the SQL92 standard.
DELETE, INSERT, REPLACE, and UPDATE return how many rows were affected.
Function names do not clash with table or column names. For example, ABS is a valid column name. The only restriction is that for a function call, no spaces are allowed between the function name and the `(' that follows it. See section 7.30 Is MySQL picky about reserved words?.
All MySQL programs can be invoked with the --help or -? options to obtain online assistance.
The server can provide error messages to clients in many languages. See section 9.1 What languages are supported by MySQL?.
Clients connect to the MySQL server using a TCP/IP connection or Unix socket.
The MySQL-specific SHOW command can be used to retrieve information about databases, tables and indexes. The EXPLAIN command can be used to check how the optimizer resolves a query.

1.5 How stable is MySQL?

This section addresses the questions, "how stable is MySQL?" and, "can I depend on MySQL in this project?"

At TcX, MySQL has worked without any problems in our projects since mid-1996. When MySQL was released to a wider public, we noticed that there were some pieces of "untested code" that were quickly found by the new users who made queries in a different manner. Each new release has had fewer portability problems than the previous one, even though each has had many new features, and we hope that it will be possible to label one of the next releases 'stable'.

Each release of MySQL has been usable and there have been problems only when users start to use code from "the gray zones". Naturally, outside users can't know what the gray zones are; this section attempts to indicate those that are currently known.

Here we will try to clarify some issues and to answer some of the more important questions that seem to concern many people. This section has been put together from information gathered from the mailing list (which is very active in reporting bugs).

The descriptions deal with the 3.21.x version of MySQL. All known and reported bugs are fixed in the latest version, with the exception of the bugs listed in the BUGS file which are things that are "design"-related.

MySQL is written in multiple layers and different independent modules. Here is a list of the different modules and how well-tested each of them is:

The ISAM table handler -- Stable: This is how all the data are stored. In all MySQL releases there hasn't been a single (reported) bug in this code. The only known way to get a corrupted table is to kill the server in the middle of an update. Even that is unlikely to destroy any data beyond rescue, because all data are flushed to disk between each query. There hasn't been a single bug report about lost data because of bugs in MySQL, either.
The parser and lexical analyser -- Stable: There hasn't been a single reported bug in this system for a couple of months.
The C client code -- Stable: No known problems. In early 3.20 releases, there were some limitations in the send/receive buffer size. In 3.21.x, the send/receive buffer is now dynamic up to a default of 512K.
mysql, mysqladmin and mysqlshow -- Stable: The command line clients have had very few bugs.
mysqldump and mysqlimport -- Beta: Rewritten for 3.21.
Basic SQL -- Stable: The basic SQL function system and string classes and dynamic memory handling. Not a single reported bug on this system.
Query optimizer -- Gamma: Some changes in 3.21.
Range optimizer -- Alpha: Totally rewritten for 3.21.x
Join optimizer -- Gamma: Small changes for 3.21.
GROUP BY, ORDER BY and related function COUNT() -- Beta: Rewritten for 3.21 and throughly tested.
Locking -- Gamma: This is very system-dependent. On some systems there are big problems using standard OS locking (fcntl()). In these cases, you should run the MySQL daemon with the --skip-locking flag. Problems are known to occur on some Linux systems and on SunOS when using NFS-mounted file systems.
Linux threads -- Gamma: The only problem found has been with the fcntl() call, which is fixed by using the --skip-locking option to mysqld. Some people have reported lockup problems with the 0.5 release.
Solaris 2.5+ pthreads -- Stable: We use this for all our production work.
MIT-pthreads (Other systems) -- Beta: There have been no reported bugs since 3.20.15 and no known bugs since 3.20.16. On some systems, there is a "misfeature" where some operations are quite slow (a 1/20 second sleep is done between each query). Of course, MIT-pthreads may slow down everything a bit, but index-based SELECT statements are usually done in one time frame so there shouldn't be a mutex locking/thread juggling.
Other thread implementions -- Alpha: The ports to other systems are still very new and may have bugs, possibly in MySQL, but most often in the thread implementation itself.
LOAD DATA ..., INSERT ... SELECT -- Stable: Some people have thought they have found bugs here, but these have turned out to be misunderstandings. So check the manual before reporting bugs!
ALTER TABLE -- Gamma: Partly rewritten for 3.21.
DBD -- Beta: Now maintained by Jochen Wiedmann
mysqlaccess -- Beta: Written and maintained by Yves Carlier
The Technical Documentation -- Beta: It is improving.
MyODBC (uses ODBC SDK 2.5) -- Beta: It seems to work well with some programs.

TcX provides email support for paying customers, but the MySQL mailing list usually provides answers to common questions. Bugs are usually fixed right away with a patch; for serious bugs, there is almost always a new release.

1.6 Year 2000 compliance

MySQL itself has no problems with Year 2000 compliance:

MySQL uses Unix time functions and has no problems with dates until 2069; all 2-digit years are regarded to be in the range 1970 to 2069, which means that if you store 01 in a year column, MySQL treats it as 2001.
All MySQL date functions are stored in one file `sql/time.cc' and coded very carefully to be year 2000-safe.
In MySQL 3.22 and later versions, the new YEAR column type can store years 0 and 1901 to 2155 in 1 byte and display them using 2 or 4 digits.

You may run into problems with applications that use MySQL in a way that is not Year 2000-safe. For example, many old applications store or manipulate years using 2-digit values (which are ambiguous) rather than 4-digit values. This problem may be compounded by applications that use values such as 00 or 99 as "missing" value indicators.

Unfortunately, these problems may be difficult to fix, since different applications may be written by different programmers, each of whom may use a different set of conventions and date-handling functions.

1.7 General SQL information and tutorials

This book has been recommended by a several people on the MySQL mailing list:

Judith S. Bowman, Sandra L. Emerson and Marcy Darnovsky
The Practical SQL Handbook: Using Structured Query Language
Second Edition
Addison-Wesley
ISBN 0-201-62623-3
http://www.awl.com

This book has also received some recommendations on the mailing list:

Martin Gruber
Understanding SQL
ISBN 0-89588-644-8
Publisher Sybex 510 523 8233
Alameda, CA USA

A SQL tutorial is available on the net at http://w3.one.net/~jhoffman/sqltut.htm.

1.8 Useful MySQL-related links

1.8.1 Web development tools that support MySQL

1.8.2 Web servers with MySQL tools

1.8.3 Examples

1.8.4 Other MySQL-related links

1.8.5 SQL and database interfaces

1.8.6 General database links

There are also many web pages that use MySQL. See section A Some MySQL users. Send any additions to this list to

2 MySQL mailing lists and how to ask questions or report errors (bugs)

2.1 The MySQL mailing lists

Requests to be added to or dropped from the main MySQL mailing list should be sent to the electronic mail address mdomo@tcx.se. Sending a one-line message saying either subscribe mysql or unsubscribe mysql suffices. If your reply address is not valid, you may specify your address explicitly using subscribe mysql your-name@your.domain or unsubscribe mysql your-name@your.domain.

Please do not send mail about subscribing or unsubscribing to forwarded automatically to hundreds of other users.

Your local site may have many subscribers to mysql@tcx.se. If so, it may have a local mailing list, so that a single message from tcx.se is sent to the site and propagated to the local list. In such cases, please contact your system administrator to be added to or dropped from the local MySQL list.

Mail to mdomo@tcx.se is handled automatically by the Majordomo mailing list processor.

The following MySQL mailing lists exist:

mysql-announce: This is for announcement of new versions of MySQL and related programs. This is a low volume list that we think all MySQL users should be on.
mysql: The main list for general MySQL discussion. Please note that some things should go to the more-specialized lists. It you post to the wrong list, you may not get an answer!
mysql-digest: The mysql list in digest form. That means you get all individual messages, sent as one large mail message once a day.
mysql-Java: Discussion about MySQL and Java. Mostly about the JDBC drivers.
mysql-win32: All things concerning MySQL on Microsoft operating systems like Windows/NT.
myodbc: All things concerning connecting to MySQL with ODBC.
msql-mysql-modules: A list about the Perl support in MySQL.
msql-mysql-modules-digest: A digest version of the msql-mysql-modules list.
mysql-developer: A list for people who work on the MySQL code.

You subscribe or unsubscribe to all lists in the same way as described above. In your subscribe or unsubscribe request, just put the appropriate mailing list name rather than mysql.

2.2 Asking questions or reporting bugs

Before you ask a question on the mailing list, it is a good idea to check this manual. If you can't find an answer here, check with your local MySQL expert. If you still can't find an answer to your question, go ahead and read the next section about how to send mail to

2.3 What to do if you think you have found a bug

If you can, please use the mysqlbug script that can be found in the `scripts' directory in the distribution. mysqlbug helps you generate a bug report in a standard format. If you cannot use mysqlbug, remember to specify those items listed below that are relevant. Note that it is possible to answer a letter with too much information, but not one with too little. You should always use mysqlbug if your question is any way related to a MySQL version you are using!

mysqlbug should automatically find most of the following information, but if something important is missing, please include it with your message!

The version of MySQL you are using (for example, MySQL 3.21.22). You can find out which version you are running by executing mysqladmin version.
The manufacturer and model of machine you are working on.
The operating system name and version. For most operating systems, you can get this from uname -a.
Sometimes the amount of memory (real and virtual) is also relevant.
If the bug occurs during compilation, include the exact error messages and also a few lines around the offending code in the file where the error occurred.
If this is a run-time bug, please describe exactly how you got the error. If you can include a test program, with tables, that shows the error, you probably will get a more explicit answer.
If you can't produce a test case in a few rows or if the test table is too big to be mailed to everyone (more than 10 rows) you should dump your tables using mysqldump and create a `README' file that describes your problem. tar and gzip or zip the files and use ftp to transfer the archive to ftp://www.tcx.se/pub/mysql/secret. Then send a short description of the problem to mysql@tcx.se.
If your question is related to the privilege system, please include the output of mysqlaccess, the output of mysqladmin reload and all error messages you get when trying to connect! You should do the tests in the above order!
Indicate in your mail message that you have checked the reference manual and mail archive so others know that you have tried to solve your problem yourself.

If you are a support customer, please post the bug report to well as to the appropriate mailing list to see if someone else has experienced (and perhaps solved) the problem.

For information on reporting bugs in MyODBC, see section 15.2 How to report problems with MyODBC.

When answers are sent to you individually and not to the mailing list, it is considered good etiquette to summarize the answers and send the summary to the mailing list to that others may have the benefit of the responses you receive.

2.3.1 What to do if MySQL keeps crashing

Since it is very hard to know why something is crashing, first try to check whether or not things that work for others crash for you. Please try the following things:

Have you tried the benchmarks? This should test MySQL rather well. You can also add code that simulates your application!
Try fork_test.pl and fork2_test.pl.
If you configure MySQL for debugging, it will be much easier to find out possible errors if something goes wrong. See section 19.10 Debugging MySQL.
Reconfigure MySQL with the --with-debug option to configure and then recompile. This causes a safe memory allocator to be included that can find some errors. It also provides a lot of output about what is happening.
Use mysqld --log and try to determine from the information in the log whether or not some specific query kills the server. 95% of all bugs are related to some specific query!
Have you applied the latest patches for your operating system?
Use the --skip-locking option to mysqld. On some systems, the lockd lock manager does not work properly; the --skip-locking option tells mysqld not to use external locking. (This means that you cannot run 2 mysqld servers on the same data and you must be careful when using isamchk, but it may be instructive to try the option as a test.)
Have you tried mysqladmin -u root processlist when mysqld appears to be dead? Sometimes mysqld is not dead even though you might think so. The problem may be that all connections are in use, or there may be some internal lock problem. mysqladmin processlist will usually be able to make a connection even in these cases, and can provide useful information about the current number of connections and their status.
Run the command mysqladmin -i 5 status in a separate window to output statistics.
Try the following:
1. Start mysqld from gdb (or another debugger).
2. Run your test scripts.
3. Do back (or the backtrace command in your debugger) when mysqld core dumps.
Try to simulate your application with a Perl script to force MySQL to crash or misbehave.
Or send a normal bug report. See section 2.3 What to do if you think you have found a bug. But be even more detailed than usual. Since MySQL works for many people it may be that the crash results from something that exists only on your computer (for example, an error that is related to your particular system libraries).

2.4 Guidelines for answering questions on the mailing list

If you consider your answer to have broad interest, you may want to post it to the mailing list instead of replying directly to the individual who asked. Try to make your answer general enough that people other than the original poster may benefit from it. When you post to the list, please make sure that your answer is not a duplication of a previous answer.

Try to summarize the essential part of the question in your reply, but don't feel obliged to quote the whole question.

3 Licensing or When do I have/want to pay for MySQL?

The basic licensing issues are:

The easiest way to pay for MySQL is to use the license form at TcX's secure server at https://www.tcx.se/license.htmy.
We hope everybody understands that you only have to pay if you are selling MySQL directly, selling a product which includes the MySQL server or installing and maintaining a MySQL server at some client site. You may not include MySQL in a distribution if you charge for some part of the distribution. For internal use, you do not have to pay us if you do not want to.
You do not need a license to include client code in commercial programs. The client access part of MySQL is in the public domain. The command line client (mysql) includes parts that are under the GNU Public License (readline).
We may add some additional functionality in the commercial version. The likely test candidate for this is the ability to create fast compressed read-only databases. The current server includes support to read such databases but not the packing tool used to create them. If we get enough revenue from support, we will probably release this under the same license as the other stuff.
But if you like MySQL and want to encourage further development you are welcome to purchase a license or support.

See section J The MySQL server license for non Microsoft operating systems.

3.1 How much MySQL costs

For normal use, MySQL costs nothing. When you sell MySQL, directly or as a part of another product, you have to pay for it. See section J The MySQL server license for non Microsoft operating systems.

If your use of MySQL requires a license (see section 3 Licensing or When do I have/want to pay for MySQL?), you only need to get a license for each machine that runs the mysqld server. A multi-CPU machine counts as one machine. There is no restriction on the number of concurrent users connected to a machine running a mysqld server.

Our current license prices are shown below. All prices are in US Dollars. If you pay by credit card, the currency is FIM (Finish Marks) so the prices will differ slightly.

Number of licenses	Price per copy	Total
1	US $200	US $200
10 pack	US $150	US $1500
50 pack	US $120	US $6000

For high volume (OEM) purchases, the following prices apply:

Number of licenses	Price per copy	Minimum at one time	Minimum payment
100-1000	$40	100	$4000
1000-2500	$25	200	$5000
2500-5000	$20	400	$8000

For OEM purchases, you must act as a middle-man for eventual problems or extension requests from users. We also require OEM customers to have a support contract.

If you have a low-margin high-volume product, you can always talk to us about other terms (for example, a percent of the sale price). If you do, please be informative about your product, pricing, market and any other information that may be relevant.

3.2 How to get commercial support

A full-price license includes really basic support. This means that we try to answer any relevant question. If the answer is in the documentation, we will direct you to the appropriate section. If you do not have a license or support, we probably will not answer at all.

If you discover what we consider a real bug, we are likely to fix it in any case. But if you pay for support we will notify you about the fix status instead of just fixing it in a later release.

More comprehensive support is sold separately. Costs for the various types of commercial support are shown below, and the following sections describe what each level of support includes. You are entitled to upgrade from any lower level of support to a higher level of support for the difference between the prices of the two support levels.

Type of support	Cost per year
Basic email support	US $200
Extended email support	US $1000
Login support	US $2000
Extended login support	US $5000

3.2.1 Basic email support

Basic email support includes the following types of service:

For MySQL-specific questions that don't belong on the MySQL mailing list (mysql@tcx.se), you can contact registration number and expiration date to ensure a quick response from mysql-support@tcx.se. You can also crosspost your questions to any of the standard MySQL mailing lists, as someone else may already have experienced and solved the problem/question you have.
We guarantee a timely answer for your email messages. We can't guarantee that we can solve any problem, but at least you will receive an answer if we can contact you by email.
We will help with unexpected problems when you install MySQL from a binary distribution on supported platforms. This level of support does not cover installing MySQL from a source distribution.
We will help you with bugs and missing features. Any bugs that are found are fixed for the next MySQL release. If the bug is critical for you, we will mail you a patch for it as soon the bug is fixed. Critical bugs always have the highest priority for us, to ensure that they are fixed as soon as possible.
Your suggestions for the further development of MySQL will be taken into consideration. By taking email support you have already helped the further development of MySQL. If you want to have more input, upgrade to a higher level of support.
If you want us to help optimize your system, you have to upgrade to a higher level of support.

3.2.2 Extended email support

Extended basic support includes everything in basic email support with these additions:

Your email will be dealt with before mail from basic email support users and non-registered users.
Your suggestions for the further development of MySQL will receive strong consideration. Simple extensions that suit the basic goals of MySQL are implemented in a matter of days. By taking extended email support you have already helped the further development of MySQL.
We include a binary version of the pack_isam tool that supports fast compressed read-only databases (no BLOB or TEXT types yet). The current server includes support to read such databases but not the packing tool.
Typical questions that are covered by extended email support are:
- We will answer and (within reason) solve questions that relate to possible bugs in MySQL. As soon as the bug is found and corrected, we will mail a patch for it.
- We will help with unexpected problems when you install MySQL from a source or binary distribution on supported platforms.
- We will answer questions about missing features and offer hints how to work around them.
- We will provide hints on optimizing mysqld for your situation.
You are allowed to influence the priority of items on the MySQL TODO. This will ensure that the features you really need will be implemented sooner than they might be otherwise.

3.2.3 Login support

Your email will be dealt with even before mail from extended support users.
Your suggestions for the further development of MySQL will be taken into very high consideration. Realistic extensions that can be implemented in a couple of hours and that suit the basic goals of MySQL will be implemented as soon as possible.
If you have a very specific problem, we can try to log in on your system to solve the problem "in place".
Like any database vendor, we can't guarantee that we can rescue any data from crashed tables, but if the worst happens we will help you rescue as much as possible. MySQL has proven itself very reliable, but anything is possible due to circumstances beyond our control (for example, if your system crashes or someone kills the server with kill -9).
We will provide hints on optimizing your system and your queries.
You are allowed to call a MySQL developer (in moderation) and discuss your MySQL-related problems.

3.2.4 Extended login support

Extended login support includes everything in login support with these additions:

Your email has the highest possible priority.
We will actively examine your system and help you optimize it and your queries. We may also optimize and/or extend MySQL to better suit your needs.

You may also request special extensions just for you. For example:

mysql> select MY_CALCULATION(col_name1,col_name2) from tbl_name;

We will provide a binary distribution of all important MySQL releases for your system, as long as we can get an account on a similar system. In the worst case, we may require access to your system to be able to create a binary distribution.
If you can provide accommodations and pay for traveler fares, you can even get a MySQL developer to visit you and offer you help with your troubles. Extended login support entitles you to one personal encounter per year, but we are as always very flexible towards our customers!

3.3 How to pay for licenses or support

Currently we can take SWIFT payments, cheques or credit cards.

Payment should be made to:

Postgirot Bank AB
105 06 STOCKHOLM, SWEDEN

T.C.X DataKonsult AB
BOX 6434
11382 STOCKHOLM, SWEDEN

SWIFT address: PGSI SESS
Account number: 96 77 06 - 3

Specify: license and/or support and your name and email address.

In Europe and Japan you can use EuroGiro (that should be less expensive) to the same account.

If you want to pay by cheque, make it payable to "Monty Program KB" and mail it to the address below:

T.C.X DataKonsult AB
BOX 6434
11382 STOCKHOLM, SWEDEN

If you want to pay with credit card over the Internet, you can use TcX's secure license form.

You can also print a copy of the above page, fill it in and send it by fax to:

+46-8-729 69 05

If you want us to bill you, you can use the license form and write "bill us" in the comment field. You can also mail a message to to bill you.

3.4 Who to contact for more information about licensing or support

For commercial licensing, or if you have any questions about any of the information in this section, please contact:

David Axmark
Detron HB
Kungsgatan 65 B
753 21 UPPSALA
SWEDEN
Voice Phone +46-18-10 22 80     (Swedish and English spoken)
Fax +46-8-729 69 05             (Email *much* preferred)
E-Mail: mysql-licensing@tcx.se

3.5 What copyrights MySQL uses

There are several different copyrights on the MySQL distribution:

The MySQL-specific source needed to build the mysqlclient library and programs in the `client' directory is in the public domain. Each file that is in the public domain has a header which clearly states so. This includes everything in the `client' directory and some parts of the mysys, mystring and dbug libraries.
Some small parts of the source (GNU getopt) are covered by the "GNU LIBRARY LIBRARY GENERAL PUBLIC LICENSE". See the `mysys/COPYING.LIB' file.
Some small parts of the source (GNU readline) are covered by the "GNU GENERAL PUBLIC LICENSE". See the `readline/COPYING' file.
Some parts of the source (the regexp library) are covered by a Berkeley style copyright.
The other source needed for the MySQL server on Unix platforms is covered by the "MySQL FREE PUBLIC LICENSE", which is based on the "Aladdin FREE PUBLIC LICENSE." See section J The MySQL server license for non Microsoft operating systems. When running MySQL on any Microsoft operating system, other licensing applies. See section K The MySQL license for Microsoft operating systems

The following points set forth the philosophy behind our copyright policy:

The SQL client library should be totally free so that it can be included in commercial products without limitations.
People who want free access to the software we have put a lot of work into can have it, so long as they do not try to make money directly by distributing it for profit.
People who want the right to keep their own software proprietary, but also want the value from our work, can pay for the privilege.
That means normal in-house use is FREE. But if you use it for something important to you, you may want to support further development of MySQL by purchasing a support contract.

3.6 When you may distribute MySQL commercially without a fee

This is a clarification of the information in the "MySQL FREE PUBLIC LICENSE" (FPL). See section J The MySQL server license for non Microsoft operating systems.

MySQL may be used freely, including by commercial entities for evaluation or unsupported internal use. However, distribution for commercial purposes of MySQL, or anything containing or derived from MySQL in whole or in part, requires a written commercial license from TcX AB, the sole entity authorized to grant such licenses.

You may not include MySQL "free" in a package containing anything for which a charge is being made, except as noted below.

The intent of the exception provided in the second clause of the license is to allow commercial organizations operating an FTP server or a bulletin board to distribute MySQL freely from it, provided that:

The organization complies with the other provisions of the FPL, which include among other things a requirement to distribute the full source code of MySQL and of any derived work, and to distribute the FPL itself along with MySQL;
The only charge for downloading MySQL is a charge based on the distribution service and not one based on the content of the information being retrieved (i.e., the charge would be the same for retrieving a random collection of bits of the same size);
The server or BBS is accessible to the general public, i.e., the phone number or IP address is not kept secret, and anyone may obtain access to the information (possibly by paying a subscription or access fee that is not dependent on or related to purchasing anything else).

If you want to distribute software in a commercial context that incorporates MySQL and you do not want to meet these conditions, you should contact TcX AB to find out about commercial licensing. Commercial licenses involve a payment, and include support and other benefits. These are the only ways you legally can distribute MySQL or anything containing MySQL: either by distributing MySQL under the requirements of the FPL, or by getting a commercial license from TcX AB.

3.7 Selling a product that can be configured to use MySQL

If you want to sell a product that can be configured to use MySQL although your customer is responsible for obtaining/installing MySQL (or some other supported alternative), does one of you owe us money if your customer chooses to use MySQL?

If your product REQUIRES MySQL to work, you would have to buy a license. If MySQL just added some new features, it should fall inside normal use. For example, if using MySQL added logging to a database rather than to a text file, it would not require a license. This would, of course, mean that the user bears the responsibility of obtaining and installing MySQL. If the program is (almost) useless without MySQL you would have to get a MySQL license to sell your product.

3.8 Running a commercial web server using MySQL

If you run a commercial web server that uses MySQL, you are not selling MySQL itself and need not purchase a license. However, in this case we would like you to purchase MySQL support. That is either your support of MySQL or our support of you (the latter is more expensive since our time is limited).

3.9 Selling commercial Perl/Tcl/PHP/etc. applications

These are the questions you should ask to determine whether or not you need a MySQL license when selling your application: Is your application designed for MySQL alone? Does it require MySQL to function at all? Or is it designed more generally for "a database" and can run under MySQL, PostgreSQL, or something else?

If you've designed it strictly around MySQL then you've really made a commercial product that requires the engine, so you need to buy a license.

If, however, you can support any database with a base level of functionality (and you don't rely on anything that only MySQL supports) you probably DO NOT have to pay.

It also depends on what you're doing for the client. Are you tying into a database you expect to already exist by the time your software is purchased? Then you probably don't have to pay. Or do you plan to distribute MySQL or give them detailed instructions on installing it with your software? Then you probably do.

One thing I'd like to suggest, folks. Look, development won't last forever if nobody pays. I agree that buying a copy for every software user is prohibitive compared to other products available, but would it not be courtesy for commercial developers to register their OWN copy that they develop with?

3.10 Possible future changes in the licensing

We may choose to distribute older versions of MySQL with the GPL in the future. However, these versions will be identified as GNU MySQL. Also, all copyright notices in the relevant files will be changed to the GPL.

4 Installing MySQL

4.1 How to get MySQL

Check the MySQL home page for information about the current version and for downloading instructions.

However, the Internet connection at TcX is not so fast; we would prefer that you do the actual downloading from one of the mirror sites listed below.

Please report bad or out of date mirrors to webmaster@tcx.se.

Europe:

Austria [Univ. of Technology/Vienna] WWW FTP
Bulgaria [Naturella] FTP
Czech Republic [CESNET] WWW
Denmark [Ake] WWW
Denmark [SunSITE] WWW FTP
Estonia [Tradenet] WWW
Germany [Wolfenbuettel] WWW
Germany [Staufen] WWW
Hungary [Xenia] WWW
Israel [Netvision] WWW
Italy [Matrice] WWW
Poland [Sunsite] WWW FTP
Russia [DirectNet] WWW
Russia [Cityline] FTP WWW
Romania [Timisoara] WWW FTP
Romania [Bucharest] WWW FTP
Sweden [Sunet] WWW FTP
UK [Omnipotent/UK] WWW FTP
UK [PLiG/UK] WWW FTP
UK [SunSITE] WWW FTP
Ukraina [Paco] WWW FTP

North America:

Canada [Polaris Computing] WWW
Canada [Tryc] WWW
Canada [Cyberus] WWW FTP
USA [Hurricane Electric/San Jose] WWW
USA [Buoy/New York] WWW
USA [Hypernet Communications/Dallas] WWW
USA [Netcasting/West Coast] FTP
USA [Circle Net/North Carolina] WWW
USA [Gina net/Florida] WWW
USA [DIGEX] FTP

South America:

Chile [Amerikanclaris] WWW FTP

Asia:

Korea [KREONet] WWW
Japan [Soft Agency] WWW
Japan [HappySize] WWW FTP
Singapore [Com5 Productions] WWW FTP
Taiwan [NCTU] WWW

Australia:

Australia [AARNet/Queensland] WWW FTP
Australia [Tas] WWW FTP
Australia [Blue Planet/Melbourne] WWW FTP

Africa:

South-Africa [The Internet Solution/Johannesburg] FTP

4.2 Operating systems supported by MySQL

We use GNU Autoconf so it is possible to port MySQL to all modern systems with working Posix threads and a C++ compiler. The client code requires C++ but not threads. We use and develop the software ourselves primarily on Sun Solaris (versions 2.5 & 2.6) and to a lesser extent on RedHat Linux 5.0.

MySQL has been reported to compile sucessfully on the following operating system/thread package combinations. Note that for many operating systems, the native thread support works only in the latest versions.

Solaris 2.5 & 2.6 with native threads
SunOS 4.x with the included MIT-pthreads package
BSDI 2.x with the included MIT-pthreads package
BSDI 3.0 and 3.1 with native threads
SGI IRIX 6.x with native threads
AIX 4.x with native threads
DEC UNIX 4.x with native threads
Linux 2.0+ with LinuxThreads 0.7.1 or glibc 2.0.7
FreeBSD 2.x with the included MIT-pthreads package
FreeBSD 3.x with native threads
SCO OpenServer with a recent port of the FSU-threads package
NetBSD 1.3 Intel and NetBSD 1.3 Alpha
OpenBSD 2.x with the included MIT-pthreads package
HP-UX 10.20 with the included MIT-pthreads package
Win95 and NT (this version is currently available only for users with a MySQL license or MySQL email support)

4.3 Which MySQL version to use

The first decision to make is whether you want to use the latest development release or the last stable release.

Normally if you are beginning to use MySQL for the first time or trying to port it to some system for which there is no binary distribution, we recommend going with the development release. This is because there are usually no really bad bugs in the development release, and you can easily test it on your machine with the crash-me and benchmark tests. See section 11 The MySQL benchmark suite.

Otherwise, if you are running an old system and want to upgrade, but don't want to take chances with 3.22, you should upgrade to 3.21.33. We have tried to fix only fatal bugs and make small, relatively safe changes in this version.

The second decision to make is whether you want to use a source distribution or a binary distribution:

If you want to run MySQL on a platform for which a current binary distribution exists, use that. It generally will be easier to install than a source distribution.
If you want to read (and/or modify) the C and C++ code that makes up MySQL, you should get a source distribution. The source code is always the ultimate manual. Source distributions also contain more tests and examples than binary distributions.

In the MySQL naming scheme, release numbers consist of three numbers and a suffix. For example, a release name like mysql-3.21.17-beta is interpreted like this:

The first number (3) describes the file format. All version 3 releases have the same file format. When a version 4 appears, every table will have to be converted to the new format (nice tools for this will be included, of course).
The second number (21) is the release level. Normally there are two to choose from. One is the release/stable branch and the other is the development branch. Normally both are stable but the development version may have quirks, missing documentation or may fail to compile on some systems.
The third number (17) is the version number within the release level. This is incremented for each new distribution. Usually you want the latest version for the release level you have choosen.
The suffix (beta) indicates the stability level of the release:
- alpha means that some new large code section exists which hasn't been 100% tested. Known bugs should be documented in the News section (usually there are none). See section D MySQL change history. There are also new commands and extensions in most alpha releases.
- beta means that all new code has been tested. No major new things are added. There should be no known bugs.
- gamma is a beta that has been around a while and seems to work fine. This is what many other companies call a release.
- If there is no suffix, it means that the version has been run for a while at many different sites with no reports of bugs other than platform-specific bugs.

All versions of MySQL are run through our standard tests and benchmarks to ensure that they are relatively safe to use. Since the standard tests are extended over time to check for all previously found bugs, the test suite keeps getting better.

Note that all releases have been tested at least with:

An internal test suite: This is part of a production system for a customer. It has many tables with hundreds of megabytes of data.
The MySQL benchmark suite: This runs a range of common queries. It is also a test to see whether the latest batch of optimizations actually made the code faster. See section 11 The MySQL benchmark suite.
The crash-me test: This tries to determine what features the database supports and what its capabilities and limitations are. See section 11 The MySQL benchmark suite.

Another test is that we use the newest MySQL version in our internal production environment, on at least one machine. We have more than 100 gigabytes of data to work with.

4.4 How and when updates are released

Well, MySQL is evolving quite rapidly here at TcX and we want to share this with other MySQL users. We try to make a release when we have a very useful feature that others seem to have a need for.

We also try to help out users who request features that are easy to implement. We also take note on what our licensed users want to have and we especially take notes of what our extended email supported customers want and try to help them out.

No one has to download a new release. The News section will tell you if the new release has something you really want. See section D MySQL change history.

We use the following policy when updating MySQL:

For each minor update, the last number in the version string is incremented. When there are major new features or minor incompatibilities with previous versions, the second number in the version string is incremented. When the file format changes, the first number is increased.
Stable tested releases are meant to appear about 1-2 times a year, but if small bugs are found, a release with only bug-fixes will be released.
Working releases are meant to appear about every 1-8 weeks.
Binary distributions for some platforms will be made by us for major releases. Other people may make binary distributions for other systems but probably less frequently.
We usually make patches available as soon as we have located and fixed small bugs.
For non-critical but annoying bugs, we will make patches available if they are sent to us. Otherwise we will combine many of them into a bigger patch.
If there is, by any chance, a fatal bug in a release we will make a new release as soon as possible. We would like other companies to do this, too. :)

The 3.21.x version incorporates major portability changes for many different systems. When the 3.21 release is stable, we will remove the alpha/beta suffix and move active development to 3.22. Bugs will still be fixed in the stable version. We don't believe in a complete freeze, as this also leaves out bug fixes and things that "must be done". "Somewhat frozen" means that we may add small things that "almost surely will not affect anything that's already working".

4.5 Installation layouts

This section describes the default layout of the directories created by installing binary and source distributions.

A binary distribution is installed by unpacking it at the installation location you choose and creates the following directories in the location you choose (typically `/usr/local/mysql'):

Directory	Contents of directory
`bin'	Client programs, the `mysqld` server
`data'	Log files, databases
`scripts'	`mysql_install_db`
`share'	Error message files
`sql-bench'	Benchmarks

A source distribution is installed after you configure and compile it. By default, the installation step installs files under `/usr/local', in the following subdirectories:

Directory	Contents of directory
`bin'	Client programs and scripts
`libexec'	The `mysqld` server
`share'	Error message files
`sql-bench'	Benchmarks
`var'	Log files, databases

The layout of a source installation differs from that of a binary installation in the following ways:

The mysqld server is installed in the `/usr/local/libexec' directory rather than in `/usr/local/mysql/bin'.
The data directory is `/usr/local/var' rather than `/usr/local/mysql/data'.
mysql_install_db is installed in the `/usr/local/bin' directory rather than in `/usr/local/mysql/scripts'.

4.6 Installing a MySQL binary distribution

The basic commands you have to do to use a MySQL binary distribution are:

scripts/mysql_install_db
bin/safe_mysqld &

Here follows a more detailed description:

You need the following tools to install a MySQL binary distribution:

GNU gunzip to uncompress the distribution.
A reasonable tar to unpack the distribution. GNU tar is known to work.

If you run into problems, PLEASE ALWAYS USE mysqlbug when posting questions to mysql@tcx.se. Even if the problem isn't a bug, mysqlbug gathers system information that will help others solve your problem. By not using mysqlbug, you lessen the likelihood of getting a solution to your problem! You will find mysqlbug in the `bin' directory after you unpack the distribution. See section 2.3 What to do if you think you have found a bug.

To install a binary distribution, follow the steps below, then proceed to section 4.14 Post-installation setup and testing, for post-installation setup and testing.

Pick the directory under which you want to unpack the distribution, and move into it. In the example below, we unpack the distribution under `/usr/local' and create a directory `/usr/local/mysql' into which MySQL is installed. (The following instructions therefore assume you have permission to create files in `/usr/local'. If that directory is protected, you will need to perform the installation as root.)
Obtain a distribution file from one of the sites listed in section 4.1 How to get MySQL. MySQL binary distributions are provided as compressed tar archives and have names like `mysql-VERSION-OS.tar.gz', where VERSION is a number (e.g., 3.21.15), and OS indicates the type of operating system for which the distribution is intended (e.g., pc-linux-gnu-i586).
Unpack the distribution and create the installation directory:
```
shell> gunzip < mysql-VERSION-OS.tar.gz | tar xvf -
shell> ln -s mysql-VERSION-OS mysql
```
The first command creates a directory named `mysql-VERSION-OS'. The second command makes a symbolic link to that directory. This lets you refer more easily to the installation directory as `/usr/local/mysql'.
Change into the installation directory:
```
shell> cd mysql
```
You will find several files and subdirectories in the mysql directory. The most important for installation purposes are the `bin' and `scripts' subdirectories.

`bin'
This directory contains client programs and the server You should add the full pathname of this directory to your PATH environment variable so that your shell finds the MySQL programs properly.
`scripts'
This directory contains the mysql_install_db script used to initialize the server access permissions
If you would like to use mysqlaccess and have the MySQL distribution in some nonstandard place, you must change the location where mysqlaccess expects to find the mysql client. Edit the `bin/mysqlaccess' script at approximately line 18. Search for a line that looks like this:
```
$MYSQL     = '/usr/local/bin/mysql';    # path to mysql executable
```
Change the path to reflect the location where mysql actually is stored on your system. If you do not do this, you will get a broken pipe error when you run mysqlaccess.
If you want to install support for the Perl DBI/DBD interface, see section 4.10 Perl installation comments.
If you would like MySQL to start automatically when you boot your machine, you can copy bin/mysql.server to where your system has its startup files. More information can be found in the bin/mysql.server script itself, and in section 4.14.3 Automatically starting and stopping MySQL.

After everything has been unpacked and installed, you should initialize and test your distribution. See section 4.14 Post-installation setup and testing.

4.6.1 Building client programs

If you compile MySQL clients that you've written yourself or that you obtain from a third party, they must be linked using the -lmysqlclient option on the link command. You may also need to specify a -L option to tell the linker where to find the library. For example, if the library is installed in `/usr/local/mysql/lib', use -L/usr/local/mysql/lib -lmysqlclient on the link command.

For clients that use MySQL header files, you may need to specify a -I option (for example, -I/usr/local/mysql/include) when you compile them, so the compiler can find the header files.

4.6.2 System-specific notes

The following sections indicate some of the issues that have been observed to occur on particular systems.

4.6.2.1 Linux notes

MySQL needs at least Linux 2.0.
The binary release is linked with -static, which means you need not worry about which version of the system libraries you have. You need not install LinuxThreads, either. A program linked with -static is slightly bigger than a dynamically-linked program but also slightly faster (3-5%). The only problem is that you can't use user definable functions (UDFs) with a statically-linked program. If you are going to write or use UDF functions (this is only something for C or C++ programmers) you must compile MySQL yourself, using dynamic linking.

The Linux-Intel binary release of MySQL is configured for the highest possible speed. We are even using the Pentium compiler, pgcc. This compiler is installed under the name gcc and the distribution is configured as follows:

shell> CC=gcc \
       CFLAGS="-O6 -mpentium -mstack-align-double -fomit-frame-pointer" \
       CXX=gcc \
       CXXFLAGS="-O6 -mpentium -mstack-align-double -fomit-frame-pointer -felide-constructors" \
       ./configure \
           --prefix=/usr/local/mysql \
           --enable-assembler \
           --with-mysqld-ldflags=-all-static

MySQL Perl support requires Perl 5.004_03 or newer.

4.6.2.2 HP-UX notes

The binary distribution of MySQL for HP-UX is distributed as an HP depot file. This means that you must be running at least HP-UX 10.x to have access to HP's software depot tools.

The HP version of MySQL was compiled on an HP 9000/8xx server under HP-UX 10.20, and uses MIT-pthreads. It is known to work well under this configuration. This version does not use HP's native thread package. It is highly unlikely that MySQL will use HP native threads on anything but HP-UX 10.30 or later.

Other configurations that may work:

HP 9000/7xx running HP-UX 10.20+
HP 9000/8xx running HP-UX 10.30 (does not use HP native threads)

The following configurations almost definitely won't work:

HP 9000/7xx or 8xx running HP-UX 10.x where x < 2
HP 9000/7xx or 8xx running HP-UX 9.x

To install the distribution, use one of the commands below, where /path/to/depot is the full path to the depot file:

To install everything, including the server, client and development tools:
```
/usr/sbin/swinstall -s /path/to/depot mysql.full
```

To install only the server:

/usr/sbin/swinstall -s /path/to/depot mysql.server

To install only the client package:

/usr/sbin/swinstall -s /path/to/depot mysql.client

To install only the development tools:

/usr/sbin/swinstall -s /path/to/depot mysql.developer

The depot places binaries and libraries in `/opt/mysql' and data in `/var/opt/mysql'. The depot also creates the appropriate entries in `/sbin/init.d' and `/sbin/rc2.d' to start the server automatically at boot time. Obviously, this entails being root to install.

4.7 Installing a MySQL source distribution

You need the following tools to build and install MySQL from source:

GNU gunzip to uncompress the distribution.
A reasonable tar to unpack the distribution. GNU tar is known to work.
A working ANSI C++ compiler. gcc >= 2.8.1, egcs >= 1.0.2, SGI C++ and SunPro C++ are some of the compilers that are known to work. libg++ is not needed when using gcc. gcc 2.7.x has a bug that makes it impossible to compile some perfectly legal C++ files, such as `sql/sql_base.cc'. If you only have gcc 2.7.x, you must upgrade your gcc to be able to compile MySQL.
A good make program. GNU make is always recommended and is sometimes required. If you have problems, we recommend trying GNU make 3.75 or newer.

If you run into problems, PLEASE ALWAYS USE mysqlbug when posting questions to mysql@tcx.se. Even if the problem isn't a bug, mysqlbug gathers system information that will help others solve your problem. By not using mysqlbug, you lessen the likelihood of getting a solution to your problem! You will find mysqlbug in the `scripts' directory after you unpack the distribution. See section 2.3 What to do if you think you have found a bug.

4.7.1 Quick installation overview

The basic commands you have to do to install MySQL from source are:

configure --prefix=/usr/local/mysql
make
make install
scripts/mysql_install_db
/usr/local/mysql/bin/safe_mysqld &

Here follows a more detailed description:

To install a source distribution, follow the steps below, then proceed to section 4.14 Post-installation setup and testing, for post-installation initialization and testing.

Pick the directory under which you want to unpack the distribution, and move into it.
Obtain a distribution file from one of the sites listed in section 4.1 How to get MySQL. MySQL source distributions are provided as compressed tar archives and have names like `mysql-VERSION.tar.gz', where VERSION is a number like 3.22.10-beta.
Unpack the distribution into the current directory:
```
shell> gunzip < mysql-VERSION.tar.gz | tar xvf -
```
This command creates a directory named `mysql-VERSION'.
Change into the top-level directory of the unpacked distribution:
```
shell> cd mysql-VERSION
```
Configure the release and compile everything:
```
shell> ./configure
shell> make
```
When you run configure, you might want to specify some options. Run ./configure --help for a list of options. section 4.7.3 Typical configure options, discusses some of the more useful options. If configure fails, and you are going to send mail to `config.log' that you think can help solve the problem. Also include the last couple of lines of output from configure if configure aborts. Post the bug report using the mysqlbug script. See section 2.3 What to do if you think you have found a bug. If the compile fails, see section 4.8 Problems compiling?, for help with a number of common problems.
Install everything:
```
shell> make install
```
You might need to run this command as root.
Create the MySQL privilege tables
```
shell> scripts/mysql_install_db
```
If you want to install support for the Perl DBI/DBD interface, see section 4.10 Perl installation comments.
If you would like MySQL to start automatically when you boot your machine, you can copy support-files/mysql.server to where your system has its startup files. More information can be found in the support-files/mysql.server script itself, and in section 4.14.3 Automatically starting and stopping MySQL.

After everything has been installed, you should initialize and test your distribution.

You can start the MySQL server with:

shell> cd mysql-install-directory
shell> bin/safe_mysqld &

Note that MySQL versions before 3.22.10 started the MySQL server when you run mysql_install_db. This is not longer true!

See section 4.14 Post-installation setup and testing.

4.7.2 Applying patches

Sometimes patches appear on the mailing list. To apply a patch, change into the top-level directory of your MySQL source tree and run these commands:

shell> gunzip < patch-file-name.gz | patch -p1
shell> rm config.cache
shell> make clean

Then follow the instructions for a normal source install, beginning with the ./configure step. After running the make install step, restart your MySQL server.

You may need to bring down any currently running server before you run make install. Some systems do not allow you to install a new version of a program if it replaces the version that is currently executing.

4.7.3 Typical `configure` options

The configure script gives you a great deal of control over how you configure your MySQL distribution. Typically you do this using options on the configure command line. You can also affect configure using certain environment variables. For a list of options supported by configure, run this command:

shell> ./configure --help

Some of the more commonly-used configure options are described below:

To compile just the MySQL client libraries and client programs, use the --without-server option:
```
shell> ./configure --without-server
```
If you don't have a C++ compiler, mysql will not compile (it is the one client program that requires C++). In this case, you can remove the code in configure that tests for the C++ compiler and then run ./configure with the --without-server option. The compile step will still try to build mysql, but you can ignore any warnings about `mysql.cc'. (If make stops, try make -k to tell it to continue with the rest of the build even if errors occur.)
If you don't want your log files and database directories located under `/usr/local/var', use a configure command something like one of these:
```
shell> ./configure --prefix=/usr/local/mysql
shell> ./configure --prefix=/usr/local \
           --localstatedir=/usr/local/mysql/data
```
The first command changes the installation prefix so that everything is installed under `/usr/local/mysql' rather than the default of `/usr/local'. The second command preserves the default installation prefix, but overrides the default location for database directories (normally `/usr/local/var') and changes it to /usr/local/mysql/data.
If you want your sockets located somewhere other than the default location (normally `/tmp' or `/var/run'), use a configure command like this:
```
shell> ./configure --with-unix-socket-path=/path/to/socket/dir
```
`/path/to/socket/dir' must be an absolute pathname.
If you want to compile statically-linked programs (e.g., to make a binary distribution, to get more speed or to work around problems with some RedHat distributions), run configure like this:
```
shell> ./configure --with-client-ldflags=-all-static \
           --with-mysqld-ldflags=-all-static
```
If you are using gcc and don't have libg++ or libstdc++ installed, you can tell configure to use gcc as your C++ compiler:
```
shell> CC=gcc CXX=gcc ./configure
```
When you use gcc as your C++ compiler, it will not attempt to link in libg++ or libstdc++. If you get errors that your compiler or linker can't create the shared library `libmysqlclient.so.#' you can work around this problem by giving the --disable-shared option to configure. In this case, configure will not build a shared libmysqlclient.so.# library.
You can configure MySQL not to use DEFAULT column values for non-NULL columns (i.e., columns that are not allowed to be NULL). This causes INSERT statements to generate an error unless you explicitly specify values for all columns that require a non-NULL value. To suppress use of default values, run configure like this:
```
shell> CXXFLAGS=-DDONT_USE_DEFAULT_FIELDS ./configure
```
By default, MySQL uses the ISO-8859-1 (Latin1) character set. To change the default set, use the --with-charset option:
```
shell> ./configure --with-charset=CHARSET
```
CHARSET may be one of big5, czech, danish, dec8, dos, german1, hp8, koi8_ru, latin1, latin2, swe7, usa7, ujis or sjis. See section 9.1.1 The character set used for data and sorting. If you want to convert characters between the server and the client, you should take a look at the SET OPTION CHARACTER SET command. See section 7.24 SET OPTION syntax. Warning: If you change character sets after having created any tables, you will have to run isamchk -r -q on every table. Your indexes may be sorted incorrectly otherwise. (This can happen if you install MySQL, create some tables, the reconfigure MySQL using a different character set and reinstall it.)
To configure MySQL with debugging code, use the --with-debug option:
```
shell> ./configure --with-debug
```
This causes a safe memory allocator to be included that can find some errors and that provides output about what is happening.
Options that pertain to particular systems can be found in the system-specific sections later in this chapter. See section 4.11 System-specific notes.

4.8 Problems compiling?

All MySQL programs compile cleanly for us with no warnings on Solaris using gcc. On other systems, warnings may occur due to differences in system include files. See section 4.9 MIT-pthreads notes, for warnings that may occur when using MIT-pthreads. For other problems, check the list below.

The solution to many problems involves reconfiguring. If you do need to reconfigure, take note of the following:

If configure is run after it already has been run, it may use information that was gathered during its previous invocation. This information is stored in `config.cache'; when configure starts up, it looks for that file and reads its contents if it exists, on the assumption that the information is still correct. That assumption is invalid when you reconfigure.
Each time you run configure, you must run make again to recompile. However, you may want to remove old object files from previous builds first, since they were compiled using different configuration options.

To prevent old configuration information or object files from being used, run these commands before rerunning configure:

shell> rm config.cache
shell> make clean

Alternatively, you can run make distclean.

The list below describes some of the problems compiling MySQL that have been found to occur most often:

If you get errors when compiling `sql_yacc.cc' such as the ones shown below, you have probably run out of memory or swap space:
```
Internal compiler error: program cc1plus got fatal signal 11
  or
Out of virtual memory
  or
Virtual memory exhausted
```
The problem is that gcc requires huge amounts of memory to compile `sql_yacc.cc' with inline functions. Try running configure with the --with-low-memory option:
```
shell> ./configure --with-low-memory
```
This option causes -fno-inline to be added to the compile line if you are using gcc and -O0 if you are using something else. You should try the --with-low-memory option even if you have so much memory and swap space that you think you can't possibly have run out. This problem has been known to occur even on systems with generous hardware configurations, and the --with-low-memory option usually fixes it.
By default, configure picks c++ as the compiler name and GNU c++ links with -lg++. If you are using gcc, this can cause problems during configuration such as this:
```
configure: error: installation or configuration problem:
C++ compiler cannot create executables.
```
You might also observe problems during compilation related to g++, libg++ or libstdc++. One cause of these problems is that you may not have g++, or you may have g++ but not libg++ or libstdc++. The `config.log' contains the exact reason why your c++ compiler didn't work! To work around these problems, you can use gcc as your C++ compiler. Try setting the environment variable CXX to "gcc -O3". For example:
```
shell> CXX="gcc -O3" ./configure
```
This works because gcc compiles C++ sources as well as g++ does, but does not link in libg++ or libstdc++ by default. Another way to fix these problems, of course, is to install g++, libg++ and libstdc++.

If your compile fails with either of the following errors, you have to upgrade your version of make to GNU make:

making all in mit-pthreads
make: Fatal error in reader: Makefile, line 18:
Badly formed macro assignment
  or
make: file `Makefile' line 18: Must be a separator (:

If your make stops with this error, you should try using GNU make:
```
Can't find Makefile.PL
```
Solaris and FreeBSD are known to have troublesome make programs.
If you get error messages from make or error messages like this, you have to upgrade your make to GNU make:
```
pthread.h: No such file or directory
```
GNU make version 3.75 is known to work.
If you want to define flags to be used by your C or C++ compilers, do so by adding the flags to the CFLAGS and CXXFLAGS environment variables. You can also specify the compiler names this way using CC and CXX. For example:
```
shell> CC=gcc
shell> CFLAGS=-O6
shell> CXX=gcc
shell> CXXFLAGS=-O6
shell> export CC CFLAGS CXX CXXFLAGS
```
See section 4.12 TcX binaries, for a list of flag definitions that have been found to be useful on various systems.
If you get an error message like this, you need to upgrade your gcc compiler:
```
client/libmysql.c:273: parse error before `__attribute__'
```
gcc 2.8.1 is known to work, but we recommend using egcs 1.0.3a or newer instead.
If you get errors when compiling mysqld that look like this, configure didn't correctly detect the type of the last argument to accept(), getsockname() or getpeername():
```
cxx: Error: mysqld.cc, line 645: In this statement, the referenced
     type of the pointer value "&length" is "unsigned long", which
     is not compatible with "int".
new_sock = accept(sock, (struct sockaddr *)&cAddr, &length);
```
To fix this, edit the `config.h' file (which is generated by configure). Look for these lines:
```
/* Define as the base type of the last arg to accept */
#define SOCKET_SIZE_TYPE XXX
```
Change XXX to size_t or int, depending on your operating system. (Note that you will have to do this each time you run configure, since configure regenerates `config.h'.)
The `sql_yacc.cc' file is generated from `sql_yacc.yy'. Normally you don't need to generate `sql_yacc.cc' yourself, since MySQL comes with an already-generated copy. However, if you do need to recreate it, you might encounter this error:
```
"sql_yacc.yy", line xxx fatal: default action causes potential...
```
This is a sign that your version of yacc is deficient. You probably need to install bison (the GNU version of yacc) and use that instead.
If you need to debug mysqld or a MySQL client, run configure with the --with-debug option, then recompile and link your clients with the new client library. Before running a client, you should set the MYSQL_DEBUG environment variable:
```
shell> MYSQL_DEBUG=d:t:O,/tmp/client.trace
shell> export MYSQL_DEBUG
```
This causes clients to generate a trace file in `/tmp/client.trace'.
If you have problems with your own client code, you should attempt to connect to the server and run your query using a client that is known to work. Do this by running mysql in debugging mode:
```
shell> mysql --debug=d:t:O,/tmp/client.trace
```
This will provide useful information in case you mail a bug report. See section 2.3 What to do if you think you have found a bug.

4.9 MIT-pthreads notes

This section describes some of the issues involved in using MIT-pthreads.

If your system does not provide native thread support, you will need to build MySQL using the MIT-pthreads package. This includes most FreeBSD systems, SunOS 4.x, Solaris 2.4 and earlier, and some others. See section 4.2 Operating systems supported by MySQL.

On most systems, you can force MIT-pthreads to be used by running configure with the --with-mit-threads option:
```
shell> ./configure --with-mit-threads
```
Building in a non-source directory is not supported when using MIT-pthreads, because we want to minimize our changes to this code.
MIT-pthreads doesn't support the AF_UNIX protocol used to implement Unix sockets. This means that if you compile using MIT-pthreads, all connections must be made using TCP/IP (which is a little slower). If you find after building MySQL that you cannot connect to the local server, it may be that your client is attempting to connect to localhost using a Unix socket as the default. Try making a TCP/IP connection by using mysql with a host option (-h or --host) to specify the local host name explicitly.
The checks that determine whether or not to use MIT-pthreads occur only during the part of the configuration process that deals with the server code. If you have configured the distribution using --without-server to build only the client code, clients will not know whether or not MIT-pthreads is being used and will use Unix socket connections by default. Since Unix sockets do not work under MIT-pthreads, you will also need to use -h or --host in such instances.
When MySQL is compiled using MIT-pthreads, system locking is disabled by default for performance reasons. You can tell the server to use system locking with the --use-locking option.
Sometimes (at least on Solaris) the pthread bind() command fails to bind to a socket without any error message. The result is that all connections to the server fail. For example:
```
shell> mysqladmin version
mysqladmin: connect to server at " failed;
error: 'Can't connect to mysql server on localhost (146)'
```
The solution to this is to kill the mysqld server and restart it. This has only happened to us when we have forced the server down and done a restart immediately.
With MIT-pthreads, the sleep() system call isn't interruptible with SIGINT (break). This is only noticeable when you run mysqladmin --sleep. You must wait for the sleep() call to terminate before the interrupt is served and the process stops.

When linking (at least on Solaris) you will receive warning messages like these; they can be ignored:

ld: warning: symbol `_iob' has differing sizes:
    (file /my/local/pthreads/lib/libpthread.a(findfp.o) value=0x4;
file /usr/lib/libc.so value=0x140);
    /my/local/pthreads/lib/libpthread.a(findfp.o) definition taken
ld: warning: symbol `__iob' has differing sizes:
    (file /my/local/pthreads/lib/libpthread.a(findfp.o) value=0x4;
file /usr/lib/libc.so value=0x140);
    /my/local/pthreads/lib/libpthread.a(findfp.o) definition taken

Some other warnings also can be ignored:

implicit declaration of function `int strtoll(...)'
implicit declaration of function `int strtoul(...)'

We haven't gotten readline to work with MIT-pthreads. (This isn't needed, but may be interesting for someone.)

4.10 Perl installation comments

MySQL support for the Perl DBI/DBD interface is distributed separately from the main MySQL distribution, as of release 3.22.8. If you want to install Perl support, check the http://www.tcx.se/Contrib for the files you will need.

The Perl client code for the DBD/DBI interface requires Perl 5.004 or later. The interface will not work if you have an older version of Perl.

The Perl distributions are provided as compressed tar archives and have names like `MODULE-VERSION.tar.gz', where MODULE is the module name and VERSION is the version number. You should get the Data-Dumper, DBI, and Msql-Mysql-modules archives. Once you have them, install them using the procedure shown below. The example shown below is for the Data-Dumper module, but the procedure is the same for all three modules.

Unpack the distribution into the current directory:
```
shell> gunzip < Data-Dumper-VERSION.tar.gz | tar xvf -
```
This command creates a directory named `Data-Dumper-VERSION'.
Change into the top-level directory of the unpacked distribution:
```
shell> cd Data-Dumper-VERSION
```

Build the distribution and compile everything:

shell> perl Makefile.PL
shell> make
shell> make install

After you've installed the three modules, run make test in the Msql-Mysql-modules directory to exercise the interface code. (The server must be running for this to work.)

4.10.1 Problems using the Perl `DBI`/`DBD` interface

If you get the following errors from DBD-mysql, you are probably using gcc (or using an old binary compiled with gcc):

/usr/bin/perl: can't resolve symbol '__moddi3'
/usr/bin/perl: can't resolve symbol '__divdi3'

Add -L/usr/lib/gcc-lib/... -lgcc to the link command when the `mysql.so' library gets built (check the output from make for `mysql.so' when you compile the Perl client). The -L option should specify the path to the directory where `libgcc.a' is located on your system.

Another cause of this problem may be that Perl and MySQL aren't both compiled with gcc. In this case, you can solve the mismatch by compiling both with gcc.

If you want to use the Perl module on a system that doesn't support dynamic linking (like SCO) you can generate a static version of Perl that includes DBI and DBD-mysql. The way this works is that you generate a version of Perl with the DBI code linked in and install it on top of your current Perl. Then you use that to build a version of Perl that additionally has the DBD code linked in, and install that.

On SCO, you must have the following environment variables set:

shell> LD_LIBRARY_PATH=/lib:/usr/lib:/usr/local/lib:/usr/progressive/lib
or
shell> LD_LIBRARY_PATH=/usr/lib:/lib:/usr/local/lib:/usr/ccs/lib:/usr/progressive/lib:/usr/skunk/lib
shell> LIBPATH=/usr/lib:/lib:/usr/local/lib:/usr/ccs/lib:/usr/progressive/lib:/usr/skunk/lib
shell> MANPATH=scohelp:/usr/man:/usr/local1/man:/usr/local/man:/usr/skunk/man:

First, you create a Perl that includes a statically-linked DBI by running these commands in the `perl/DBI' directory:

shell> perl Makefile.PL LINKTYPE=static
shell> make
shell> make install
shell> make perl

After this you must install the new Perl. The output of make perl will indicate the exact make command you will need to execute to perform the installation. On SCO, this is make -f Makefile.aperl inst_perl MAP_TARGET=perl.

Next you create Perl that includes a statically-linked DBD::mysql by running these commands in the `perl/Mysql-modules' directory:

shell> perl Makefile.PL LINKTYPE=static
shell> make
shell> make install
shell> make perl

You should also install this new Perl. Again, the output of make perl indicates the command to use.

4.11 System-specific notes

The following sections indicate some of the issues that have been observed to occur on particular systems.

4.11.1 Solaris notes

On Solaris, you may run into trouble even before you get the MySQL distribution unpacked! Solaris tar can't handle long file names, so you may see an error like this when you unpack MySQL:

x mysql-3.22.8-beta/bench/Results/ATIS-mysql_odbc-NT_4.0-cmp-db2,informix,ms-sql,mysql,oracle,solid,sybase, 0 bytes, 0 tape blocks
tar: directory checksum error

In this case, you must use GNU tar (gtar) to unpack the distribution. You can find a precompiled copy for Solaris at http://www.tcx.se/Downloads/.

Sun native threads work only on Solaris 2.5 and higher. For 2.4 and earlier versions, you can use MIT-pthreads. See section 4.9 MIT-pthreads notes.

If you have the Sun Workshop 4.2 compiler, you can run configure like this:

shell> CC=cc CFLAGS="-Xa -fast -xstrconst -mt" \
       CXX=CC CXXFLAGS="-xsb -noex -fast -mt" \
       ./configure

You may also have to edit the configure script to change this line:

#if !defined(__STDC__) || __STDC__ != 1

to this:

#if !defined(__STDC__)

If you turn on __STDC__ with the -Xc option, the Sun compiler can't compile with the Solaris `pthread.h' header file. This is a Sun bug (broken compiler or broken include file).

If mysqld issues the error message shown below when you run it, you have tried to compile MySQL with the Sun compiler without enabling the multi-thread option -mt:

libc internal error: _rmutex_unlock: rmutex not held

Add -mt to CFLAGS and CXXFLAGS and try again.

If you get the following error when compiling MySQL with gcc, it means that your gcc is not configured for your version of Solaris!

shell> gcc -O3 -g -O2 -DDBUG_OFF  -o thr_alarm ...
./thr_alarm.c: In function `signal_hand':
./thr_alarm.c:556: too many arguments to function `sigwait'

The proper thing to do in this case is to get the newest version of egcs or gcc and compile it with your current gcc compiler! At least for Solaris 2.5, almost all binary versions of gcc have old, unusable include files that will break all programs that use threads (and possibly other programs)!

As Solaris doesn't provide static versions of all system libraries (libpthreads and libdl), you can't compile MySQL with --static. If you try to do this, you will get the error:

ld: fatal: library -ldl: not found

If too many processes try to connect very rapidly to mysqld, you will see this error in the MySQL log:

Error in accept: Protocol error

You might try starting the server with the --set-variable back_log=50 option as a workaround for this.

4.11.2 SunOS 4 notes

On SunOS 4, MIT-pthreads is needed. This in turn means you will need GNU make to compile MySQL.

Some SunOS 4 systems have problems with dynamic libraries and libtool. You can use the following configure line to avoid this problem.

./configure --disable-shared --with-mysqld-ldflags=-all-static

When compiling readline, you may get warnings about duplicate defines. These may be ignored.

When compiling mysqld, there will be some implicit declaration of function warnings. These may be ignored.

4.11.3 Linux notes (all Linux versions)

If you can't start mysqld or if mysql_install_db doesn't work, please continue reading! This only happens on Linux system with problems in the LinuxThreads or libc/glibc libraries. There are a lot of simple workarounds to get MySQL to work! The simplest is to use the binary version of MySQL (not the RPM) for Linux x86; One nice aspect of this version is that it's probably 10% faster than any version you would compile yourself! See section 10.3 How compiling and linking affects the speed of MySQL.

isamchk hangs with libc.so.5.3.12. Upgrading to the newest libc fixes this problem.

When using LinuxThreads you will see a minimum of three processes running. These are in fact threads. There will be one thread for the LinuxThreads manager, one thread to handle connections, and one thread to handle alarms and signals.

If you are using LinuxThreads and mysqladmin shutdown doesn't work, you have to upgrade to LinuxThreads 0.7.1 or newer.

If you are using RedHat, you might get errors like this:

/usr/bin/perl is needed...
/usr/sh is needed...
/usr/sh is needed...

If so, you should upgrade your version of rpm to `rpm-2.4.11-1.i386.rpm' and `rpm-devel-2.4.11-1.i386.rpm' (or later).

You can get the upgrades of libraries to RedHat 4.2 from ftp://ftp.redhat.com/updates/4.2/i386. Or http://www.sunsite.unc.edu/pub/Linux/distributions/redhat/code/rpm/ for other distributions.

4.11.3.1 Linux-x86 notes

LinuxThreads should be installed before configuring MySQL!

MySQL requires libc version 5.4.12 or newer. It's known to work with libc 5.4.46. glibc version 2.0.6 and later should also work. There have been some problems with the glibc RPMs from RedHat so if you have problems, check whether or not there are any updates! The glibc 2.0.7-19 RPM is known to work.

On some older Linux distributions, configure may produce an error like this:

Syntax error in sched.h. Change _P to __P in the /usr/include/sched.h file.
See the Installation chapter in the Reference Manual.

Just do what the error message says and add an extra underscore to the _P macro that has only one underscore, then try again.

You may get some warnings when compiling; those shown below can be ignored:

mysqld.cc -o objs-thread/mysqld.o
mysqld.cc: In function `void init_signals()':
mysqld.cc:315: warning: assignment of negative value `-1' to `long unsigned int'
mysqld.cc: In function `void * signal_hand(void *)':
mysqld.cc:346: warning: assignment of negative value `-1' to `long unsigned int'

In Debian GNU/Linux, if you want MySQL to start automatically when the system boots, do the following:

shell> cp mysql.server /etc/init.d/mysql.server
shell> /usr/sbin/update-rc.d mysql.server defaults 99

mysql.server can be found in the `share/mysql' directory under the MySQL installation directory, or in the `support-files' directory of the MySQL source tree.

If mysqld always core dumps when it starts up, the problem may be that you have an old `/lib/libc.a'. Try renaming it, then remove `sql/mysqld' and do a new make install and try again. This problem has been reported on some Slackware installations. RedHat 5.0 has also a similar problem with some new glibc versions. See section 4.11.3.2 RedHat 5.0 notes.

If you get the following error when linking mysqld, it means that your `libg++.a' is not installed correctly:

/usr/lib/libc.a(putc.o): In function `_IO_putc':
putc.o(.text+0x0): multiple definition of `_IO_putc'

You can avoid using `libg++.a' by running configure like this:

shell> CXX=gcc ./configure

4.11.3.2 RedHat 5.0 notes

If you have any problems with MySQL on RedHat, you should start by upgrading glibc to the newest possible version!

If you install all the official RedHat patches (including glibc-2.0.7-19 and glibc-devel-2.0.7-19), both the binary and source distributions of MySQL should work without any trouble!

The updates are needed since there is a bug in glibc 2.0.5 in how pthread_key_create variables are freed. With glibc 2.0.5, you must use a statically-linked MySQL binary distribution. If you want to compile from source, you must install the corrected version of LinuxThreads from http://www.tcx.se/Downloads/Linux or upgrade your glibc.

If you have an incorrect version of glibc or LinuxThreads, the symptom is that mysqld crashes after each connection. For example, mysqladmin version will crash mysqld when it finishes!

Another symptom of incorrect libraries is that mysqld crashes at once when it starts. On some Linux systems, this can be fixed by configuring like this:

shell> ./configure --with-mysqld-ldflags=-all-static

On Redhat 5.0, the easy way out is to install the glibc 2.0.7-19 RPM and run configure without the --with-mysqld-ldflags=-all-static option.

For the source distribution of glibc 2.0.7, a patch that is easy to apply and is tested with MySQL may be found at http://www.tcx.se/Download/Linux/glibc-2.0.7-total-patch.tar.gz.

If you experience crashes like these when you build MySQL, you can always download the newest binary version of MySQL. This is statically-linked to avoid library conflicts and should work on all Linux systems!

MySQL comes with an internal debugger that can generate trace files with a lot of information that can be used to find and solve a wide range of different problems. See section 19.10 Debugging MySQL.

4.11.3.3 RedHat 5.1 notes

The glibc of RedHat 5.1 (glibc 2.0.7-13) has a memory leak, so to get a stable MySQL version, you must upgrade glibc to 2.0.7-19, downgrade glibc or use a binary version of mysqld. If you don't do this, you will encounter memory problems (out of memory, etc., etc.). The most common error in this case is:

Can't create a new thread (errno 11). If you are not out of available
memory, you can consult the manual for any possible OS dependent bug

After you have upgraded to glibc 2.0.7-19, you can configure MySQL with dynamic linking (the default), but you cannot run configure with the --with-mysqld-ldflags=-all-static option until you have installed glibc 2.0.7-19 from source!

You can check which version of glibc you have with rpm -q glibc.

4.11.3.4 Linux-Sparc notes

In some implementations, readdir_r() is broken. The symptom is that SHOW DATABASES always returns an empty set. This can be fixed by removing HAVE_READDIR_R from `config.h' after configuring and before compiling.

Some problems will require patching your Linux installation. The patch can be found at http://www.tcx.se/patches/Linux-sparc-2.0.30.diff. This patch is against the Linux distribution `sparclinux-2.0.30.tar.gz' that is available at vger.rutgers.edu (a version of Linux that was never merged with the official 2.0.30). You must also install LinuxThreads 0.6 or newer.

Thanks to jacques@solucorp.qc.ca for this information.

4.11.3.5 Linux-Alpha notes

The first problem is LinuxThreads. The RedHat distribution uses an old (broken) LinuxThreads version, so you must patch LinuxThreads for Alpha. Use the following procedure:

Obtain the glibc2.5c source from any GNU FTP site.
Get the file ftp://www.tcx.se/pub/mysql/linux/patched-glibc-linuxthreads-0.6.tar.gz. This includes a fixed .c file. Copy this to the glibc `./linuxthreads' directory.
Configure and compile glibc (You have to read the manual how to do this together with LinuxThreads), but don't install it!
In the `/usr/lib' directory, rename your old version of `libpthread.a' to `libpthread.a-old'.
Copy the file `glibc.../linuxthreads/libpthread.a' to `/usr/lib'.

Configure MySQL with the following command:

shell> CC=gcc CCFLAGS="-Dalpha_linux_port" \
       CXX=gcc CXXFLAGS="-O3 -Dalpha_linux_port" \
       ./configure --prefix=/usr/local/mysql

Try to compile mysys/thr_lock and mysys/thr_alarm. Test that these programs work! (Invoke each one with no arguments. Each should end with test_succeeded if everything was okay.)
Recompile mysqld.

Note that Linux-Alpha is still an alpha-quality platform for MySQL. With RedHat 5.0 and the patched LinuxThreads, you have a very good chance of it working.

If you have problems with signals (MySQL dies unexpectedly under high load) you may have found an OS bug with threads and signals. In this case you can tell MySQL not to use signals by configuring with:

shell> CFLAGS=-DDONT_USE_THR_ALARM CXXFLAGS=-DDONT_USE_THR_ALARM ./configure ...

This doesn't affect the performance of MySQL, but has the side effect that you can't kill clients that are "sleeping" on a connection with mysqladmin kill or mysqladmin shutdown. The client will instead die when it issues its next command.

4.11.3.6 MkLinux notes

MySQL should work on MkLinux with the newest glibc package (tested with glibc 2.0.7).

4.11.4 Linux RPM notes

The recommended way to install MySQL on linux is by a RPM. The MySQL RPMS is currently being build on a RedHat 5.1 system but should work on other versions of Linux that supports RPM as well.

MySQL-VERSION.i386.rpm The MySQL server. Unless you only want to connect to another MySQL server running on another machice yo will need this.
MySQL-client-VERSION.i386.rpm The standard MySQL client programs. You probably always want to install this package.
MySQL-bench-VERSION.i386.rpm Test and benchmarks. Requires perl and msql-mysql-modules RPMS.
MySQL-devel-VERSION.i386.rpm Libraries and include files nesesarry if you want to compile other MySQL clients.
MySQL-VERSION.src.rpm This contains the source code to all of the above packages. It can also be used to try to build RPMS for other architectures (for example Alpha or SPARC).

To make a standard minimal installation:

rpm -i MySQL-VERSION.i386.rpm MySQL-client-VERSION.i386.rpm

To install only the client package:
```
rpm -i MySQL-client-VERSION.i386.rpm
```

The RPM places data in `/var/lib/mysql'. The RPM also creates the appropriate entries in `/sbin/rc.d/' to start the server automatically at boot time.

4.11.5 Alpha-DEC-Unix notes

When compiling threaded programs under Digital UNIX, the documentation recommends the -pthread option for cc and cxx and the libraries -lmach -lexc (in addition to -lpthread). You should run configure something like this:

shell> CC="cc -pthread" CXX="cxx -pthread -O" \
       ./configure -with-named-thread-libs="-lpthread -lmach -lexc -lc"

When compiling mysqld, you may see a couple of warnings like this:

mysqld.cc: In function void handle_connections()':
mysqld.cc:626: passing long unsigned int *' as argument 3 of
accept(int,sockadddr *, int *)'

You can safely ignore these warnings. They occur because configure can't detect warnings, only errors.

If you start the server directly from the command line, you may have problems with it dying when you log out. (When you log out, your outstanding processes receive a SIGHUP signal.) If so, try starting the server like this:

shell> nohup mysqld [options] &

nohup causes the command following it to ignore any SIGHUP signal sent from the terminal. Alternatively, start the server by running safe_mysqld, which invokes mysqld using nohup for you.

4.11.6 Alpha-DEC-OSF1 notes

If you have problems compiling and have DEC CC and gcc installed, try running configure like this:

shell> CC=cc CFLAGS=-O CXX=gcc CXXFLAGS=-O3 \
       ./configure --prefix=/usr/local/mysql

On OSF1 V4.0D and compiler "DEC C V5.6-071 on Digital UNIX V4.0 (Rev. 878)" the compiler had some strange behavior (undefined asm symbols). /bin/ld also appears to be broken (problems with _exit undefined when linking mysqld). On this system, we have managed to compile MySQL with the following configure line, after replacing /bin/ld with the version from OSF 4.0C:

shell> CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql

In some versions of OSF1, the alloca() function is broken. Fix this by removing the line in `config.h' that defines 'HAVE_ALLOCA'.

The alloca() function also may have an incorrect prototype in /usr/include/alloca.h. This warning resulting from this can be ignored.

configure will use the following thread libraries automatically: -with-named-thread-libs="-lpthread -lmach -lexc -lc".

When using gcc, you can also try running configure like this:

shell> CFLAGS=-D_PTHREAD_USE_D4 CXX=gcc CXXFLAGS=-O3 ./configure ....

shell> CFLAGS=-DDONT_USE_THR_ALARM CXXFLAGS=-DDONT_USE_THR_ALARM ./configure ...

4.11.7 SGI-IRIX notes

You may have to undefine some things in `config.h' after running configure and before compiling.

In some Irix implementations, the alloca() function is broken. If the mysqld server dies on some SELECT statements, remove the lines from `config.h' that define HAVE_ALLOC and HAVE_ALLOCA_H. If mysqladmin create doesn't work, remove the line from `config.h' that defines HAVE_READDIR_R. You may have to remove the HAVE_TERM_H line as well.

Irix 6.2 doesn't support POSIX threads out of of the box. You must install these patches, which are available from SGI if you have support: 1403, 1404, 1644, 1717, 1918, 2000, 2044.

If you get the something like the following error when compiling `mysql.cc':

"/usr/include/curses.h", line 82: error(1084): invalid combination of type

Then type the following in the top-level directory of your MySQL source tree:

shell> extra/replace bool curses_bool < /usr/include/curses.h > include/curses.h
shell> make

There have also been reports of scheduling problems. If only one thread is running, things go slow. Avoid this by starting another client. This may lead to a 2-to-10-fold increase in execution speed thereafter for the other thread.

This is a poorly-understood problem with IRIS threads; you may have to improvise to find solutions until this can be fixed.

If you are compiling with gcc, you can use the following configure command:

shell> CC=gcc CXX=gcc CXXFLAGS=-O3 \
       ./configure --prefix=/usr/local/mysql --with-thread-safe-client

4.11.8 FreeBSD notes

If you notice that configure will use MIT-pthreads, you should read the MIT-pthreads notes. See section 4.9 MIT-pthreads notes.

If you get an error on make install that it can't find `/usr/include/pthreads', configure didn't detect that you need MIT-pthreads on FreeBSD. This is fixed by doing:

shell> rm config.cache
shell> ./configure --with-mit-threads

The FreeBSD make behavior is slightly different from that of GNU make. If you have make-related problems, you should install GNU make.

If mysql or mysqladmin takes a long time to respond, a user said the following:

Are you running the ppp user process? On one FreeBSD box (2.2.5) MySQL clients takes a couple of seconds to connect to mysqld if the ppp process is running.

FreeBSD is also known to have a very low default file handle limit. See section 16.9 File not found.

If you have a problem with SELECT NOW() returning values in GMT and not your local time, you have to set the TZ environment variable to your current timezone. This should be done for the environment in which the server runs, for example in safe_mysqld or mysql.server.

Make sure that you modify the /etc/hosts file so that the localhost entry is correct (otherwise you will have problems connecting to the database).

If you are using FreeBSD 2.2.6, don't forget to apply the ttcp and mmap-22 patches to the OS (for security reasons). Please see http://www.freebsd.org for these CERT patches.

4.11.8.1 FreeBSD-3.0 notes

You have to run configure with the --with-named-thread-libs=-lc_r option.

The pthreads library for FreeBSD doesn't contain the sigwait() function and there are some bugs in it. To fix this, get the `FreeBSD-3.0-libc_r-1.0.diff' file and apply this in the `/usr/src/lib/libc_r/uthread' directory. Then follow the instructions that can be found with man pthread about how to recompile the libc_r library.

You can test if you have a 'modern' libpthread.a with this command:

shell> nm /usr/lib/libc_r.a | grep sigwait

If the above doesn't find sigwait, you must use the patch above and recompile libc_r.

4.11.9 BSD/OS 2.# notes

If you get the following error when compiling MySQL, your ulimit for virtual memory is too low:

item_func.h: In method `Item_func_ge::Item_func_ge(const Item_func_ge &)':
item_func.h:28: virtual memory exhausted
make[2]: *** [item_func.o] Error 1

Try using ulimit -v 80000 and run make again. If this doesn't work and you are using bash, try switching to csh or sh; Some BSDI users have reported problems with bash and ulimit.

If you are using gcc, you may also use have to use the --with-low-memory flag to configure to be able to compile `sql_yacc.cc'.

4.11.9.1 BSD/OS 3.# notes

Upgrade to BSD/OS 3.1. If that is not possible, install BSDIpatch M300-038.

Use the following command when configuring MySQL:

shell> env CXX=shlicc++ CC=shlicc2 \
       ./configure \
           --prefix=/usr/local/mysql \
           --localstatedir=/var/mysql \
           --without-perl \
           --with-unix-socket-path=/var/mysql/mysql.sock

The following is also known to work:

shell> env CC=gcc CXX=gcc CXXFLAGS=-O3 \
       ./configure \
           --prefix=/usr/local/mysql \
           --with-unix-socket-path=/var/mysql/mysql.sock

You can change the directory locations if you wish, or just use the defaults by not specifying any locations.

If you have problems with performance under heavy load, try using the --skip-thread-priority option to safe_mysqld! This will run all threads with the same priority; on BSDI 3.1, this gives better performance. (At least until BSDI fixes their thread scheduler).

If you get the error virtual memory exhausted while compiling, you should try using ulimit -v 80000 and run make again. If this doesn't work and you are using bash, try switching to csh or sh; Some BSDI users have reported problems with bash and ulimit.

4.11.10 SCO notes

The current port is tested only on a 'sco3.2v5.0.4' system. There has also been a lot of progress on a port to 'sco 3.2v4.2'.

For OpenServer 5.0.X You need to use GDS in Skunkware 95 (95q4c). This is necessary because GNU gcc 2.7.2 in Skunkware 97 does not have GNU as.
You need the port of GCC 2.5.? for this product and the Development system. They are required on this version of SCO UNIX. You cannot just use the GCC Dev system.
You should get FSU thread package and install this first. This can be found at http://www.cs.wustl.edu/~schmidt/ACE_wrappers/FSU-threads.tar.gz. You can also get a precompiled package from ftp://www.tcx.se/pub/mysql/Downloads/SCO/FSU-threads-3.5c.tar.gz.
FSU pthreads can be compiled with SCO UNIX 4.2 with tcpip. Or OpenServer 3.0 or Open Desktop 3.0 (OS 3.0 ODT 3.0), with the SCO Development System installed using a good port of GCC 2.5.X ODT or OS 3.0 you will need a good port of GCC 2.5.? There are a lot of problems without a good port. The port for this product requires the SCO UNIX Development system. Without it, you are missing the libraries and the linker that is needed.
To build FSU pthreads in your system, do the following:
1. Run ./configure in the `threads/src' directory and select the SCO OpenServer option. This command copies `Makefile.SCO5' to `Makefile'.
2. Run make.
3. To install in the default `/usr/include' directory, login as root and cd to `thread/src' directory, and run make install.
Remember to use GNU make when making MySQL.
If you don't start safe_mysqld as root, you will probably only get the default 110 open files per process. mysqld will write a note about this in the log file.
With SCO 3.2V4.2, you must use a FSU-pthreads version 3.5c or newer. The following configure command should work:
```
shell> CFLAGS="-D_XOPEN_XPG4" CXX=gcc CXXFLAGS="-D_XOPEN_XPG4" \
       ./configure \
           --with-debug --prefix=/usr/local/mysql \
           --with-named-thread-libs="-lgthreads -lsocket -lgen -lgthreads" \
           --with-named-curses-libs="-lcurses" \
           --without-perl
```
You may get some problems with some include files. In this case you can find new SCO-specific include files at ftp://www.tcx.se/pub/mysql/Downloads/SCO/SCO-3.2v4.2-includes.tar.gz. You should unpack this file in the `include' directory of your MySQL source tree.

SCO development notes:

MySQL should automatically detect FSU-threads and link mysqld with -lgthreads -lsocket -lgthreads.
The SCO development libraries are reentrant in FSU pthreads. SCO claims that its libraries function are reentrant so they must be reentrant with FSU pthreads. FSU pthreads on OpenServer tries to use the SCO scheme to make reentrant library.
FSU threads (at least the version at www.tcx.se) comes linked with GNU malloc. If you encounter problems with memory usage, make sure that `gmalloc.o' is included in `libgthreads.a' and `libgthreads.so'.
In FSU pthreads, the following system calls are pthreads aware: read(), write(), getmsg(), connect(), accept(), select() and wait().

4.11.11 SCO Unixware 7.0 notes

Unixware 7.x, scheduled to be released in December 1998, will have Posix threads and MySQL will be very easy to install then. The following reflects the UnixWare 7.0.1 RAF version.

MySQL 3.22.5 fixes some portability problems under Unixware so you must use at least this version.

You must configure with at least the following options:

shell> CFLAGS=-DUNIXWARE_7 CXXFLAGS=-DUNIXWARE_7 \
       ./configure --with-named-thread-libs=-Kthread

As the Unixware compiler doesn't support compilation with -O and -g, you must edit `configure' and change CFLAGS="-g -O2" to CFLAGS="-O2".

4.11.12 IBM-AIX notes

Automatic detection of xlC is missing from Autoconf, so something like this is needed when using the IBM compiler:

shell> CC="xlc_r -ma -O3 -qstrict" CXX="xlC_r -ma -O3 -qstrict" \
       ./configure

If you are using egcs to compile MySQL, you MUST use the -fno-exceptions flag, as the exception handling in egcs is not thread-safe! (This is tested with egcs 1.1). We recommend the following configure line with egcs and gcc on AIX:

shell> CXX=gcc CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti" \
       ./configure --prefix=/home/monty --with-debug --with-low-memory

shell> CFLAGS=-DDONT_USE_THR_ALARM CXX=gcc \
       CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti -DDONT_USE_THR_ALARM" \
       ./configure --prefix=/home/monty --with-debug --with-low-memory

4.11.13 HP-UX notes

There are a couple of "small" problems when compiling MySQL on HP-UX. Below we describe some problems and workarounds when using the HP-UX compiler and gcc 2.8.0.

gcc 2.8.0 can't compile readline on HP-UX (an internal compiler error occurs). On the other hand, MIT-pthreads can't be compiled with the HP-UX compiler, because it can't compile .S (assembler) files. We got MySQL to compile on HP-UX 10.20 by doing the following:

shell> CC=cc CFLAGS="+z +e -Dhp9000s800 -D__hpux__" \
       CXX=gcc CXXFLAGS=-O3 \
       ./configure --prefix=/usr/local/mysql --with-low-memory
shell> cd mit-pthreads
shell> rm config.cache
shell> CC=gcc CXX=gcc ./configure
shell> cd ..
shell> make
shell> make install
shell> scripts/mysql_install_db
shell> /usr/local/mysql/bin/safe_mysqld &

This compiles MySQL with the HP-UX compiler, except for the MIT-pthreads part of the distribution, which is compiled with gcc.

4.12 TcX binaries

As a service, TcX provides a set of binary distributions of MySQL that are compiled at TcX or at sites where customers kindly have given us access to their machines.

These distributions are generated with scripts/make_binary_distribution and are configured with the following compilers and options.

SunOS 4.1.4 2 sun4c with gcc 2.7.2.1: CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --disable-shared
SunOS 5.5.1 sun4u with egcs 1.0.3a: CC=gcc CFLAGS="-O6 -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O6 -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-low-memory
SunOS 5.6 sun4u with egcs 2.90.27: CC=gcc CFLAGS="-O6 -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O6 -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-low-memory
SunOS 5.6 i86pc with gcc 2.8.1: CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-low-memory
Linux 2.0.33 i386 with pgcc 2.90.29 (egcs 1.0.3a): CFLAGS="-O6 -mpentium -mstack-align-double -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O6 -mpentium -mstack-align-double -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static
SCO 3.2v5.0.4 i386 with gcc 2.7-95q4: CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql
AIX 2 4 with gcc 2.7.2.2: CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql
OSF1 V4.0 564 alpha with gcc 2.8.1: CC=gcc CFLAGS=-O CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-low-memory
IRIX 6.3 IP32 with gcc 2.8.0: CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql
BSDI BSD/OS 3.1 i386 with gcc 2.7.2.1: CC=gcc CXX=gcc CXXFLAGS=-O ./configure --prefix=/usr/local/mysql
BSDI BSD/OS 2.1 i386 with gcc 2.7.2: CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql

Anyone who has more optimal options for any of the configurations listed above can always mail them to us at mysql-developer@tcx.se.

RPM distributions prior to MySQL 3.22 are user-contributed. Beginning with 3.22, some RPMs are TcX-generated.

4.13 Win32 notes

The MySQL-Win32 version has by now proven itself to be very stable. The Win32 version of MySQL has the same features as the corresponding Unix version with the following exceptions:

Table cache size limitations

Win32 can handle only a very limited number of open files at the same time (about 255). Because of this, you shouldn't increase the number of open connections or number of cached tables very much on Win32.

Win95 and threads

Win95 leaks about 200 bytes of main memory for each thread creation. Because of this, you shouldn't run mysqld for an extended time on Win95 if you do many connections, since each connection in MySQL creates a new thread! NT doesn't suffer from this bug.

Blocking read

MySQL uses a blocking read for each connection. This means that:

A connection will not be disconnected automatically after 8 hours, as happens with the Unix version of MySQL.
If a connection "hangs," it's impossible to break it without killing MySQL.
mysqladmin kill will not work on a sleeping connection.
mysqladmin shutdown can't abort as long as there are sleeping connections.

We plan to fix this in the near future.

UDF functions

For the moment, MySQL-Win32 does not support user definable functions.

DROP DATABASE

You can't drop a database that is in use by some thread.

Killing MySQL from the task manager

You can't kill MySQL from the task manager or with the shutdown utility in Windows95. You must take it down with mysqladmin shutdown.

Case-insensitive names

Filenames are case insensitive on Win32, so database and table names are also case insensitive in MySQL for Win32. The only restriction is that database and table names must be given in the same case throughout a given statement. The following query would not work because it refers to a table both as my_table and as MY_TABLE:

SELECT * FROM my_table WHERE MY_TABLE.col=1;

The `\' directory character

Pathname components in Win95 are separated by `\', which is also the escape character in MySQL. If you are using LOAD DATA INFILE or SELECT ... INTO OUTFILE, you must double the `\' character or use Unix style filenames with `/':

SELECT * FROM skr INTO OUTFILE 'C:/tmp/skr.txt';
LOAD DATA INFILE "C:\\tmp\\skr.txt" INTO TABLE skr;

Can't open named pipe error

If you use the shareware version of MySQL-Win32 on NT with the newests mysql-clients you will get the following error: error 2017: can't open named pipe to host: . pipe... This is because the release version of MySQL uses named pipes on NT by default. You can avoid this error by using the --host=localhost option to the new MySQL clients or create a file, `C:\my.cnf', that contains the following information:

[client]
host = localhost

If you get the error

Access denied for user: 'some-user@unknown'
to database 'mysql'

when accessing a MySQL server on the same machine, this means that your MySQL can't resolve your host name properly. To fix this you should create a file `\windows\hosts' with the following information:

127.0.0.1       localhost

Here are some open issues for anyone who might want to help us with the Win32 release:

When you suspend a laptop running Win95, the mysqld daemon doesn't accept new connections when the laptop is resumed. We don't know if this is a problem with Win95, TCP/IP or MySQL.
It would also be real nice to be able to kill mysqld from the task manager. For the moment, you must use mysqladmin shutdown.
When registering mysqld as a service with -install (on NT) it would be nice if you could also add default options on the command line. For the moment, the workaround is to update the `C:\my.cnf' file instead.
Port readline to Win32 for use in the mysql command line tool.
GUI versions of the standard MySQL clients (mysql, mysqlshow, mysqladmin, and mysqldump) would be nice.
It would be nice if the 'read' and 'write' to sockets in `net.c' were interruptible. This would make it possible to kill open threads with mysqladmin kill on Win32.
Documentation of which Windows programs work with MySQL-Win32/MyODBC and what must be done to get them working.
mysqld always starts in the "C" locale and not in the default locale. We would like to have mysqld use the current locale for the sort order.
Add more options to MysqlManager
Change the communication protocol between the server and client to use Windows internal communication instead of sockets and TCP/IP.
Implement UDF functions with .DLLs
pthread_cond_timedwait() ignores the time argument. Until this is fixed, the MySQL SQL get_lock() code ignores the time argument.

Other Win32-specific issues are described in the `README' file that comes with the MySQL-Win32 distribution.

4.14 Post-installation setup and testing

Once you've installed MySQL (from either a binary or source distribution), you need to initialize the grant tables, start the server and make sure that the server works okay. You may also wish to arrange for the server to be started and stopped automatically when your system starts up and shuts down.

Normally you install the grants and start the server with:

./scripts/mysql_install_db
cd mysql_installation_directory
./bin/safe_mysqld &

This is described in detail below:

Testing is most easily done from the top-level directory of the MySQL distribution. For a binary distribution, this is your installation directory (typically something like `/usr/local/mysql'). For a source distribution, this is the main directory of your MySQL source tree.

In the commands shown below in this section and the following subsections, BINDIR is the path to the location in which programs like mysqladmin and safe_mysqld are installed. For a binary distribution, this is the `bin' directory within the distribution. For a source distribution, BINDIR is probably `/usr/local/bin', unless you specified an installation directory other than `/usr/local' when you ran configure. EXECDIR is the location in which the mysqld server is installed. For a binary distribution, this is the same as BINDIR. For a source distribution, EXECDIR is probably `/usr/local/libexec'.

If necessary, start the mysqld server and set up the initial MySQL grant tables containing the privileges that determine how users are allowed to connect to the server. This is normally done with the mysql_install_db script. Normally, mysql_install_db needs to be run only the first time you install MySQL. Therefore, if you are upgrading an existing installation, you can skip this step. mysql_install_db is quite safe to use and will not update any tables that already exists, so if you are unsure what to do, you can always run mysql_install_db.
```
shell> scripts/mysql_install_db
```
If you don't set up the grant tables, the following error will appear in the log file when you start the server:
```
mysqld: Can't find file: 'host.frm'
```
You might need to run mysql_install_db as root. However, if you prefer, you can run the MySQL server as an unprivileged (non-root) user, provided that user can read and write files in the database directory. Instructions for running MySQL as an unprivileged user are given in section 16.7 How to run MySQL as a normal user. mysql_install_db creates four tables (user, db, host and func) in the mysql database. A description of the initial privileges is given in section 6.7 Setting up the initial MySQL privileges. Briefly, the these privileges allow the MySQL root user to do anything, and allow anybody to create or use databases with a name of 'test' or starting with 'test_'. If you have problems with mysql_install_db, see section 4.14.1 Problems running mysql_install_db. There are some alternatives to running the mysql_install_db script as is:
- You may want to edit mysql_install_db before running it, to change the initial privileges that are installed into the grant tables.
- If you want to change things in the grant tables after installing them, you can run mysql_install_db, then use mysql -u root mysql to connect to the grant tables as the MySQL root user and issue SQL statements to modify the grant tables directly.
- It is possible to recreate the grant tables completely after they have already been created. You might want to do this if you've already installed the tables but then want to recreate them after editing mysql_install_db.
For more information about these alternatives, see section 6.7 Setting up the initial MySQL privileges.
Start the MySQL server with:
```
shell> cd mysql_installation_directory
shell> bin/safe_mysqld &
```
See section 4.14.2 Problems starting the MySQL server

Verify that the server is running using mysqladmin. The following commands provide a simple test to check that the server is up and responding to connections:

shell> BINDIR/mysqladmin version
shell> BINDIR/mysqladmin variables

For example, the output from mysqladmin version varies slightly depending on your platform and version of MySQL, but should be similar to that shown below:

shell> BINDIR/mysqladmin version
mysqladmin  Ver 6.3 Distrib 3.22.9-beta, for pc-linux-gnu on i686
TCX Datakonsult AB, by Monty

Server version          3.22.9-beta
Protocol version        10
Connection              Localhost via UNIX socket
TCP port                3306
UNIX socket             /tmp/mysql.sock
Uptime:                 16 sec

Running threads: 1  Questions: 20  Reloads: 2  Open tables: 3

To get a feeling for what else you can do with BINDIR/mysqladmin, invoke it with the --help option.

Verify that you can shut down the server:

shell> BINDIR/mysqladmin -u root shutdown

Verify that you can restart the server. Do this using safe_mysqld or by invoking mysqld directly. For example:
```
shell> BINDIR/safe_mysqld --log &
```
If safe_mysqld fails, try running it from the MySQL installation directory (if you are not already there). If that doesn't work, see section 4.14.2 Problems starting the MySQL server.
Run some simple tests to verify that the server is working. The output should be similar to what is shown below:
```
shell> BINDIR/mysqlshow
+-----------+
| Databases |
+-----------+
| mysql     |
+-----------+

shell> BINDIR/mysqlshow mysql
Database: mysql
+--------+
| Tables |
+--------+
| db     |
| host   |
| user   |
+--------+

shell> BINDIR/mysql -e "select host,db,user from db" mysql
+------+--------+------+
| host | db     | user |
+------+--------+------+
| %    | test   |      |
| %    | test_% |      |
+------+--------+------+
```
There is also a benchmark suite in `sql-bench' that you can use to compare how MySQL performs on different platforms. In the `sql-bench/Results' directory you can find the results from many runs against different databases and platforms. To run all tests, execute these commands:
```
shell> cd sql-bench
shell> run-all-tests
```
If you don't have the `sql-bench' directory, you are probably using a RPM for a binary distribution. (Source distribution RPMs include the benchmark directory.) In this case, you must first install the benchmark suite before you can use it. Beginning with MySQL 3.22, there are benchmark RPM files named `mysql-bench-VERSION-i386.rpm' that contain benchmark code and data. You can also run the tests in the `tests' subdirectory. For example, to run `auto_increment.tst', do this:
```
shell> BINDIR/mysql -vvf test < ./tests/auto_increment.tst
```
The expected results are shown in the file `./tests/auto_increment.res'.

4.14.1 Problems running `mysql_install_db`

This section lists some of the problems you might encounter when you run mysql_install_db:

mysql_install_db doesn't install the privilege tables

You may find that mysql_install_db doesn't install the privilege tables, but terminates after displaying the following messages:

starting mysqld daemon with databases from XXXXXX
mysql daemon ended

In this case, you should examine the log file very carefully! The log should be located in the directory `XXXXXX' named by the error message, and should indicate why mysqld didn't start. If you don't understand what happened, include the log when you post a bug report using mysqlbug! See section 2.3 What to do if you think you have found a bug.

There is already a mysqld daemon running

In this case, you have probably don't have to run mysql_install_db at all. You have to run mysql_install_db only once, when you install MySQL the first time.

Installing a second mysqld daemon doesn't work when one daemon is running

This can happen when you already have an existing MySQL installation, but want to put a new installation in a different place (e.g., for testing, or perhaps you simply want to run two installations at once). Generally the problem that occurs when you try to run the second server is that it tries to use the same socket and port as the old one. In this case you will get the error message:

Can't start
server: Bind on TCP/IP port: Address already in use

Can't
start server : Bind on unix socket...

You can start the new server with a different socket and port as follows:

shell> MYSQL_UNIX_PORT=/tmp/mysqld-new.sock
shell> MYSQL_TCP_PORT=3307
shell> export MYSQL_UNIX_PORT MYSQL_TCP_PORT
shell> scripts/mysql_install_db
shell> bin/safe_mysqld &

After this, you should edit your server boot script to start both daemons with different sockets and ports. For example, it could invoke safe_mysqld twice, but with different --socket, --port and --basedir options for each invocation.

mysqld crashes at once

If you are running RedHat 5.0 with a version of glibc older than 2.0.7-5, you should make sure you have installed all glibc patches! There is a lot of information about this in the MySQL mail archives. See section 4.11.3 Linux notes (all Linux versions). Links to the mail archives are available at the online MySQL documentation page. You can also start mysqld manually using the --skip-grant option and add the privilege information yourself using mysql:

shell> BINDIR/safe_mysqld --skip-grant &
shell> BINDIR/mysql -u root mysql

From mysql, manually execute the SQL commands in mysql_install_db. Make sure you run mysqladmin reload afterward to tell the server to reload the grant tables.

4.14.2 Problems starting the MySQL server

Generally, you start the mysqld server in one of three ways:

By invoking mysql.server. This script is used primarily at system startup and shutdown, and is described more fully in section 4.14.3 Automatically starting and stopping MySQL.
By invoking safe_mysqld, which tries to determine the proper options for mysqld and then runs it with those options.
By invoking mysqld directly.

Whichever method you use to start the server, if it fails to start up correctly, check the log file to see if you can find out why. Log files are located in the data directory (typically `/usr/local/mysql/data' for a binary distribution, `/usr/local/var' for a source distribution). Look in the data directory for a file with a name of the form `host_name.log', where host_name is the name of your server host. Then check the last few lines of that file:

shell> tail host_name.log

When the mysqld daemon starts up, it changes directory to the data directory. This is where it expects to write log files and the pid (process ID) file, and where it expects to find databases.

The data directory location is hardwired in when the distribution is compiled. However, if mysqld expects to find the data directory somewhere other than where it really is on your system, it will not work properly. If you have problems with incorrect paths, you can find out what options mysqld allows and what the default path settings are by invoking mysqld with the --help option. You can override the defaults by specifying the correct pathnames as command-line arguments to mysqld. (These options can be used with safe_mysqld as well.)

Normally you should need to tell mysqld only the base directory under which MySQL is installed. You can do this with the --basedir option. You can also use --help to check the effect of changing path options (note that --help must be last). For example:

shell> EXECDIR/mysqld --basedir=/usr/local --help

Once you determine the path settings you want, start the server without the --help option.

If you get the following error:

Can't start server: Bind on TCP/IP port: Address already in use
  or
Can't start server : Bind on unix socket...

this means that some other program (or another mysqld server) is already using the TCP/IP port or socket mysqld tries to use! You should check with ps that you don't have another mysqld server running. If you can't find another server running, you can try doing telnet your-host-name tcp-ip-port-number and press RETURN a couple of times. If you don't get a error message like telnet: Unable to connect to remote host: Connection refused, something is using the TCP/IP port mysqld is trying to use. See section 4.14.1 Problems running mysql_install_db, See section 17.3 Running multiple MySQL servers on the same machine.

The safe_mysqld script is written so that it normally is able to start a server that was installed from either a source or a binary version of MySQL, even if these install the server in slightly different locations. safe_mysqld expects one of these conditions to be true:

The server and databases can be found relative to the directory from which safe_mysqld is invoked. safe_mysqld looks under its working directory for `bin' and `data' directories (for binary distributions) or for `libexec' and `var' directories (for source distributions). This condition should be met if you execute safe_mysqld from your MySQL installation directory (for example, `/usr/local/mysql' for a binary distribution).
If the server and databases cannot be found relative to its working directory, safe_mysqld attempts to locate them by absolute pathnames. Typical locations are `/usr/local/libexec' and `/usr/local/var'. The actual locations are determined when the distribution was built from which safe_mysqld comes. They should be correct if MySQL was installed in a standard location.

Since safe_mysqld will try to find the server and databases relative to its own working directory, you can install a binary distribution of MySQL anywhere, as long as you start safe_mysqld from the MySQL installation directory:

shell> cd mysql_installation_directory
shell> BINDIR/safe_mysqld &

If safe_mysqld fails, even when invoked from the MySQL installation directory, you can modify it to use the path to mysqld and the pathname options that are correct for your system. Note that if you upgrade MySQL sometime, your modified version will be overwritten, so you should make a copy of your edited version that you can reinstall.

If mysqld is currently running, you can find out what path settings it is using by executing this command:

shell> mysqladmin variables

If safe_mysqld starts the server but you can't connect to it, you should make sure you have an entry in `/etc/hosts' that looks like this:

127.0.0.1       localhost

This problem occurs only on systems that don't have a working thread library and for which MySQL must be configured to use MIT-pthreads.

If you don't have write access to create a socket file at the default place (in `/tmp') or permission to create temporary files in /tmp, you will get an error when starting/using mysqld.

You can specify a different socket, port and temporary directory as follows:

shell> MYSQL_UNIX_PATH=/some_tmp_dir/mysqld.sock
shell> MYSQL_TCP_PORT=3306
shell> TMPDIR=/some_tmp_dir/
shell> export MYSQL_UNIX_PATH MYSQL_TCP_PORT TMPDIR

`some_tmp_dir' should be the path to some directory for which you have write permission.

After this you should be able to run mysql_install_db and start the server with:

shell> scripts/mysql_install_db
shell> BINDIR/safe_mysqld &

4.14.3 Automatically starting and stopping MySQL

The mysql.server script can be used to start or stop the server, by invoking it with start or stop arguments:

shell> mysql.server stop
shell> mysql.server start

mysql.server can be found in the `share/mysql' directory under the MySQL installation directory, or in the `support-files' directory of the MySQL source tree.

Before mysql.server starts the server, it changes directory to the MySQL installation directory, then invokes safe_mysqld. You might need to edit mysql.server if you have a binary distribution that you've installed in a non-standard location. Modify it to cd into the proper directory before it runs safe_mysqld. You can also modify mysql.server to pass other options to safe_mysqld. For example, if you want the server to run as some specific user, change the --user option in the safe_mysqld invocation. (You must run mysql.server as the Unix root user for this to work.)

mysql.server stop brings down the server by issuing a mysqladmin shutdown command. You should make the script unreadable to anyone but root since you will need to put the password for the MySQL root user in the script. Alternatively, you could edit mysql.server to read the server pid file from the data directory to get the server process ID, and kill that process.

You can take down the server manually by executing mysqladmin shutdown yourself.

You might want to add these start and stop commands to the appropriate places in your `/etc/rc*' files when you start using MySQL for production applications. Note that if you modify mysql.server, then if you upgrade MySQL sometime, your modified version will be overwritten, so you should make a copy of your edited version that you can reinstall.

4.14.4 Option files

MySQL 3.22 can read default startup options for the server and for clients from option files.

MySQL reads default options from the following files on Unix:

Filename Purpose

/etc/my.cnf Global options

DATADIR/my.cnf Server-specific options

~/.my.cnf User-specific options

DATADIR is the MySQL data directory (typically `/usr/local/mysql/data' or `/usr/local/var'). Note that this is the directory that was specified at configuration time, not the one specified with --datadir when mysqld starts up! (The server looks for option files before it processes any command-line arguments, so --datadir has no effect on where the server looks for those files.)

MySQL reads default options from the following files on Win32:

Filename	Purpose
`C:\my.cnf`	Global options
`C:\mysql\data\my.cnf`	Server-specific options

MySQL tries to read option files in the order listed above. If multiple option files exist, an option specified in a file read later overrides the same option specified in a file read earlier. Options specified on the command line override options specified in any option file. Some options can be specified using environment variables. Options specified on the command line or in option files override environment variable values.

The following programs support option files: mysql, mysqladmin, mysqld, mysqldump, mysqlimport, isamchk and pack_isam.

In option files, you can specify any long option that a program supports! Run the program with --help to get a list of available options.

An option file can contain lines of the following forms:

#comment: Comment lines starts with `#' or `;'. Empty lines are ignored.
[group]: group is the name of the program or group for which you want to set options. After a group line, any option or set-variable lines apply to the named group, until the end of the option file or another group line is given.
option: This is equivalent to --option on the command line.
option=value: This is equivalent to --option=value on the command line.
set-variable = variable=value: This is equivalent to --set-variable variable=value on the command line. This syntax must be used to set a mysqld variable.

The client group allows you to specify options that apply to all MySQL clients (not mysqld). This is the perfect group to use to specify the password you use to connect to the server. (But make sure the option file is readable and writable only to yourself.)

Note that for options and values, all leading and trailing blanks are automatically deleted. You may use the escape sequences `\b', `\t', `\n', `\r', `\\' and `\s' in your value string (`\s' == blank).

Here is a typical global option file:

[client]
port=3306
socket=/tmp/mysql.sock

[mysqld]
port=3306
socket=/tmp/mysql.sock
set-variable = key_buffer=16M
set-variable = max_allowed_packet=1M

[mysqldump]
quick

Here is typical user option file:

[client]
# The following password will be sent to all standard MySQL clients
password=my_password

[mysql]
no-auto-rehash

If you have a source distribution, you will find a sample configuration file named `my-example.cnf' in the `support-files' directory. If you have a binary distribution, look in the `DIR/share/mysql' directory, where DIR is the pathname to the MySQL installation directory (typically `/usr/local/mysql'). You can copy `my-example.cnf' to your home directory (rename the copy to `.my.cnf') to experiment with.

To tell a MySQL program not to read any option files, specify --no-defaults as the first option on the command line. This MUST be the first option or it will have no effect! If you want to check which options are used, you can give the option --print-defaults as the first option.

Note for developers: Option file handling is implemented simply by processing all matching options (i.e., options in the appropriate group) before any command line arguments. This works nicely for programs that use the last instance of an option that is specified multiple times. If you have an old program that handles multiply-specified options this way but doesn't read option files, you need add only two lines to give it that capability. Check the source code of any of the standard MySQL clients to see how to do this.

4.15 Is there anything special to do when upgrading/downgrading MySQL?

You can always move the MySQL form and data files between different versions on the same architecture as long as you have the same base version of MySQL. The current base version is 3. If you change the character set by recompiling MySQL (which may also change the sort order), you must run isamchk -r -q on all tables. Otherwise your indexes may not be ordered correctly.

If you are paranoid and/or afraid of new versions, you can always rename your old mysqld to something like mysqld-'old-version-number'. If your new mysqld then does something unexpected, you can simply shut it down and restart with your old mysqld!

When you do an upgrade you should also backup your old databases, of course. Sometimes it's good to be a little paranoid!

After an upgrade, if you experience problems with recompiled client programs, like Commands out sync or unexpected core dumps, you probably have used an old header or library file when compiling your programs. In this case you should check the date for your `mysql.h' file and `libmysql.a' library to verify that they are from the new MySQL distribution. If not, please recompile your programs!

4.15.1 Upgrading from a 3.21 version to 3.22

Nothing that affects compatibility has changed between 3.21 and 3.22. The only pitfall is that new tables that are created with DATE type columns will use the new way to store the date. You can't access these new fields from an old version of mysqld.

The C API interface to mysql_real_connect() has changed. If you have an old client program that calls this function, you must place a 0 for the new db argument (or recode the client to send the db element for faster connections).

4.15.2 Upgrading from a 3.20 version to 3.21

If you already have a version older than 3.20.28 running and want to switch to 3.21.x, you need to do the following:

You can start the mysqld 3.21 server with safe_mysqld --old-protocol to use it with clients from the 3.20 distribution. In this case, the new client function mysql_errno() will not return any server error, only CR_UNKNOWN_ERROR, (but it works for client errors) and the server uses the old password() checking rather than the new one.

If you are NOT using the --old-protocol option to mysqld, you will need to make the following changes:

All client code must be recompiled. If you are using ODBC, you must get the new MyODBC 2.x driver.
The script scripts/add_long_password must be run to convert the password field in the mysql.user table to CHAR(16).
All passwords must be reassigned in the mysql.user table (to get 62-bit rather than 31-bit passwords).
The table format hasn't changed, so you don't have to convert any tables.

MySQL 3.20.28 and above can handle the new user table format without affecting clients. If you have a MySQL version earlier than 3.20.28, passwords will no longer work on it if you convert the user table. So to be safe, you should first upgrade to at least 3.20.28 and then upgrade to 3.21.x.

The new client code works with a 3.20.x mysqld server, so if you experience problems with 3.21.x, you can use the old 3.20.x server without having to recompile the clients again.

If you are not using the --old-protocol option to mysqld, old clients will issue the error message:

ERROR: Protocol mismatch. Server Version = 10 Client Version = 9

The new Perl DBI/DBD interface also supports the old mysqlperl interface. The only change you have to make if you use mysqlperl is to change the arguments to the connect() function. The new arguments are: host, database, user, password (the user and password arguments have changed places).

The following changes may affect queries in old applications:

HAVING must now be specified before any ORDER BY clause.
The parameters to LOCATE() has been swapped.
There are some new reserved words. The most notable are DATE, TIME and TIMESTAMP.

4.15.3 Upgrading to another architecture

Currently the MySQL data and index files (`*.ISD' and `*.ISM' files) are architecture-dependent and in some case OS-dependent. If you want to move your applications to another machine that has a different architecture/OS than your current machine, you should not try to move a database by simply copying the files to the other machine. You should use mysqldump instead.

By default, mysqldump will create a file full of SQL statements. You can then transfer the file to the other machine and feed it as input to the mysql client.

Try mysqldump --help to see what options are available. If you are moving the data to a newer version of MySQL, you should use mysqldump --opt with the newer version to get a fast, compact dump.

The easiest (although not the fastest) way to move a database between two machines is to run the following commands on the machine on which the database is located:

shell> mysqladmin -h 'other hostname' create db_name
shell> mysqldump --opt db_name \
        | mysql -h 'other hostname' db_name

If you want to copy a database from a remote machine over a slow network, you can use:

shell> mysqladmin create db_name
shell> mysqldump -h 'other hostname' --opt --compress db_name \
        | mysql db_name

You can also store the result in a file (compressed in this example):

shell> mysqldump --quick db_name | gzip > db_name.contents.gz

Transfer the file containing the database contents to the target machine and run these commands there:

shell> mysqladmin create db_name
shell> gunzip < db_name.contents.gz | mysql db_name

You can also use mysqldump and mysqlimport to accomplish the database transfer. For big tables, this is much faster than simply using mysqldump. In the commands shown below, DUMPDIR represents the full pathname of the directory you use to store the output from mysqldump.

First, create the directory for the output files and dump the database:

shell> mkdir DUMPDIR
shell> mysqldump --tab=DUMPDIR db_name

Then transfer the files in the DUMPDIR directory some corresponding directory on the target machine and load the files into MySQL there:

shell> mysqladmin create db_name
shell> cat DUMPDIR/*.sql | mysql db_name
shell> mysqlimport db_name PATH/*.txt

Also, don't forget to copy the mysql database, since that's where the grant tables (user, db, host) are stored. You may have to run commands as the MySQL root user on the new machine until you have the mysql database in place.

After you import the mysql database on the new machine, execute mysqladmin reload so that the server reloads the grant table information.

5 How standards-compatible is MySQL?

5.1 MySQL extensions to ANSI SQL92

MySQL includes some extensions that you probably will not find in other SQL databases. Be warned that if you use them, your code will not be portable to other SQL servers. In some cases, you can still write portable code that includes MySQL extensions by using comments of the form /*! ... */. In this case, MySQL will execute the code within the comment. For example:

SELECT /*! STRAIGHT_JOIN */ col_name from table1,table2 WHERE ...

MySQL extensions are listed below:

The field types MEDIUMINT, SET, ENUM and the different BLOB and TEXT types.
The field attributes AUTO_INCREMENT, BINARY, UNSIGNED and ZEROFILL.
All string comparisons are case insensitive by default, with sort ordering determined by the current character set (ISO-8859-1 Latin1 by default). If you don't like this, you should declare your columns with the BINARY attribute, which causes comparisons to be done according to the ASCII order used on the MySQL server host.
MySQL maps all tables to filenames. This has two implications:
- Table names are case sensitive in MySQL on operating systems that have case sensitive filenames (like most Unix systems). If you have a problem remembering table names, create everything in lowercase.
- You can use standard system commands to backup, rename, move, delete and copy tables. For example, to rename a table, rename the `.ISD', `.ISM' and `.frm' files to which the table corresponds.
LIKE is allowed on numeric columns.
Use of INTO OUTFILE and STRAIGHT_JOIN in a SELECT statement. See section 7.11 SELECT syntax.
EXPLAIN SELECT to get a description on how tables are joined.
Use of index names, indexes on a subpart of a field, and use of INDEX or KEY in a CREATE TABLE statement. See section 7.6 CREATE TABLE syntax.
Use of CHANGE col_name, DROP col_name or DROP INDEX in an ALTER TABLE statement. See section 7.7 ALTER TABLE syntax.
Use of IGNORE in an ALTER TABLE statement.
Use of multiple ADD, ALTER, DROP or CHANGE clauses in an ALTER TABLE statement.
Use of DROP TABLE with the keywords IF EXISTS.
Use of DROP TABLE with more than one table.
Use of LOAD DATA INFILE. In many cases, this syntax is compatible with Oracle's LOAD DATA INFILE. See section 7.15 LOAD DATA INFILE syntax.
Use of OPTIMIZE TABLE.
Strings may be enclosed by either `"' or `'', not just by `''.
Using the escape `\' character.
The SET OPTION statement. See section 7.24 SET OPTION syntax.
You don't need to have all columns in the GROUP BY part. This gives better performance for some very specific, but quite normal queries. See section 7.3.12 Functions for use with GROUP BY clauses.
To make it easier for a user that comes from different SQL environments MySQL supports aliases for many functions. For example, all string functions support both the ANSI SQL and the ODBC syntax.
MySQL understands the || and && operators to mean logical OR and AND, as in the C programming language. In MySQL, || and OR are synonyms, as are && and AND. Because of this nice syntax, MySQL doesn't support the ANSI SQL operator || for string concatenation; use CONCAT() instead. Since CONCAT() takes any number of arguments, it's easy to convert use of the || operator to MySQL.
To support use of MySQL-specific keywords as STRAIGHT_JOIN in SQL code that should be portable, you can embed them in a /* */ comment that starts with a '!'. In this case MySQL will parse the comment as it would any other MySQL statement, but other SQL servers will ignore the extensions. For example:
```
SELECT /*! STRAIGHT_JOIN */ * from table1,table2 WHERE ...
```
Use of any of the following functions or commands:
- CREATE DATABASE or DROP DATABASE. See section 7.4 CREATE DATABASE syntax.
- % instead of MOD(). % is supported for C programmers and for compatibility with PostgreSQL.
- =, <>, <= ,<, >=,>, <<, >>, AND, OR or LIKE in a column statement.
- LAST_INSERT_ID(). See section 18.4.49 How can I get the unique ID for the last inserted row?.
- REGEXP or NOT REGEXP.
- CONCAT() or CHAR() with one or more than two arguments. In MySQL these functions can take any number of arguments.
- BIT_COUNT(), ELT(), FROM_DAYS(), FORMAT(), IF(), PASSWORD(), ENCRYPT(), PERIOD_ADD(), PERIOD_DIFF(), TO_DAYS(), or WEEKDAY().
- Use of TRIM() to trim substrings. ANSI SQL only supports removal of single characters.
- The STD(), BIT_OR() and BIT_AND() group functions.
- Use of REPLACE instead of DELETE + INSERT. See section 7.14 REPLACE syntax.
- The FLUSH flush_option command.

5.2 Functionality missing from MySQL

The following functionality is missing in the current version of MySQL. For the priority of new extensions, you should consult the MySQL TODO list. That is the latest version of the TODO list in this manual. See section F List of things we want to add to MySQL in the future (The TODO).

5.2.1 Sub-selects

The following will not work in MySQL:

SELECT * FROM table1 WHERE id IN (SELECT id FROM table2);

MySQL only supports INSERT ... SELECT ... and REPLACE ... SELECT ... Independent sub-selects will be probably be available in 3.23.0. You can now use the function IN() in other contexts, however.

5.2.2 `SELECT INTO TABLE`

MySQL doesn't yet support SELECT ... INTO TABLE .... Currently, MySQL only supports SELECT ... INTO OUTFILE ..., which is basically the same thing.

5.2.3 Transactions

Transactions are not supported. MySQL shortly will support atomic operations, which are like transactions without rollback. With atomic operations, you can execute a group of insert/select/whatever commands and be guaranteed that no other thread will interfere. In this context, you won't usually need rollback. Currently, you can prevent interference from other threads with the help of the LOCK TABLES and UNLOCK TABLES commands. See section 7.23 LOCK TABLES/UNLOCK TABLES syntax.

5.2.4 Stored procedures and triggers

A stored procedure is a set of SQL commands that can be compiled and stored in the server. Once this has been done, clients don't need to keep reissuing the entire query but can refer to the stored procedure. This provides more speed because the query has to be parsed only once and less data need be sent between the server and the client. You can also raise the conceptual level by having libraries of functions in the server.

A trigger is a stored procedure that is invoked when a particlar event occurs. For example, you can install a stored procedure that is triggered each time a record is deleted from a transaction table and that automatically deletes the corresponding customer from a customer table when all his transactions are deleted.

The planned update language will be able to handle stored procedures, but without triggers. Triggers usually slow down everything, even queries for which they are not needed.

To see when MySQL might get stored procedures, see section F List of things we want to add to MySQL in the future (The TODO).

5.2.5 Foreign Keys

Note that foreign keys in SQL are not used to join tables, but are used mostly for checking referential integrity. If you want to get results from multiple tables from a SELECT statement, you do this by joining tables! See section 7.12 JOIN syntax.

The FOREIGN KEY syntax in MySQL exists only for compatibility with other SQL vendors' CREATE TABLE commands; it doesn't do anything. The FOREIGN KEY syntax without ON DELETE ... is mostly used for documentation purposes. Some ODBC applications may use this to produce automatic WHERE clauses, but this is usually easy to override. FOREIGN KEY is sometimes used as a constraint check, but this check is unnecessary in practice if rows are inserted into the tables in the right order. MySQL only supports these clauses because some applications require them to exist (regardless of whether or not they work!).

In MySQL, you can work around the problem of ON DELETE ... not being implemented by adding the appropriate DELETE statement to an application when you delete records from a table that has a foreign key. In practice this is as quick (in some cases quicker) and much more portable than using foreign keys.

In the near future we will extend the FOREIGN KEY implementation so that at least the information will be saved and may be retrieved by mysqldump and ODBC.

5.2.5.1 Reasons NOT to use foreign keys

There are so many problems with FOREIGN KEYs that we don't know where to start:

Foreign keys make life very complicated, because the foreign key definitions must be stored in a database and implementing them would destroy the whole "nice approach" of using files that can be moved, copied and removed.
The speed impact is terrible fore INSERT and UPDATE statements, and in this case almost all FOREIGN KEY checks are useless because one usually inserts records in the right tables in the right order.
There is also a need to hold locks on many more tables when updating one table, because the side effects can cascade through the entire database. It's MUCH faster to delete records from one table first and subsequently delete them from the other tables.
You can no longer restore a table by doing a full delete from the table and then restoring all records (from a new source or from a backup).
If you have foreign keys you can't dump and restore tables unless you do so in a very specific order.
It's very easy to do "allowed" circular definitions that make the tables impossible to recreate each table with a single create statement, even if the definition works and is usable.

The only nice aspect of foreign key is that it gives ODBC and some other client programs the ability to see how a table is connected and use this to show connection diagrams and to help building applicatons.

MySQL will soon store FOREIGN KEY definitions so that a client can ask for and receive an answer how the original connection was made. The current `.frm' file format does not have any place for it.

5.2.6 Views

MySQL doesn't support views, but this is on the TODO.

5.2.7 `--' as the start of a comment

Some other SQL databases use `--' to start comments. MySQL has `#' as the start comment character, even if the mysql command line tool removes all lines that start with `--'. You can also use the C comment style /* this is a comment */ with MySQL. See section 7.28 Comment syntax.

MySQL will not support `--'; this degenerate comment style has caused many problems with automatically-generated SQL queries that have used something like the following code, where we automatically insert the value of the payment for !payment!:

UPDATE tbl_name SET credit=credit-!payment!

What do you think will happen when the value of payment is negative?

Because 1--1 is legal in SQL, we think it is terrible that `--' means start comment.

If you have a SQL program in a text file that contains `--' comments you should use:

shell> replace " --" " #" < text-file-with-funny-comments.sql \
         | mysql database

instead of the normal:

shell> mysql database < text-file-with-funny-comments.sql

You can also change the `--' comments to `#' comments in the command file:

shell> replace " --" " #" -- text-file-with-funny-comments.sql

Change them back with this command:

shell> replace " #" " --" -- text-file-with-funny-comments.sql

5.3 What standards does MySQL follow?

Entry level SQL92. ODBC level 0-2.

5.4 What functions exist only for compatibility?

GRANT. See section 7.25 GRANT syntax (Compatibility function). This always succeeds, because GRANT in MySQL does nothing. You should use the MySQL privilege tables instead. See section 6.4 How the privilege system works.

5.5 Limitations of `BLOB` and `TEXT` types

If you want to use GROUP BY or ORDER BY on a BLOB or TEXT field, you must make the field into a fixed-length object. The standard way to do this is with the SUBSTRING function. For example:

mysql> select comment from tbl_name order by SUBSTRING(comment,20);

If you don't do this, only the first max_sort_length bytes (default=1024) are considered when sorting.

BLOB and TEXT cannot have DEFAULT values and will also always be NULL columns.

5.6 How to cope without `COMMIT`-`ROLLBACK`

MySQL doesn't support COMMIT-ROLLBACK. The problem is that handling COMMIT-ROLLBACK efficiently would require a completely different table layout than MySQL uses today. MySQL would also need extra threads that do automatic cleanups on the tables and the disk usage would be much higher. This would make MySQL about 2-4 times slower than it is today. MySQL is much faster than almost all other SQL databases (typically at least 2-3 times faster). One of the reasons for this is the lack of COMMIT-ROLLBACK.

For the moment, we are much more for implementing the SQL server language (something like stored procedures). With this you would very seldom really need COMMIT-ROLLBACK. This would also give much better performance.

Loops that need transactions normally can be coded with the help of LOCK TABLES, and you don't need cursors when you can update records on the fly.

We have transactions and cursors on the TODO but not quite prioritized. If we implement these, it will be as an option to CREATE TABLE. That means that COMMIT-ROLLBACK will only work on those tables and only those tables will be slower.

We at TcX have a greater need for a real fast database than a 100% general database. Whenever we find a way to implement these features without any speed loss, we will probably do it. For the moment, there are many more important things to do. Check the TODO for how we prioritize things at the moment. Customers with higher levels of support can alter this, so things may be reprioritized.

The current problem is actually ROLLBACK. Without ROLLBACK, you can do any kind of COMMIT action with LOCK TABLES. To support ROLLBACK, MySQL would have to be changed to store all old records that were updated and revert everything back to the starting point if ROLLBACK was issued. For simple cases, this isn't that hard to do (the current isamlog could be used for this purpose), but it would be much more difficult to implement ROLLBACK for ALTER/DROP/CREATE TABLE.

To avoid using ROLLBACK, you can use the following strategy:

Use LOCK TABLES ... to lock all the tables you want to access.
Test conditions.
Update if everything is okay.
UNLOCK TABLES

This is usually a much faster method than using transactions with possible ROLLBACKs, although not always. The only situation this solution doesn't handle is when someone kills the threads in the middle of an update. In this case, all locks will be released but some of the updates may not have been executed.

You can also use functions to update records in a single operation. You can get a very efficient application by using the following techniques:

Modify fields relative to their current value
Update only those fields that actually have changed

For example, when we are doing updates on some customer information, we update only the customer data that have changed and test only that none of the changed data, or data that depend on the changed data, have changed compared to the original row. The test for changed data is done with the WHERE clause in the UPDATE statement. If the record wasn't updated, we give the client a message: "Some of the data you have changed have been changed by another user". Then we show the old row versus the new row in a window, so the user can decide which version of the customer record he should use.

This gives us something that is similar to "column locking" but is actually even better, because we only update some of the columns with values that are relative to their current values. This means that typical UPDATE statements look something like these:

UPDATE tablename SET pay_back=pay_back+'relative change';

UPDATE customer
  SET
    customer_date='current_date',
    address='new address',
    phone='new phone',
    money_he_owes_us=money_he_owes_us+'new_money'
  WHERE
    customer_id=id AND address='old address' AND phone='old phone';

As you can see, this is very efficient and works even if another client has changed the values in the pay_back or money_he_owes_us columns.

In many cases, users have wanted ROLLBACK and/or LOCK TABLES to manage unique identifiers for some tables. This can be handled much more efficiently by using an AUTO_INCREMENT column and either the SQL LAST_INSERT_ID() function or the C API function mysql_insert_id(). See section 18.4.49 How can I get the unique ID for the last inserted row?.

At TcX, we have never had any need for row-level locking because we have always been able to code around it. Some cases really need row locking, but they are very few. If you want row-level locking, you can use a flag column in the table and do something like this:

UPDATE tbl_name SET row_flag=1 WHERE id=ID;

MySQL returns 1 for the number of affected rows if the row was found and row_flag wasn't already 1 in the original row.

6 The MySQL access privilege system

MySQL has an advanced but non-standard security/privilege system. This section describes how it works.

6.1 What the privilege system does

The primary function of the MySQL privilege system is to associate a user name on a host with select, insert, update and delete privileges on a database.

Additional functionality includes the ability to have an anonymous user and to grant privileges for MySQL-specific functions such as LOAD DATA INFILE and administrative operations.

Please note that user names, as used by MySQL for authentication purposes, have nothing to do with Unix user names (login names) or Windows user names. Most MySQL clients try to log in using the current Unix user name as the MySQL user name, but that is for convenience only. Client programs allow a different name to be specified with the -u or --user options. This means that you can't make a database secure in any way unless all MySQL user names have passwords. Anyone may attempt to connect to the server using any name, and they will succeed if you don't have a password for each name.

MySQL user names can be up to 16 characters long, whereas Unix user names typically are limited to 8 characters.

MySQL passwords have nothing to do with Unix passwords, either. There is no necessary connection between the password you use to log in on a Unix machine and the password you use to access a database on that machine. MySQL also encrypts passwords using a different algorithm than the one used during the Unix login process.

6.2 Connecting to the MySQL server

MySQL client programs generally require that you specify connection parameters: the host you want to connect to, your user name and your password. For example, the mysql client can be started like this (optional arguments are enclosed between `[' and `]'):

shell> mysql [-h host_name] [-u user_name] [-pyour_pass]

Note that there is no space between -p and the password following it.

Alternate forms of the -h, -u and -p options are --host=host_name, --user=user_name and --password=your_pass.

mysql uses default values for connection parameters that are missing from the command line. The default hostname is localhost and the default user name is your Unix login name. (No password is supplied if -p is missing.) Thus, for a Unix user joe, the following commands are equivalent:

shell> mysql -h localhost -u joe
shell> mysql -h localhost
shell> mysql -u joe
shell> mysql

Other MySQL clients behave similarly.

On Unix systems, you can specify different default values to be used when you make a connection, so that you need not enter them on the command line each time you invoke a client program:

You can specify connection parameters in the [client] section of the `.my.cnf' configuration file in your home directory. The relevant section of the file might look like this:
```
[client]
host=host_name
user=user_name
password=your_pass
```
See section 4.14.4 Option files.
You can specify connection parameters using environment values. The host can be specified using MYSQL_HOST. The MySQL user name can be specified using USER, LOGNAME or LOGIN (although these variables might already be set to your Unix login name, and it might be wise not to change them). The password can be specified using MYSQL_PWD (but this is insecure; see next section).

If connection parameters are specified in multiple ways, values specified on the command line override values specified in configuration files and environment variables, and values in configuration files override values in environment variables.

6.2.1 Keeping your password secure

It is inadvisable to specify your password in a way that exposes it to discovery by other users. The methods you can use to specify your password when you run client programs are listed below, along with an assessment of the risks of each method:

Use a -pyour_pass or --password=your_pass option on the command line. This is convenient but insecure, since your password becomes visible to system status programs (such as ps) that may be invoked by other users to display command lines. (MySQL clients typically overwrite the command line argument with zeroes during their initialization sequence, but there is still a brief interval during which the value is visible.)
Use a -p or --password option (with no your_pass value specified). In this case, the client program will solicit the password from the terminal:
```
shell> mysql -u user_name -p
Enter password: ********
```
The client echoes `*' characters to the terminal as you enter your password so that onlookers cannot see it. It is more secure to enter your password this way than to specify it on the command line because it is not visible to other users. However, this method of entering a password is unsuitable if you want to invoke a client from a script that runs non-interactively.
Store your password in a configuration file. For example, you can list your password in the [client] section of the `.my.cnf' file in your You can specify connection parameters using environment values. The host can be specified using MYSQL_HOST. The MySQL user name can be specified using USER, LOGNAME or LOGIN (although these variables might already be set to your Unix login name, and it might be wise not to change them). The password can be specified using MYSQL_PWD (but this is insecure; see next section).

6.2.2 Keeping your password secure

Use a -pyour_pass or --password=your_pass option on the command line. This is convenient but insecure, since your password becomes visible to system status programs (such as ps) that may be invoked by other users to display command lines. (MySQL clients typically overwrite the command line argument with zeroes during their initialization sequence, but there is still a brief interval during which the value is visible.)
Use a -p or --password option (with no your_pass value specified). In this case, the client program will solicit the password from the terminal:
```
shell> mysql -u user_name -p
Enter password: ********
```
The client echoes `*' characters to the terminal as you enter your password so that onlookers cannot see it. It is more secure to enter your password this way than to specify it on the command line because it is not visible to other users. However, this method of entering a password is unsuitable if you want to invoke a client from a script that runs non-interactively.
Store your password in a configuration file. For example, you can list your password in the [client] section of the `.my.cnf' file in your home directory:
```
[client]
password=your_pass
```
If you store your password in `.my.cnf', the file should not be group or world readable or writable. Make sure the file's access mode is 400 or 600.
You can store your password in the MYSQL_PWD environment variable, but this method must be considered extremely insecure and should not be used. Some versions of ps include an option to display the environment of running processes; your password will be in plain sight for all to see if you set MYSQL_PWD. Even on systems without such a version of ps, it is unwise to assume there is no other method to observe process environments.

All in all, the safest methods are probably to have the client program prompt for the password or to specify the password in a well-protected `.my.cnf' file.

6.3 Privileges provided by MySQL

Privilege information is stored in the user, db and host tables in the mysql database (that is, in the database named mysql). The MySQL server reads the contents of these tables when it starts up or when you explicitly tell it to reread the tables by executing a mysqladmin reload command. This means that whenever you make changes to those tables, the changes have no effect until you execute mysqladmin reload or restart the server.

The names used in this manual to refer to the privileges provided by MySQL are shown below, along with the table column name associated with each privilege and the context in which the privilege applies.

Privilege	Column	Context
select	`Select_priv`	tables
insert	`Insert_priv`	tables
update	`Update_priv`	tables
delete	`Delete_priv`	tables
create	`Create_priv`	databases, tables or indexes
drop	`Drop_priv`	databases or tables
reload	`Reload_priv`	server administration
shutdown	`Shutdown_priv`	server administration
process	`Process_priv`	server administration
file	`File_priv`	file access on server

The select, insert, update and delete privileges allow you to perform operations on rows in existing tables in a database.

SELECT statements require the select privilege only if they actually retrieve rows from a table. You can execute certain SELECT statements even without permission to access any of the databases on the server. For example, you could use the mysql client as a simple calculator:

mysql> SELECT 1+1;
mysql> SELECT PI()*2;

The create and drop privileges allow you to create new databases and tables, or to drop (delete) existing databases and tables.

The create privilege also allows you to create or drop indexes. You need create (rather than drop) to drop an index because that is actually done by recreating the table minus the index.

Note that if you grant the drop privilege for the mysql database to a user, that user can drop the database in which the MySQL access privileges are stored!

The file privilege gives you permission to read and write files on the server using the LOAD DATA INFILE and SELECT ... INTO OUTFILE statements. Any user to whom this privilege is granted can read or write any file that the MySQL server can read or write.

The remaining privileges are used for administrative operations, which are performed using the mysqladmin program. The table below shows which mysqladmin commands each administrative privilege allows you to execute:

Privilege	Commands permitted to privilege holders
reload	`reload`, `refresh`, `flush-hosts`, `flush-logs`, `flush-tables`
shutdown	`shutdown`
process	`processlist`, `kill`

The reload command tells the server to reread the privilege tables. The refresh command flushes all tables and opens and closes the log files. The flush-* commands perform functions similar to refresh but are more limited in scope, and may be preferable in some instances. For example, if you just want to flush the log files, flush-logs is a better choice than refresh.

The processlist command displays information about the threads executing within the server. The kill command kills server threads. You can always display or kill your own threads, but you need the process privilege to display or kill threads initiated by other users.

Certain privileges should be granted sparingly:

The process privilege can be used to view the plain text of currently executing queries, including queries that set or change passwords.
The file privilege could be abused to read any world-readable file on the server into a database table, the contents of which can then be accessed using SELECT.
Privileges on the mysql database can be used to change passwords and other access privilege information. (Even though passwords are stored encrypted, a malicious user can, with sufficient privileges, replace a password with a different one.)

There are some things that you cannot do with the MySQL privilege system:

You cannot explicitly specify that a given user should be denied access. That is, you cannot explicitly match a user and then refuse the connection.
You cannot grant privileges on a table-specific basis. For example, you cannot allow a user to access some tables within a database but not others.
You cannot specify that a given user should be able to drop some tables within a database but not others.

6.4 How the privilege system works

The MySQL privilege system ensures that all users may do exactly the things that they are supposed to be allowed to do. When you connect to a MySQL server, your identity is determined by the host from which you connect and the user name you specify. The system grants privileges according to your identity and what you want to do.

MySQL considers both your hostname and user name in identifying you because there is little reason to assume that a given user name belongs to the same person everywhere on the Internet. For example, the user bill who connects from whitehouse.gov need not be the same person as the user bill who connects from microsoft.com. MySQL handles this by allowing you to distinguish users on different hosts that happen to have the same name: you can grant bill one set of privileges for connections from whitehouse.gov, and a different set of privileges for connections from microsoft.com.

MySQL access control involves two stages:

Stage 1: The server checks whether or not you are even allowed to connect.
Stage 2 (assuming the server lets you connect): The server checks each request you issue to see whether or not you have sufficient privileges to perform it. For example, if you select rows from a table in a database or drop a table from the database, the server makes sure you have the select or drop privilege for the database.

The server bases decisions at both stages of access control on the contents of three grant tables (user, db and host) in the mysql database. The fields in the grant tables are shown below:

Table name	`user`	`db`	`host`
Scope fields	`Host`	`Host`	`Host`
	`User`	`Db`	`Db`
	`Password`	`User`
Privilege fields	`Select_priv`	`Select_priv`	`Select_priv`
	`Insert_priv`	`Insert_priv`	`Insert_priv`
	`Update_priv`	`Update_priv`	`Update_priv`
	`Delete_priv`	`Delete_priv`	`Delete_priv`
	`Create_priv`	`Create_priv`	`Create_priv`
	`Drop_priv`	`Drop_priv`	`Drop_priv`
	`Reload_priv`
	`Shutdown_priv`
	`Process_priv`
	`File_priv`

Each table contains two types of fields: scope fields and privilege fields.

Scope fields determine the scope of each entry in the tables, i.e., the context in which the entry applies. For example, a user table entry with Host and User values of 'thomas.loc.gov' and 'bob' would be used for authenticating connections made to the server by bob from the host thomas.loc.gov. Similarly, a db table entry with Host, User and Db fields of 'thomas.loc.gov', 'bob' and 'reports' would be used when bob connects from the host thomas.loc.gov to access the reports database.

For access-checking purposes, Host values are compared in case-insensitive fashion and User, Password and Db values are compared in case-sensitive fashion.

Privilege fields indicate the privileges granted by a table entry and what operations can be performed. The server combines the information in the user, db and host tables to form a complete description of a user's privileges. The rules used to do this are described in section 6.6 Access control, stage 2: Request verification.

Scope fields are strings, declared as shown below; the default value for each is the empty string:

Field name	Type
`Host`	`CHAR(60)`
`User`	`CHAR(16)`
`Password`	`CHAR(16)`
`Db`	`CHAR(64)`

All privilege fields are declared as ENUM('N','Y') -- each can have a value of 'N' or 'Y', and the default value is 'N'.

Briefly, the server uses the grant tables like this:

The user table scope fields determine whether to allow or reject incoming connections. For allowed connections, the privilege fields indicate the user's global (superuser) privileges.
The db table scope fields determine which users can access which databases from which hosts. The privilege fields determine which operations are allowed.
The host table is used as an extension of the db table when you want a given db table entry to apply to several hosts. For example, if you want a user to be able to use a database from several hosts in your network, leave the Host value empty in the user's db table entry, then populate the host table with an entry for each of those hosts. This mechanism is described more detail in section 6.6 Access control, stage 2: Request verification.

Note that administrative privileges (reload, shutdown, etc.) are specified only in the user table. This is because administrative operations are operations on the server itself and are not database-specific, so there is no reason to list such privileges in the db or host tables. In fact, only the user table need be consulted to determine whether or not you can perform an administrative operation.

The file privilege is specified only in the user table, too. It is not an administrative privilege as such, but your ability to read or write files on the server host is not dependent on the database you are accessing.

The mysqld server reads the contents of the grant tables once, when it starts up. If you make changes to the tables, the server will not notice unless you execute mysqladmin reload or restart the server.

When you modify the contents of the grant tables, it is a good idea to make sure that your changes set up privileges the way you want. A useful diagnostic tool is the mysqlaccess script, which Yves Carlier has provided for the MySQL distribution. Invoke mysqlaccess with the --help option to find out how it works. Also, see section 6.10 Causes of Access denied errors and section 6.11 How to make MySQL secure against crackers.

6.5 Access control, stage 1: Connection verification

When you attempt to connect to a MySQL server, the server accepts or rejects the connection based on your identity and whether or not you can verify your identity by supplying the correct password. If not, the server completely denies access to you. Otherwise, the server accepts the connection, then enters stage 2 and waits for requests.

Your identity is based on two pieces of information:

The host from which you connect
Your MySQL user name

Identity checking is performed using the three user table scope fields (Host, User and Password). The server accepts the connection only if a user table entry matches your hostname and user name, and you supply the correct password.

You can specify user table entries as follows:

A Host value may be 'localhost' to indicate the local host, a hostname or an IP number.
You can use the wildcard characters `%' and `_' in the Host field.
A Host value of '%' matches any hostname. A blank Host value is equivalent to '%'. Note that these values match any host that can create a connection to your server!
Wildcard characters are not allowed in the User field, but you can specify a blank value, which matches any name. If the matching entry for an incoming connection has a blank user name, the user is considered to be the anonymous user (i.e., the user with no name), rather than the name that the client actually specified.
The Password field can be blank. This does not mean that any password matches, it means the user must connect without specifying a password.

The table below shows some examples of the types of connections allowed by various combinations of Host and User values in user table entries. (Access is contingent on the user supplying the correct password, of course.)

`Host` value	`User` value	Connections matched by entry
`'thomas.loc.gov'` @tab `'fred'` @tab `fred`, connecting from `thomas.loc.gov`
`'thomas.loc.gov'` @tab `"` @tab Any user, connecting from `thomas.loc.gov`
`'%'` @tab `'fred'` @tab `fred`, connecting from any host
`'%'` @tab `"` @tab Any user, connecting from any host
`'%.loc.gov'` @tab `'fred'` @tab `fred`, connecting from any host in the `loc.gov` domain
`'x.y.%'` @tab `'fred'` @tab `fred`, connecting from `x.y.net`, `x.y.com`, `x.y.edu`, etc. (this is probably not useful)
`'144.155.166.177'` @tab `'fred'` @tab `fred`, connecting from the host with IP address `144.155.166.177`
`'144.155.166.%'` @tab `'fred'` @tab `fred`, connecting from any host in the class C `144.155.166` subnet

Since you can use IP wildcard values in the Host field (e.g., '144.155.166.%' to match every host on a subnet), there is the possibility that someone might try to exploit this capability by naming a host 144.155.166.somewhere.com. To foil such attempts, MySQL disallows matching on hostnames that start with digits and a dot. Thus, if you have a host named something like 1.2.foo.com, its name will never match the Host column of the grant tables. Only an IP number can match an IP wildcard value.

How does the server choose which user table entry to use if more than one matches? This question is resolved by the way the user table is sorted, which is done at server startup time. Suppose the user table looks like this:

+-----------+----------+-
| Host      | User     | ...
+-----------+----------+-
| %         | root     | ...
| %         | jeffrey  | ...
| localhost | root     | ...
| localhost |          | ...
+-----------+----------+-

When the server reads in the table, it orders the entries with the most-specific Host values first ('%' in the Host column means "any host" and is least specific). Entries with the same Host value are ordered with the most-specific User values first (a blank User value means "any user" and is least specific). As a result, the sorted user table looks like this:

+-----------+----------+-
| Host      | User     | ...
+-----------+----------+-
| localhost | root     | ...
| localhost |          | ...
| %         | jeffrey  | ...
| %         | root     | ...
+-----------+----------+-

The matching algorithm looks through the sorted entries and uses the first match found. For a connection from localhost by jeffrey, the entries with localhost in the Host column match first. Of those, the entry with the blank user name matches both the connecting hostname and user name. (The '%'/'jeffrey' entry would have matched, too, but it is not the first match in the table.)

Here is another example. Suppose the user table looks like this:

+----------------+----------+-
| Host           | User     | ...
+----------------+----------+-
| %              | jeffrey  | ...
| thomas.loc.gov |          | ...
+----------------+----------+-

The sorted table looks like this:

+----------------+----------+-
| Host           | User     | ...
+----------------+----------+-
| thomas.loc.gov |          | ...
| %              | jeffrey  | ...
+----------------+----------+-

A connection from thomas.loc.gov by jeffrey is matched by the first entry, whereas a connection from whitehouse.gov by jeffrey is matched by the second.

If you have problems connecting to the server, print out the user table and sort it by hand to see where the first match is being made.

6.6 Access control, stage 2: Request verification

Once you establish a connection, the server enters stage 2. For each request that comes in on the connection, the server checks whether you have sufficient privileges for it, based on the type of operation you wish to perform. This is where the privilege fields in the grant tables come into play. These privileges can come from any of the user, db or host tables. (You may find it helpful to refer to the table shown earlier that lists the fields present in each of the grant tables; ssee section 6.4 How the privilege system works.)

The user table grants privileges that are assigned to you on a global basis and apply no matter what the current database is. For example, if the user table grants you the delete privilege, you can delete rows from any database on the server host! In other words, user table privileges are superuser privileges. For this reason, you should leave the privileges in the user table set to 'N' for most users and grant privileges on a database-specific basis only, using the db and host tables. It is wise to grant privileges in the user table only to superusers.

The db and host tables, by contrast, grant database-specific privileges. The wildcard characters `%' and `_' can be used in the Host and Db fields of each table. Blank values are allowed in any of the scope fields. A '%' or blank Host or Db value means "any host" or "any database." A blank User value matches the anonymous user.

The db and host tables are read in and sorted when the server starts up (at the same time that it reads the user table). The db table is sorted on the Host, Db and User scope fields, and the host table is sorted on the Host and Db scope fields. As with the user table, sorting puts the most-specific values first and least-specific values last, and when the server looks for matching entries, it uses the first match that it finds.

The request verification process is described below. If you are familiar with the access-checking source code, you will notice that the description here differs slightly from the algorithm used in the code. The description is equivalent to what the code actually does; it differs only to make the explanation simpler.

For administrative requests (shutdown, reload, etc.), the server checks only the user table entry, since that is the only table that specifies administrative privileges. Access is granted if the entry allows the requested operation and denied otherwise. For example, if you want to execute mysqladmin shutdown but your user table entry doesn't grant the shutdown privilege to you, access is denied without even checking the db or host tables (there is no need to, because they contain no Shutdown_priv column).

For database-related requests (insert, update, etc.), the server first checks the user's global (superuser) privileges by looking in the user table entry. If the entry allows the requested operation, access is granted.

If the global privileges in the user table are insufficient, the server determines the user's database-specific privileges by checking the db and host tables:

The server looks in the db table for a match on the Host, Db and User fields. Host and User are matched to the connecting user's hostname and MySQL user name. The Db field is matched to the database the user wants to access. If no db table entry matches, access is denied.
If there is a matching db table entry and its Host field is not blank, that entry defines the user's database-specific privileges.
If the matching db table entry's Host field is blank, it signifies that the host table enumerates which hosts should be allowed access to the database. In this case, a further lookup is done in the host table to find a match on the Host and Db fields. If no host table entry matches, access is denied. If there is a match, the user's database-specific privileges are computed as the intersection of the privileges in the db and host table entries, i.e., the privileges that are 'Y' in both entries. (This way you can grant general privileges in the db table entry and then selectively remove them on a host-by-host basis using the host table entries.)

After determining the database-specific privileges granted by the db and host table entries, the server adds them to the global privileges granted by the user table. Access is granted if the result allows the requested operation and denied otherwise.

It may not be apparent why the server adds the global user entry privileges to the database-specific privileges from the db and host entries for those cases in which the user privileges are initially found to be insufficient for the requested operation. The reason is that the operation might require more than one type of privilege. For example, if you execute an INSERT ... SELECT statement, you need both insert and select privileges. Your privileges might be such that the user table entry grants one privilege and the db table entry grants the other. In this case, you have the necessary privileges, but the server cannot tell that from either table by itself; the privileges granted by both entries must be combined.

The host table can be used to maintain a list of "secure" servers. At TcX, the host table contains a list of all machines on the local network. These are granted all privileges.

You can also use the host table to indicate hosts that are not so secure. Suppose you have a machine public.your.domain that is located in a public area that you do not consider secure. You can allow access to hosts on your network except that machine with host table entries like this:

+--------------------+----+-
| Host               | Db | ...
+--------------------+----+-
| public.your.domain | %  | ... (all privileges set to 'N')
| %.your.domain      | %  | ... (all privileges set to 'Y')
+--------------------+----+-

Naturally, you should always test your entries in the grant tables (e.g., using mysqlaccess) to make sure your access privileges are actually set up the way you think they are.

6.7 Setting up the initial MySQL privileges

After installing MySQL, you set up the initial access privileges by running scripts/mysql_install_db. See section 4.7.1 Quick installation overview. The scripts/mysql_install_db script starts up the mysqld server, then initializes the grant tables to contain the following set of privileges:

The MySQL root user is a superuser and can do anything. Connections must be made from the local host. Note: The initial root password is empty, so anyone can connect as root without a password and be granted all privileges.
Anonymous-user privileges allow anything to be done with databases that have a name of 'test' or starting with 'test_'. Any user can connect from the local host and be treated as the anonymous user.
Other privileges are denied. For example, normal users can't use mysqladmin shutdown or mysqladmin processlist.

Since your installation is initially wide open, one of the first things you will probably want to do is specify a password for the MySQL root user. You can do so as follows (note that you specify the password using the PASSWORD() function):

shell> mysql -u root mysql
mysql> UPDATE user SET Password=PASSWORD('new_password')
           WHERE user='root';

Then tell the server to reread the grant tables since your change will go unnoticed otherwise:

shell> mysqladmin -u root reload

From this point on, you must supply the password whenever you connect to the server as root.

You may wish to defer setting up a root password so that you don't need to specify it while you perform additional setup or testing, but you should be sure to set it before using your installation for any real production work.

See the scripts/mysql_install_db script to see how it sets up the default privileges. You can use this as a basis to see how to add other users.

If you want the initial privileges to be different than those just described above, you can modify mysql_install_db before you run it.

To recreate the grant tables completely, remove all the `*.ISM' and `*.ISD' files in the directory containing the mysql database. (This is the directory named `mysql' under the database directory, which is listed when you run mysqld --help.) Then edit the mysql_install_db script to have the privileges you want and run it.

6.8 Adding new user privileges to MySQL

The example below shows how to use the mysql client to set up new users and mysqladmin to cause the server to reload the privilege tables. This example assumes that the current user has the insert privilege for the mysql database and also has the reload administrative privilege. The example also assumes that privileges are set up according to the defaults described in the previous section. This means that to make changes, you must be on the same machine where mysqld is running, and you must connect as the MySQL root user. If you have changed the root user password, you must also specify that for the mysql and mysqladmin commands below.

shell> mysql --user=root mysql
mysql> INSERT INTO user VALUES('%','monty',PASSWORD('something'),
                               'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO user VALUES('%','admin',",
                               'N','N','N','N','N','N','Y','N','Y','N');
mysql> INSERT INTO user (host,user,password)
                        VALUES('localhost','dummy',");
mysql> quit
shell> mysqladmin --user=root reload

These commands set up three new users:

monty: A full superuser who can connect to the server from anywhere, but who must use a password to do so. Note that to set up a superuser, you need only create a user table entry with the privilege fields set to 'Y'. No db or host table entries are necessary.
admin: A user who can connect from anywhere without a password and who is granted the reload and process administrative privileges. This allows the user to execute the mysqladmin reload, mysqladmin refresh and mysqladmin flush-* commands, as well as mysqladmin processlist . No database-related privileges are granted. They could be granted later by adding entries to the db table.
dummy: A user who can connect without a password, but only from the local host. The user is granted no privileges in the user table and thus must be granted individual database privileges through the db table. (The privilege columns in the user table were not set explicitly in the INSERT statement, so those columns are assigned the default value of 'N'.)

The following example adds a user custom who can connect from hosts localhost, server.domain and whitehouse.gov. He wants to access the bankaccount database only from localhost and he wants to access the customer database from all three hosts. He wants to have password stupid on all three hosts.

shell> mysql --user=root mysql
mysql> INSERT INTO user (host,user,password)
       VALUES('localhost','custom',PASSWORD('stupid'));
mysql> INSERT INTO user (host,user,password)
       VALUES('server.domain','custom',PASSWORD('stupid'));
mysql> INSERT INTO user (host,user,password)
       VALUES('whitehouse.gov','custom',PASSWORD('stupid'));
mysql> INSERT INTO db
       (host,db,user,Select_priv,Insert_priv,Update_priv,Delete_priv,
        Create_priv,Drop_priv)
       VALUES
       ('localhost','bankaccount','custom','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO db
       (host,db,user,Select_priv,Insert_priv,Update_priv,Delete_priv,
        Create_priv,Drop_priv)
       VALUES('%','customer','custom','Y','Y','Y','Y','Y','Y');
mysql> quit
shell> mysqladmin --user=root reload

The first three INSERT statements add user table entries that allow user custom to connect from the various hosts with the given password, but grant no permissions to him (all privileges are set to the default value of 'N'). The next two INSERT statements add db table entries that grant privileges to custom for the bankaccount and customer databases. The bankaccount database can be accessed only through connections from localhost. As usual, mysqladmin reload is necessary to tell the server to reload the grant tables so that the changes take effect.

If you want to give a specific user access from any machine in a given domain, you can use a statement like the following:

mysql> INSERT INTO user VALUES ('%.mydomainname.com', 'myusername', 
           PASSWORD('mypassword'),....);

You can also use xmysqladmin, mysql_webadmin and even xmysql to insert, change and update values in the grant tables. You can find these utilities at http://www.tcx.se/Contrib.

6.9 How to set up passwords

The examples in the preceding section illustrate an important principle: when you store a non-empty password, you must use the PASSWORD() function to encrypt it. This is because the user table stores passwords in encrypted form, not as plaintext. If you forget that fact, you are likely to attempt to set passwords like this:

shell> mysql -u root mysql
mysql> INSERT INTO user (host,user,password)
       VALUES('%','jeffrey','bLa81m0');
mysql> quit
shell> mysqladmin -u root reload

The result is that the plaintext value 'bLa81m0' is stored as the password in the user table. When the user jeffrey attempts to connect to the server using his password, the mysql client encrypts the password and sends it to the server. The server compares the encrypted password to the value in the user table (which is the plaintext value 'bLa81m0'). The comparison fails and the server rejects the connection:

shell> mysql -u jeffrey -pbLa81m0 test
Access denied

Passwords must be encrypted when they are inserted in the user table, so the INSERT statement should have been specified like this instead:

mysql> INSERT INTO user (host,user,password)
       VALUES('%','jeffrey',PASSWORD('bLa81m0'));

Note: PASSWORD() performs password encryption, but it does not do so in the same way that Unix passwords are encrypted. You should not assume that if your Unix password and your MySQL password are the same, PASSWORD() will result in the same encrypted value as is stored in the Unix password file.

6.10 Causes of `Access denied` errors

If you encounter Access denied errors when you try to connect to the MySQL server, the list below indicates some courses of action you can take to correct the problem:

Did you run the mysql_install_db script after installing MySQL, to set up the initial grant table contents? If not, do so. See section 6.7 Setting up the initial MySQL privileges. Test these initial privileges by executing this command:
```
shell> mysql -u root test
```
The server should let you connect without error. You should can also make sure you have a file `user.ISD' in the MySQL database directory (ordinarily, this is `PATH/var/mysql/user.ISD', where PATH is the pathname to the MySQL installation root).
After a fresh installation, you should connect to the server and set up your users and access permissions:
```
shell> mysql -u root mysql
```
The server should let you connect because the MySQL root user has no password initially. Since that is also a security risk, setting the root password is something you may want to do while you're setting up your other MySQL users. If you in this case get the error: Access denied for user: '@unknown' to database mysql this means that you don't have an entry in the user table with the User column = root and that mysqld can resolve the hostname for your client. In this case, you must restart the server with the option --skip-grant-tables and edit your `/etc/hosts' or `\windows\hosts' file and add a entry for your host.
If you make changes to the grant tables and they seem to be ignored, remember that you must execute mysqladmin reload to cause the server to reread the tables. Otherwise your changes have no effect until the next time the server is restarted. Remember that after you set the root password, you'll need to specify the password to mysqladmin on subsequent reloads. (You won't need it for the first reload after you change the password, because the server still won't know about it yet!)
For testing, start the mysqld daemon with the --without-grant-tables option. Then you can change the MySQL grant tables and use the mysqlaccess script to check whether or not your modifications have the desired effect. When you are satisfied with your changes, execute mysqladmin reload to tell the mysqld server to start using the new grant tables. Note: reloading the grant tables overrides the --without-grant-tables option.
If you have access problems with a Perl, Python or ODBC program, try to connect to the server with mysql -u user_name db_name or mysql -u user_name -pyour_pass db_name. (Notice that there is no space between -p and the password; you can also use the --password=your_pass syntax to specify the password.) If you are able to connect using the mysql client, then there is a problem with your program and not with the access privileges.
If you can't get your password to work, remember that passwords must be inserted with the PASSWORD() function. See section 6.9 How to set up passwords.
localhost is a synonym for your local hostname, and is also the default host that clients try to connect to if you specify no host explicitly. However, connections to localhost do not work if you are running on a system that uses MIT-pthreads (localhost connections are made using Unix sockets, which are not supported by MIT-pthreads). To avoid this problem on such systems, you should use the --host option to name the server host explicitly. This will make a TCP/IP connection to the mysqld server. In this case, you must have your real hostname in user table entries on the server host. (This is true even if you are running a client program on the same host as the server.)
If you get an Access denied error when trying to connect to the database with mysql -u user_name db_name, then you may have a problem with the user table. Check this by executing mysql -u root mysql and issuing the SQL statement SELECT * FROM user. The result should include an entry with the Host and User columns matching your computer's hostname and your MySQL user name.
The Access denied error message will tell you who you are trying to log in as, the host from which you are trying to connect, and whether or not you were using a password. Normally, you should have one entry in the user table that exactly matches the hostname and user name that were given in the error message.
If you get the error Host ... is not allowed to connect to this MySQL server when you try to connect to a MySQL server on another machine, then there is no row in the user table on the remote machine that matches the host from which you are trying to connect. You can fix this by using the command line tool mysql (on the remote machine!) to add a row to the user table for the host/user combination from which you are trying to connect. If you are not running MySQL 3.22 and you don't know the IP number or hostname of the machine from which you are connecting, you should put an entry with '%' as the Host column value in the user table and restart mysqld with the --log option on the server machine. After trying to connect from the client machine, the information in the MySQL log will indicate how you really did connect. Then replace the '%' in the user table entry with the actual hostname that shows up in the log. (Otherwise, you'll have a system that is insecure.)
If mysql -u root test works but mysql -h your_hostname -u root test gives Access denied, then you may not have the correct name for your host in the user table. A common problem here is that the entry in the host table specifies an unqualified hostname, but your system's name resolution routines return a fully-qualified domain name. For example, if you have an entry with host 'tcx' in the user table, but your DNS tells MySQL that your hostname is 'tcx.subnet.se', the entry will not work. Try adding a record with the IP number of your host to the user table. (Alternatively, you could add an entry to the user table with a Host value that contains a wildcard--for example, 'tcx.%'. However, use of hostnames ending with `%' is insecure and is not recommended!)
If mysql -u user_name db_name works when executed on the server machine, but mysql -u host_name -u user_name db_name doesn't work when executed on another client machine, you don't have the client machine listed in the user table or the db table.
If you can't figure out why you get Access denied, remove from the user table all entries that have Host values containing wildcards (entries that contain `%' or `_'). A very common error is to insert a new entry with Host='%' and User='some user', thinking that this will allow you to specify localhost to connect from the same machine. The reason that this doesn't work is that the default privileges include a more specific entry with Host='localhost' and User=" that is used in preference to the new entry when connecting from localhost! The correct procedure is to insert a second entry with Host='localhost' and User='some_user', or to remove the entry with User=".
If you get the error Access to database denied, you may have a problem with the db table. If the entry selected from the db table has an empty value in the Host column, make sure there is at least one corresponding entry in the host table specifying which hosts the db table applies to.
If mysql -u user_name test works but mysql -u user_name other_db_name doesn't work, you don't have an entry for other_db_name listed in the db table.
If you get Access to database denied when using the SELECT ... INTO OUTFILE or LOAD DATA INFILE SQL commands, then your entry in the user table probably doesn't have the file privilege enabled.
If everything else fails, start the mysqld daemon with a debugging option--for example --debug=d,general,query. This will print host and user information about attempted connections, as well as information about each command issued. See section 19.10 Debugging MySQL.
If you have any other problems with the MySQL grant tables and feel you must post the problem to the mailing list, always provide a dump of the MySQL grant tables. You can dump the tables with the mysqldump mysql command. As always, post your problem using the mysqlbug script.
If you get the error Can't connect to local MySQL server or Can't connect to MySQL server on some_hostname, either the daemon mysqld is not running or you are trying to connect to the wrong socket or port. Check that the socket (normally `/tmp/mysql.sock') exists or try to connect to the port with telnet: telnet hostname 3306. You can also try mysqladmin version and mysqladmin -h host_name version to get some more information. Also, check the error logs in the database directory to see if they can provide some hints.
Remember that client programs will use connection parameters specified in configuration files or environment variables. If a client seems to be sending the wrong default connection parameters when you don't specify them on the command line, check your environment and the `.my.cnf' file in your home directory. You might also check the system-wide MySQL configuration files, though it is far less likely that connection parameters will be specified there. If you get Access denied when you run a client without any options, make sure you haven't specified an old password in any of your option files! See section 4.14.4 Option files.

6.11 How to make MySQL secure against crackers

To make a MySQL system secure, you should strongly consider the following suggestions:

Use passwords for all MySQL users. Remember that anyone can log in as any other person as simply as mysql -u other_user db_name. This is common behavior with client/server applications. You can change the password of all users by editing the mysql_install_db script before you run it, or only the password for the MySQL root user like this:
```
shell> mysql -u root mysql
mysql> UPDATE user SET Password=PASSWORD('new_password')
           WHERE user='root';
```
Don't start the MySQL daemon as the Unix root user. mysqld can be run as any user. You can also create a new Unix user mysql to make everything even more secure. If you run mysqld as another Unix user, you don't need to change the root user name in the user table, because MySQL user names have nothing to do with Unix user names. You can edit the mysql.server script to start mysqld as another Unix user. Normally this is done with the su command.
Check that the user that mysqld runs as is the only Unix user with read/write privileges in the database directories.
Don't give the process privilege to all users. The output of mysqladmin processlist shows the text of the currently executing queries, so any user who is allowed to execute that command might be able to see if another user issues an UPDATE user SET password=PASSWORD('not_secure') query. mysqld saves an extra connection for users who have the process privilege, so that a MySQL root user can log in and check things even if all normal connections are in use.
Don't give the file privilege to all users. Any user that has this privilege can write a file anywhere in the file system with the privileges of the mysqld daemon! To make this a bit safer, all files generated with SELECT ... INTO OUTFILE are readable to everyone, and you can't overwrite existing files.
If you don't trust your DNS, you should use IP numbers instead of hostnames in the grant tables. In principle, the --secure option to mysqld should make hostnames safe. In any case, you should be very careful about using wildcards with hostnames!
If you put a password for the Unix root user in the mysql.server script, make sure this script is readable only by root.

The following mysqld options affect security:

--secure: IP numbers returned by the gethostbyname() system call are checked to make sure they resolve back to the original hostname. This makes it harder for someone on the outside to get access by simulating another host. This option also adds some sanity checks of hostnames. The option is turned off by default in MySQL 3.21 since it sometimes takes a long time to perform backward resolutions. MySQL 3.22 caches hostnames and has this option enabled by default.
--skip-grant-tables: This option causes the server not to use the privilege system at all. This gives everyone full access to all databases! (You can tell a running server to start using the grant tables again by executing mysqladmin reload.)
--skip-name-resolve: Hostnames are not resolved. All Host column values in the grant tables must be IP numbers or localhost.
--skip-networking: Don't allow TCP/IP connections over the network. All connections to mysqld must be made via Unix sockets. This option doesn't work very well on systems that use MIT-pthreads, because the MIT-pthreads package doesn't support Unix sockets.

7 MySQL language reference

7.1 Literals: how to write strings and numbers

7.1.1 Strings

A string is a sequence of characters, surrounded by either single quote (`'') or double quote (`"') characters. Examples:

'a string'
"another string"

Within a string, certain sequences have special meaning. Each of these sequences begins with a backslash (`\'), known as the escape character. MySQL recognizes the following escape sequences:

\0: An ASCII 0 (NUL) character.
\n: A newline character.
\t: A tab character.
\r: A carriage return character.
\b: A backspace character.
\': A single quote (`'') character.
\": A double quote (`"') character.
\\: A backslash (`\') character.
\%: A `%' character. This is used to search for literal instances of `%' in contexts where `%' would otherwise be interpreted as a wildcard character.
\_: A `_' character. This is used to search for literal instances of `_' in contexts where `_' would otherwise be interpreted as a wildcard character.

There are several ways to include quotes within a string:

A `'' inside a string quoted with `'' may be written as `"'.
A `"' inside a string quoted with `"' may be written as `""'.
You can precede the quote character with an escape character (`\').
A `'' inside a string quoted with `"' needs no special treatment and need not be doubled or escaped. In the same way, `"' inside a string quoted with `'' needs no special treatment.

The SELECT statements shown below demonstrate how quoting and escaping work:

mysql> SELECT 'hello', '"hello"', '""hello""', 'hel"lo', '\'hello';
+-------+---------+-----------+--------+--------+
| hello | "hello" | ""hello"" | hel'lo | 'hello |
+-------+---------+-----------+--------+--------+
| hello | "hello" | ""hello"" | hel'lo | 'hello |
+-------+---------+-----------+--------+--------+

mysql> SELECT "hello", "'hello'", ""hello"", "hel""lo", "\"hello";
+-------+---------+-----------+--------+--------+
| hello | 'hello' | "hello" | hel"lo | "hello |
+-------+---------+-----------+--------+--------+
| hello | 'hello' | "hello" | hel"lo | "hello |
+-------+---------+-----------+--------+--------+

mysql> SELECT "This\nIs\nFour\nlines";
+--------------------+
| This
Is
Four
lines |
+--------------------+
| This
Is
Four
lines |
+--------------------+

If you want to insert binary data into a BLOB column, the following characters must be represented by escape sequences:

NUL: ASCII 0. Should be represented by `\0' (a backslash and an ASCII `0' character).
\: ASCII 92, backslash
': ASCII 39, single quote
": ASCII 34, double quote

If you write C code, you can use the C API function mysql_escape_string() to escape characters for the INSERT clause. See section 18.3 C API function overview. In Perl, you can use the quote method of the DBI package to convert special characters to the proper escape sequences. See section 18.5.1.1 The DBI interface.

You should use an escape function on every possible string that may contain any of the special characters listed above!

7.1.2 Numbers

Integers are just a sequence of digits. Floats use `.' as a decimal separator.

Examples of valid numbers:

1221
294.42
-32032.6809e+10

7.1.3 `NULL` values

When using the text file export formats (SELECT ... INTO OUTFILE), NULL may be represented by \N. See section 7.15 LOAD DATA INFILE syntax.

Note that NULL means `no data' and is different from values such as 0 for numeric types and the empty string for string types. See section 16.12 Problems with NULL values.

7.1.4 Database, table, index, column and alias names

Database, table, index, column and alias names all follow the same rules in MySQL:

A name consists of alphanumeric characters from the current character set and also `_' and `$'. The default character set is ISO-8859-1 Latin1; this may be changed by recompiling MySQL.
A database, table, index or column name can have up to 64 characters. An alias name can have up to 256 characters.
A name may start with any character that is legal in a name. In particular, a name may start with a number (which is different than many other systems!). However, a name cannot consist only of numbers.
It is recommended that you do not use names like 1e, because an expression like 1e+1 is ambiguous. It may be interpreted as the expression 1e + 1 or as the number 1e+1.
You cannot use the `.' character in names because it is used to extend the format by which you can refer to columns (see below).

In MySQL you can refer to a column using any of the following forms:

Column reference	Meaning
`col_name`	Column `col_name` from whichever table that is used in the query contains a column named `col_name`
`tbl_name.col_name`	Column `col_name` from table `tbl_name` of the current database
`db_name.tbl_name.col_name`	Column `col_name` from table `tbl_name` of the database `db_name`. This form is not available in versions of MySQL prior to 3.22.

You need not specify a tbl_name or db_name.tbl_name prefix for a column reference in a statement unless the reference would be ambiguous. For example, suppose tables t1 and t2 each contain a column c, and you retrieve c in a SELECT statement that uses both t1 and t2. In this case, c is ambiguous because it is not unique among the tables used in the statement, so you must indicate which table you mean by writing t1.c or t2.c. Similarly, if you are retrieving from a table t in database db1 and from a table t in database db2, you must refer to columns in those tables as db1.t.col_name and db2.t.col_name.

7.1.4.1 Case sensitivity in names

Database and table names are case sensitive in Unix and case insensitive in Win32, because directory and file names are case sensitive in Unix but not in Win32. (In MySQL, databases and tables correspond to directories and files within those directories, so the case sensitivity of the underlying operating system determines how MySQL behaves.)

Note: although database and file names are case insensitive for Win32, you should not refer to a given database or table using different cases within the same query.

Column names are case insensitive in all cases.

Aliases on tables are case sensitive and aliases on columns are case insensitive.

7.2 Column types

MySQL supports a number of column types, which may be grouped into three categories: numeric types, date and time types, and string (or character) types. This section first gives an overview of the types available, then summarizes the storage requirements for each column type and provides a more detailed description of the properties of the types in each category. The overview is intentionally brief. The more detailed descriptions should be consulted for additional information about particular column types, such as the allowable formats in which you can specify values.

The column types supported by MySQL are listed below. The following code letters are used in the descriptions:

M indicates the maximum display size.
D applies to floating-point types and indicates the number of digits following the decimal point.

Square brackets (`[' and `]') indicate parts of type specifiers that are optional.

TINYINT[(M)] [UNSIGNED] [ZEROFILL]: A very small integer. The signed range is -128 to 127. The unsigned range is 0 to 255.
SMALLINT[(M)] [UNSIGNED] [ZEROFILL]: A small integer. The signed range is -32768 to 32767. The unsigned range is 0 to 65535.
MEDIUMINT[(M)] [UNSIGNED] [ZEROFILL]: A medium-size integer. The signed range is -8388608 to 8388607. The unsigned range is 0 to 16777215.
INT[(M)] [UNSIGNED] [ZEROFILL]: A normal-size integer. The signed range is -2147483648 to 2147483647. The unsigned range is 0 to 4294967295.
INTEGER[(M)] [UNSIGNED] [ZEROFILL]: This is a synonym for INT.
BIGINT[(M)] [UNSIGNED] [ZEROFILL]: A large integer. The signed range is -9223372036854775808 to 9223372036854775807. The unsigned range is 0 to 18446744073709551615. Note that all arithmetic is done using signed BIGINT or DOUBLE values, so you shouldn't use unsigned big integers larger than 9223372036854775807 (63 bits) except with bit functions! Note that -, + and * will use BIGINT arithmetic when both arguments are INTEGERs! This means that if you multiply two big integers (or results from functions that returns integers) you may get unexpected results if the result is bigger than 9223372036854775807.
FLOAT(precision) [ZEROFILL]: A floating-point number. Cannot be unsigned. precision can be 4 or 8. FLOAT(4) is a single-precision number and FLOAT(8) is a double-precision number. These types are like the FLOAT and DOUBLE types described immediately below. FLOAT(4) and FLOAT(8) have the same ranges as the corresponding FLOAT and DOUBLE types, but their display size and number of decimals is undefined. This syntax is provided for ODBC compatibility.
FLOAT[(M,D)] [ZEROFILL]: A small (single-precision) floating-point number. Cannot be unsigned. Allowable values are -3.402823466E+38 to -1.175494351E-38, 0 and -1.175494351E-38 to 3.402823466E+38.
DOUBLE[(M,D)] [ZEROFILL]: A normal-size (double-precision) floating-point number. Cannot be unsigned. Allowable values are -1.7976931348623157E+308 to -2.2250738585072014E-308, 0 and 2.2250738585072014E-308 to 1.7976931348623157E+308.
DOUBLE PRECISION[(M,D)] [ZEROFILL]
REAL[(M,D)] [ZEROFILL]: These are synonyms for DOUBLE.
DECIMAL(M,D) [ZEROFILL]: An unpacked floating-point number. Cannot be unsigned. Behaves like a CHAR column (`unpacked' means the number is stored as a string, using one character for each digit of the value, the decimal point, and, for negative numbers, the `-' sign). If D is 0, values will have no decimal point or fractional part. The maximum range of DECIMAL values is the same as for DOUBLE, but the actual range for a given DECIMAL column may be constrained by the choice of M and D.
NUMERIC(M,D) [ZEROFILL]: This is a synonym for DECIMAL.
DATE: A date. The supported range is '1000-01-01' to '9999-12-31'. MySQL displays DATE values in 'YYYY-MM-DD' format, but allows you to assign values to DATE columns using either strings or numbers.
DATETIME: A date and time combination. The supported range is '1000-01-01 00:00:00' to '9999-12-31 23:59:59'. MySQL displays DATETIME values in 'YYYY-MM-DD HH:MM:SS' format, but allows you to assign values to DATETIME columns using either strings or numbers.
TIMESTAMP[(M)]: A timestamp. The range is '1970-01-01 00:00:00' to sometime in the year 2106. MySQL displays TIMESTAMP values in YYYYMMDDHHMMSS, YYMMDDHHMMSS, YYYYMMDD or YYMMDD format, depending on whether M is 14 (or missing), 12, 8 or 6, but allows you to assign values to TIMESTAMP columns using either strings or numbers. A TIMESTAMP column is useful for recording the time of an INSERT or UPDATE operation because it can be set automatically to the time of the operation by setting it to NULL. See section 7.2.6 Date and time types.
TIME: A time. The range is '-838:59:59' to '838:59:59'. MySQL displays TIME values in 'HH:MM:SS' format, but allows you to assign values to TIME columns using either strings or numbers.
YEAR: A year. The allowable values are 1901 to 2155, and 0000. MySQL displays YEAR values in YYYY format, but allows you to assign values to YEAR columns using either strings or numbers. (YEAR is a new type for MySQL 3.22.)
CHAR(M) [BINARY]: A fixed-length string that is always right-padded with spaces to the specified length when stored. The range of M is 1 to 255 characters. Trailing spaces are removed when the value is retrieved. CHAR values are sorted and compared in case-insensitive fashion unless the BINARY keyword is given.
VARCHAR(M) [BINARY]: A variable-length string. NOTE: Trailing spaces are removed when the value is stored (this differs from the ANSI SQL specification). The range of M is 1 to 255 characters. VARCHAR values are sorted and compared in case-insensitive fashion unless the BINARY keyword is given.
TINYBLOB
TINYTEXT: A BLOB or TEXT column with a maximum length of 255 (2^8 - 1) characters.
BLOB
TEXT: A BLOB or TEXT column with a maximum length of 65535 (2^16 - 1) characters.
MEDIUMBLOB
MEDIUMTEXT: A BLOB or TEXT column with a maximum length of 16777215 (2^24 - 1) characters.
LONGBLOB
LONGTEXT: A BLOB or TEXT column with a maximum length of 4294967295 (2^32 - 1) characters.
ENUM('value1','value2',...): An enumeration. A string object that can have only one value, chosen from the list of values 'value1', 'value2',... (or NULL). An ENUM can have a maxiumum of 65535 distinct values.
SET('value1','value2',...): A set. A string object that can have zero or more values, each of which must be chosen from the list of values 'value1', 'value2',... A SET can have a maximum of 64 members.

7.2.1 Column type storage requirements

The storage requirements for each of the column types supported by MySQL are listed below by category.

7.2.2 Numeric types

Column type	Storage required
`TINYINT`	1 byte
`SMALLINT`	2 bytes
`MEDIUMINT`	3 bytes
`INT`	4 bytes
`INTEGER`	4 bytes
`BIGINT`	8 bytes
`FLOAT(4)`	4 bytes
`FLOAT(8)`	8 bytes
`FLOAT`	4 bytes
`DOUBLE`	8 bytes
`DOUBLE PRECISION`	8 bytes
`REAL`	8 bytes
`DECIMAL(M,D)`	`M` bytes (`D`+2, if `M < D`)
`NUMERIC(M,D)`	`M` bytes (`D`+2, if `M < D`)

7.2.3 Date and time types

Column type	Storage required
`DATETIME`	8 bytes
`DATE`	3 bytes
`TIMESTAMP`	4 bytes
`TIME`	3 bytes
`YEAR`	1 byte

7.2.4 String types

Column type	Storage required
`CHAR(M)`	`M` bytes, `1 <= M <= 255`
`VARCHAR(M)`	`L`+1 bytes, where `L <= M` and `1 <= M <= 255`
`TINYBLOB`, `TINYTEXT`	`L`+1 bytes, where `L` < 2^8
`BLOB`, `TEXT`	`L`+2 bytes, where `L` < 2^16
`MEDIUMBLOB`, `MEDIUMTEXT`	`L`+3 bytes, where `L` < 2^24
`LONGBLOB`, `LONGTEXT`	`L`+4 bytes, where `L` < 2^32
`ENUM('value1','value2',...)`	1 or 2 bytes, depending on the number of enumeration values (65535 values maximum)
`SET('value1','value2',...)`	1, 2, 3, 4 or 8 bytes, depending on the number of set members (64 members maximum)

VARCHAR and the BLOB and TEXT types are variable-length types, for which the storage requirements depend on the actual length of column values (represented by L in the preceding table), rather than on the type's maximum possible size. For example, a VARCHAR(10) column can hold a string with a maximum length of 10 characters. The actual storage required is the length of the string (L), plus 1 byte to record the length of the string. For the string 'abcd', L is 4 and the storage requirement is 5 bytes.

The BLOB and TEXT types require 1, 2, 3 or 4 bytes to record the length of the column value, depending on the maxiumum possible length of the type.

If a table includes any variable-length column types, the record format will also be variable-length. Note that when a table is created, MySQL may under certain conditions change a column from a variable-length type to a fixed-length type, and vice-versa. See section 7.6 CREATE TABLE syntax.

The size of an ENUM object is determined by the number of different enumeration values. 1 byte is used for enumerations with up to 255 possible values. 2 bytes are used for enumerations with up to 65535 values.

The size of a SET object is determined by the number of different set members. If the set size is N, the object occupies (N+7)/8 bytes, rounded up to 1, 2, 3, 4 or 8 bytes. A SET can have a maximum of 64 members.

7.2.5 Numeric types

All integer types can have an optional attribute UNSIGNED. Unsigned values can be used when you want to allow only positive numbers in a column and you need a little bigger numeric range for the column.

All numeric types can have an optional attribute ZEROFILL. Values for ZEROFILL columns are left-padded with zeroes up to the maximum display length when they are displayed. For example, for a column declared as INT(5) ZEROFILL, a value of 4 is retrieved as 00004.

When asked to store a value in a numeric column that is outside the column type's allowable range, MySQL clips the value to the appropriate endpoint of the range and stores the resulting value instead.

For example, the range of an INT column is -2147483648 to 2147483647. If you try to insert -9999999999 into an INT column, the value is clipped to the lower endpoint of the range, and -2147483648 is stored instead. Similarly, if you try to insert 9999999999, 2147483647 is stored instead.

If the INT column is UNSIGNED, the size of the column's range is the same but its endpoints shift up to 0 and 4294967295. If you try to store -9999999999 and 9999999999, the values stored in the column become 0 and 4294967296.

Conversions that occur due to clipping are reported as `warnings' for ALTER TABLE, LOAD DATA INFILE, UPDATE and multi-row INSERT statements.

The maximum display size (M) and number of decimals (D) are used for formatting and calculation of maximum column width.

MySQL will store any value that fits a column's storage type even if the value exceeds the display size. For example, an INT(4) column has a display size of 4. Suppose you insert a value which has more than 4 digits into the column, such as 12345. The display size is exceeded, but the allowable range of the INT type is not, so MySQL stores the actual value, 12345. When retrieving the value from the column, MySQL returns the actual value stored in the column.

The DECIMAL type is considered a numeric type (as is its synonym, NUMERIC), but such values are stored as strings. One character is used for each digit of the value, the decimal point (if D > 0) and the `-' sign (for negative numbers). If D is 0, DECIMAL and NUMERIC values contain no decimal point or fractional part.

The maximum range of DECIMAL values is the same as for DOUBLE, but the actual range for a given DECIMAL column may be constrained by the choice of M and D. For example, a type specification such as DECIMAL(4,2) indicates a maximum length of four characters with two digits after the decimal point. Due to the way the DECIMAL type is stored, this specification results in an allowable range of -.99 to 9.99, much less than the range of a DOUBLE.

To avoid some rounding problems, MySQL always rounds everything that it stores in any floating-point column according to the number of decimals. Suppose you have a column type of FLOAT(8,2). The number of decimals is 2, so a value such as 2.333 is rounded to two decimals and stored as 2.33.

7.2.6 Date and time types

The date and time types are DATETIME, DATE, TIMESTAMP, TIME and YEAR. Each of these has a range of legal values, as well as a `zero' value that is used when you specify an illegal value.

Here are some general considerations to keep in mind when working with date and time types:

MySQL retrieves values for a given date or time type in a standard format, but it attempts to interpret a variety of formats for values that you supply (e.g., when you specify a value to be assigned to or compared to a date or time type). Nevertheless, only the formats described in the following sections are supported. It is expected that you will supply legal values, and unpredictable results may occur if you use values in other formats.
Although MySQL tries to interpret values in several formats, it always expects the year part of date values to be leftmost. Dates must be given in year-month-day order (e.g., '98-09-04'), rather than in the month-day-year or day-month-year orders commonly used elsewhere (e.g., '09-04-98', '04-09-98').
MySQL automatically converts a date or time type value to a number if the value is used in a numeric context, and vice versa.
When MySQL encounters a value for a date or time type that is out of range or otherwise illegal for the type, it converts the value to the `zero' value for that type. (The exception is that out-of-range TIME values are clipped to the appropriate endpoint of the TIME range.) The table below shows the format of the `zero' value for each type:

Column type `Zero' value

DATETIME '0000-00-00 00:00:00'

DATE '0000-00-00'

TIMESTAMP 00000000000000 (length depends on display size)

TIME '00:00:00'

YEAR 0000
The `zero' values are special, but you can store or refer to them explicitly using the values shown in the table. You can also do this using the values '0' or 0, which are easier to write.
`Zero' date or time values used through MyODBC are converted automatically to NULL in MyODBC 2.50.12 and above, because ODBC can't handle such values.

7.2.6.1 The `DATETIME`, `DATE` and `TIMESTAMP` types

The DATETIME, DATE and TIMESTAMP types are related. This section describes how they are similar and how they differ.

The DATETIME type is used when you need values that contain both date and time information. MySQL retrieves and displays DATETIME values in 'YYYY-MM-DD HH:MM:SS' format. The supported range is '1000-01-01 00:00:00' to '9999-12-31 23:59:59'. ("Supported" means that although earlier values might work, they are not guaranteed to.)

The DATE type is used when you need only a date value, without a time part. MySQL retrieves and displays DATE values in 'YYYY-MM-DD' format. The supported range is '1000-01-01' to '9999-12-31'.

The TIMESTAMP column type provides a type that you can use to automatically mark INSERT or UPDATE operations with the current time. (`Current time' means `current date and time' in TIMESTAMP contexts.) A TIMESTAMP column is updated automatically under either of the following conditions:

The column is not specified explicitly in an INSERT or LOAD DATA INFILE statement.
The column is not specified explicitly in an UPDATE statement and some other column changes value. (Note that an UPDATE that sets a column to the value it already has will not cause the TIMESTAMP column to be updated, because if you set a column to its current value, MySQL ignores the update for efficiency.)
You explicitely set the TIMESTAMP column to NULL.

If you have multiple TIMESTAMP columns, only the first one is updated automatically. However, you can set any TIMESTAMP column to the current time by setting it to NULL (or by setting it to NOW(), obviously).

TIMESTAMP values may range from the beginning of 1970 to sometime in the year 2106, with a resolution of one second. Values are displayed as numbers.

The format in which MySQL retrieves and displays TIMESTAMP values depends on the display size, as illustrated by the table below. The `full' TIMESTAMP format is 14 digits, but TIMESTAMP columns may be created with shorter display sizes:

Column type	Display format
`TIMESTAMP(14)`	`YYYYMMDDHHMMSS`
`TIMESTAMP(12)`	`YYMMDDHHMMSS`
`TIMESTAMP(10)`	`YYMMDDHHMM`
`TIMESTAMP(8)`	`YYYYMMDD`
`TIMESTAMP(6)`	`YYMMDD`
`TIMESTAMP(4)`	`YYMM`
`TIMESTAMP(2)`	`YY`

All TIMESTAMP columns have the same storage size, regardless of display size. The most common display sizes are 6, 8, 12, and 14. (You can specify an arbitrary display size at table creation time, but values of 0 or greater than 14 are coerced to 14. Odd-valued sizes in the range from 1 to 13 are coerced to the next higher even number.)

You can specify DATETIME, DATE and TIMESTAMP values using any of a common set of formats:

As a string in either 'YYYY-MM-DD HH:MM:SS' or 'YY-MM-DD HH:MM:SS' format. A `relaxed' syntax is allowed--any non-numeric character may be used as the delimiter between date parts or time parts. For example, '98-12-31 11:30:45', '98.12.31 11+30+45', '98/12/31 11*30*45' and '98@12@31 11^30^45' are equivalent.
As a string in either 'YYYY-MM-DD' or 'YY-MM-DD' format. A `relaxed' syntax is allowed here, too. For example, '98-12-31', '98.12.31', '98/12/31' and '98@12@31' are equivalent.
As a string with no delimiters in either 'YYYYMMDDHHMMSS' or 'YYMMDDHHMMSS' format, provided that the string makes sense as a date. For example, '19970523091528' and '970523091528' are interpreted as '1997-05-23 09:15:28', but '971122459015' is illegal (it has a nonsensical minute part) and becomes '0000-00-00 00:00:00'.
As a string with no delimiters in either 'YYYYMMDD' or 'YYMMDD' format, provided that the string makes sense as a date. For example, '19970523' and '970523' are interpreted as '1997-05-23', but '971332' is illegal (it has nonsensical month and day parts) and becomes '0000-00-00'.
As a number in either YYYYMMDDHHMMSS or YYMMDDHHMMSS format, provided that the number makes sense as a date. For example, 19830905132800 and 830905132800 are interpreted as '1983-09-05 13:28:00'.
As a number in either YYYYMMDD or YYMMDD format, provided that the number makes sense as a date. For example, 19830905 and 830905 are interpreted as '1983-09-05'.
As the result of a function that returns a value that is acceptable in a DATETIME, DATE or TIMESTAMP context, such as NOW() or CURRENT_DATE.

For values specified as strings that include date part delimiters, it is not necessary to specify two digits for month or day values that are less than 10. '1979-6-9' is the same as '1979-06-09'. Similarly, for values specified as strings that include time part delimiters, it is not necessary to specify two digits for hour, month or second values that are less than 10. '1979-10-30 1:2:3' is the same as '1979-10-30 01:02:03'.

Values specified as numbers should be 6, 8, 12 or 14 digits long. If the number is 8 or 14 digits long, it is assumed to be in YYYYMMDD or YYYYMMDDHHMMSS format and that the year is given by the first 4 digits. If the number is 6 or 12 digits long, it is assumed to be in YYMMDD or YYMMDDHHMMSS format and that the year is given by the first 2 digits. Numbers that are not one of these lengths are interpreted as though padded with leading zeros to the closest length.

Values specified as non-delimited strings are interpreted using their length as given. If the string is 8 or 14 characters long, the year is assumed to be given by the first 4 characters. Otherwise the year is assumed to be given by the first 2 characters. The string is interpreted from left to right to find year, month, day, hour, minute and second values, for as many parts as are present in the string.

Year values specified as two digits are ambiguous, since the century is unknown. MySQL interprets 2-digit year values using the following rules:

Year values in the range 00-69 are converted to 2000-2069.
Year values in the range 70-99 are converted to 1970-1999.

You can to some extent assign values of one date type to an object of a different date type. However, there may be some alteration of the value or loss of information:

If you assign a DATE value to a DATETIME or TIMESTAMP object, the time part of the resulting value is set to '00:00:00', because the DATE value contains no time information.
If you assign a DATETIME or TIMESTAMP value to a DATE object, the time part of the resulting value is deleted, because the DATE type stores no time information.

TIMESTAMP values are stored to full precision regardless of the display size. However, the only function that operates directly on the underlying stored value is UNIX_TIMESTAMP(). Other functions operate on the formatted retrieved value. This means you cannot use functions such as HOUR() or SECOND() unless the relevant part of the TIMESTAMP value is included in the formatted value. For example, the HH part of a TIMESTAMP column is not displayed unless the display size is at least 10, so trying to use HOUR() on shorter TIMESTAMP values produces a meaningless result.

Illegal DATETIME, DATE or TIMESTAMP values are converted the `zero' value of the appropriate type ('0000-00-00 00:00:00', '0000-00-00' or 00000000000000).

Be aware of certain pitfalls when specifying date values:

The relaxed format allowed for values specified as strings can be deceiving. For example, a value such as '10:11:12' might look like a time value because of the `:' delimiter, but if used in a date context will be interpreted as the year '2010-11-12'. The value '10:45:15' will be converted to '0000-00-00' because '45' is not a legal month.
Remember that although DATETIME, DATE and TIMESTAMP values all can be specified using the same set of formats, the types do not all have the same range of values. For example, TIMESTAMP values cannot be earlier than 1970 or later than 2036. For example, '1968-01-01', while legal as a DATETIME or DATE value, is not a valid TIMESTAMP value and will be converted to 0 if assigned to such an object.

7.2.6.2 The `TIME` type

MySQL retrieves and displays TIME values in 'HH:MM:SS' format (or 'HHH:MM:SS' format for large hours values). TIME values may range from '-838:59:59' to '838:59:59'. The reason the hours part may be so large is that the TIME type may be used not only to represent a time of day (which must be less than 24 hours), but also elapsed time or a time interval between two events (which may be much greater than 24 hours, or even negative).

You can specify TIME values in a variety of formats:

As a string in 'HH:MM:SS' format. A `relaxed' syntax is allowed--any non-numeric character may be used as the delimiter between time parts. For example, '10:11:12' and '10.11.12' are equivalent.
As a string with no delimiters in 'HHMMSS' format, provided that it makes sense as a time. Example: '101112' is understood as '10:11:12', but '109712' is illegal (it has a nonsensical minute part) and becomes '00:00:00'.
As a number in HHMMSS format, provided that it makes sense as a time. Example: 101112 is understood as '10:11:12'.
As the result of a function that returns a value that is acceptable in a TIME context, such as CURRENT_TIME.

For TIME values specified as strings that include a time part delimiter, it is not necessary to specify two digits for hours, minutes or seconds values that are less than 10. '8:3:2' is the same as '08:03:02'.

If you assign a `short' TIME value to a TIME column, MySQL interprets the value as specifying seconds, or minutes and seconds. For example, '12' and 12 are interpreted as '00:00:12', whereas '11:12', '1112' and 1112 are interpreted as '00:11:12'.

Values that lie outside the TIME range but are otherwise legal are clipped to the appropriate endpoint of the range. For example, '-850:00:00' and '850:00:00' are converted to '-838:59:59' and '838:59:59'.

Illegal TIME values are converted to '00:00:00'. Note that since '00:00:00' is itself a legal TIME value, there is no way to distinguish a value of '00:00:00' that was specified explicitly from one that resulted from an illegal value.

7.2.6.3 The `YEAR` type

The YEAR type is a 1-byte type used for representing years.

MySQL retrieves and displays YEAR values in YYYY format. The range is 1901 to 2155.

You can specify YEAR values in a variety of formats:

As a four-digit string in the range '1901' to '2155'.
As a four-digit number in the range 1901 to 2155.
As a two-digit string in the range '00' to '99'. Values in the ranges '00' to '69' and '70' to '99' are converted to YEAR values in the ranges 2000 to 2069 and 1970 to 1999, and are sorted as such.
As a two-digit number in the range 1 to 99. Values in the ranges 1 to 69 and 70 to 99 are converted to YEAR values in the ranges 2001 to 2069 and 1970 to 1999, and are sorted as such. Note that the range for two-digit numbers is slightly different than the range for two-digit strings, since you cannot specify zero directly as a number and have it be interpreted as 2000. You must specify it as a string '0' or '00' or it will be interpreted as 0000.
As the result of a function that returns a value that is acceptable in a YEAR context, such as NOW().

Illegal YEAR values are converted to 0000.

7.2.7 String types

The string types are CHAR, VARCHAR, BLOB, TEXT, ENUM and SET.

7.2.7.1 The `CHAR` and `VARCHAR` types

The CHAR and VARCHAR types are similar, but differ in the way they are stored and retrieved.

The length of a CHAR column is fixed to the length that you declare it when you create the table. You can declare it to be any length between 1 and 255; when values are stored, they are right-padded with spaces to the specified length. When CHAR values are retrieved, trailing spaces are removed.

Values in VARCHAR columns are variable-length strings. You can declare a VARCHAR to be any length between 1 and 255 as well. This length is the maximum length, but in contrast to CHAR, values are stored using only as many characters as are needed. Values are not padded; instead, trailing spaces are removed when values are stored. (This space removal differs from the ANSI SQL specification.)

If you assign a value to a CHAR or VARCHAR column that exceeds the column's maximum length, the value is truncated to fit.

To illustrate the differences between the two types of columns, the table below shows the result of storing various string values into CHAR(4) and VARCHAR(4) columns:

Value	`CHAR(4)`	`VARCHAR(4)`
`"`	`' '`	`"`
`'ab'`	`'ab '`	`'ab'`
`'abcd'`	`'abcd'`	`'abcd'`
`'abcdef'`	`'abcd'`	`'abcd'`

The values retrieved from the CHAR(4) and VARCHAR(4) columns will be the same in each case, because trailing spaces are removed from CHAR columns upon retrieval.

Values in CHAR and VARCHAR columns are sorted and compared in case-insensitive fashion, unless the BINARY attribute was specified when the table was created. The BINARY attribute means that column values are sorted and compared in case-sensitive fashion according to the ASCII order of the machine where the MySQL server is running.

The BINARY attribute is 'sticky'. This means that if a column marked BINARY is used in an expression, the whole expression is compared as a BINARY value.

MySQL may silently change the type of a CHAR or VARCHAR column at table creation time. See section 7.6 CREATE TABLE syntax.

7.2.7.2 The `BLOB` and `TEXT` types

A BLOB is a binary large object that can hold a variable amount of data. The four BLOB types TINYBLOB, BLOB, MEDIUMBLOB and LONGBLOB differ only in the maximum length of the values they can hold.

The four TEXT types TINYTEXT, TEXT, MEDIUMTEXT and LONGTEXT correspond to the four BLOB types and have the same maximum lengths and storage requirements. The only difference between BLOB and TEXT types is that sorting and comparison is performed in case-sensitive fashion for BLOB values and case-insensitive fashion for TEXT values. In other words, a TEXT is a case-insensitive BLOB.

BLOB and TEXT columns cannot be indexed, unlike all other MySQL column types.

If you assign a value to a BLOB or TEXT column that exceeds the column type's maximum length, the value is truncated to fit.

There is no trailing space truncation for BLOB and TEXT columns as there is for VARCHAR columns.

In most respects, you can regard a TEXT column as a VARCHAR column that can be as big as you like. Similarly, you can regard a BLOB column as a VARCHAR BINARY column. The differences are that you cannot index BLOB or TEXT columns, and there is no trailing-space removal for BLOB and TEXT columns when values are stored. BLOB and TEXT can not have DEFAULT values and will also always be NULL columns.

MyODBC defines BLOB values as LONGVARBINARY and TEXT values as LONGVARCHAR.

Because BLOB and TEXT values may be extremely long, you may run up against some contraints when using them:

When you sort or group BLOB or TEXT values, only the first max_sort_length bytes of the column are used. The default value of max_sort_length is 1024; this value can be changed by the -O option when starting the mysqld daemon. You can group on an expression involving BLOB or TEXT values:
```
mysql> SELECT id,substr(blob_col,1,100) GROUP BY 2;
```
The maximum size of a BLOB or TEXT object is determined by its type, but the largest value you can actually transmit between the client and server is determined by the amount of available memory and the size of the communications buffers. You can change the message buffer size, but you must do so on both the server and client ends. See section 10.1 Changing the size of MySQL buffers.

Note that each BLOB/TEXT column is represented internally by a unique alloced object. This is in contrast to all other column types that are alloced once when the table is opened.

7.2.7.3 The `ENUM` type

An ENUM (enumeration) is a string object that can have only one value, chosen from a list of allowed values, or NULL. For example, a column specified as ENUM("one", "two", "three") can have any of these values:

NULL
"one"
"two"
"three"

An enumeration can have a maximum of 65535 elements.

When you assign a value to an ENUM column, the case of the value to be stored does not matter; the stored value is converted to the case that was used to specify the ENUM column when the table was created.

If you retrieve an ENUM in a numeric context, the column value's index is returned. If you store a number into an ENUM, the value stored is the enumeration member whose index is that number. Enumeration values are indexed beginning with 1 (0 is reserved for incorrect enumeration values).

Sorting of ENUM values is done according to the order in which the enumeration members were listed in the column specification. For example, "a" sorts before "b" for ENUM("a", "b"), but "a" sorts after "b" for ENUM("b", "a"). NULL values sort before other enumeration values.

If an ENUM is declared NOT NULL, the default value is the first value, otherwise the default value is NULL.

7.2.7.4 The `SET` type

A SET is a string object that can have zero or more values, each of which must be chosen from a list of allowed values. SET column values that are composed of multiple set members are specified with members separated by commas (`,'). For example, a column specified as SET("one", "two") NOT NULL can have any of these values:

""
"one"
"two"
"one,two"

A SET can have a maximum of 64 different members.

MySQL stores SET values numerically, with the low-order bit of the stored value corresponding to the first set member. If you retrieve a set value into a numeric context, the value retrieved has the bit (or bits) set corresponding to the set member (or members) that make up the column value. If a number is stored into a SET column, the bit (or bits) that are set in the number determine the set member (or members) in the column value. Sorting of SET values is done numerically. NULL values sort before other set members.

Normally, you perform a SELECT on a SET column using LIKE or FIND_IN_SET():

mysql> SELECT * FROM tbl_name WHERE set_col LIKE '%value%';
mysql> SELECT * FROM tbl_name WHERE FIND_IN_SET('value',set_col)>0;

But the following will also work:

mysql> SELECT * FROM tbl_name WHERE set_col = 'val1,val2'; # Exact match
mysql> SELECT * FROM tbl_name WHERE set_col & 1;           # Is in first group

7.2.8 Choosing the right type for a column

Try to use the most precise type in all cases. For example, if an integer column will be used for values in the range between 1 and 99999, MEDIUMINT UNSIGNED is the best type.

Accurate representation of monetary values is a common problem. In MySQL you should use the DECIMAL type. This is stored as a string, so no loss of accuracy should occur. If accuracy is not too important, the DOUBLE type may also be good enough.

For high precision, you can always convert to a fixed-point type stored in a BIGINT. This allows you to do all calculations with integers and convert results back to floating-point values only when necessary.

See section 10.14 What are the different row formats? Or, when should VARCHAR/CHAR be used?.

7.2.9 Column indexes

All MySQL column types can be indexed except BLOB and TEXT types. Use of indexes on the relevant columns is the best way to improve the performance of SELECT operations.

A table may have up to 16 indexes. The maximum index length is 256 bytes, although this may be changed when compiling MySQL.

For CHAR and VARCHAR columns, you can index a prefix of a column. This is much faster and requires less disk space than indexing the whole column.

The syntax to use in the CREATE TABLE statement to index a column prefix looks like this:

KEY index_name (col_name(length))

The example below creates an index for the first 10 characters of the name column:

CREATE TABLE test (
    name CHAR(200) NOT NULL,
    KEY index_name (name(10)));

7.2.10 Multiple-column indexes

MySQL can create indexes from multiple columns.

A multiple-column index can be considered a sorted array where the values of the indexed columns are concatenated.

MySQL uses multiple-column indexes in such a way that queries are fast when you specify a known quantity for the first column of the index in a WHERE clause, even if you don't specify values for the other columns.

An index may consist up up to 15 columns (or column prefixes, for CHAR and VARCHAR columns).

Suppose you have a table that has the following specification:

CREATE TABLE test (
    id INT NOT NULL,
    last_name CHAR(30) NOT NULL,
    first_name CHAR(30) NOT NULL,
    PRIMARY KEY (id),
    INDEX name (last_name,first_name));

Then the index name is an index over last_name and first_name. The index will be used for queries that specify values in a known range for last_name, or for both last_name and first_name. Therefore, the name index will be used in the following queries:

mysql> SELECT * FROM test WHERE last_name="Widenius";

mysql> SELECT * FROM test WHERE last_name="Widenius"
                          AND first_name="Michael";

mysql> SELECT * FROM test WHERE last_name="Widenius"
                          AND (first_name="Michael" OR first_name="Monty");

mysql> SELECT * FROM test WHERE last_name="Widenius"
                          AND first_name >="M" AND first_name < "N";

However, the name index will NOT be used in the following queries:

mysql> SELECT * FROM test WHERE first_name="Michael";

mysql> SELECT * FROM test WHERE last_name="Widenius" or first_name="Michael";

For more information on the manner in which MySQL uses indexes to improve query performance, see section 10.4 How MySQL uses indexes.

7.2.11 Using column types from other database engines

To make it easier to use code written for SQL implementations from other vendors, MySQL supports the column type mappings shown in the table below. These mappings make it easier to move table definitions from other database engines to MySQL:

Other vendor type	MySQL type
`BINARY(NUM)`	`CHAR(NUM) BINARY`
`CHAR VARYING(NUM)`	`VARCHAR(NUM)`
`FLOAT4`	`FLOAT`
`FLOAT8`	`DOUBLE`
`INT1`	`TINYINT`
`INT2`	`SMALLINT`
`INT3`	`MEDIUMINT`
`INT4`	`INT`
`INT8`	`BIGINT`
`LONG VARBINARY`	`MEDIUMBLOB`
`LONG VARCHAR`	`MEDIUMTEXT`
`MIDDLEINT`	`MEDIUMINT`
`VARBINARY(NUM)`	`VARCHAR(NUM) BINARY`

Column type mapping occurs at table creation time, so if you create a table with types used by other vendors and then issue a DESCRIBE tbl_name statement, MySQL reports the table structure using the equivalent MySQL types.

7.3 Functions for use in `SELECT` and `WHERE` clauses

A select_expression or where_definition can consist of any expression using the functions described below.

Note: there must be no whitespace between a function name and the parenthesis following it. This helps the MySQL parser distinguish between function calls and references to tables or columns that happen to have the same name as a function.

For the sake of brevity, the examples shown below display the output from the mysql program in abbreviated form. So this:

mysql> select MOD(29,9);
1 rows in set (0.00 sec)

+-----------+
| mod(29,9) |
+-----------+
|         2 |
+-----------+

Is displayed like this:

mysql> select MOD(29,9);
        -> 2

7.3.1 Grouping functions

( ... )

Parentheses. Use these to force the order of evaluation in an expression.

mysql> select 1+2*3;
        -> 7
mysql> select (1+2)*3;
        -> 9

7.3.2 Normal arithmetic operations

Note that in the case of -, + and *, the result is calculated with BIGINT precession if both arguments are integers!

+

Addition

mysql> select 3+5;
        -> 8

-

Subtraction

mysql> select 3-5;
        -> -2

*

Multiplication

mysql> select 3*5;
        -> 15
mysql> select 18014398509481984*18014398509481984.0;
        -> 324518553658426726783156020576256.0
mysql> select 18014398509481984*18014398509481984;
        -> 0

The result of the last expression is wrong because the result of the integer multiplication is over the 64 bit range.

/

Division. A division by zero produces a NULL result.

mysql> select 3/5;
        -> 0.60
mysql> select 102/(1-1);
        -> NULL

A division will be calculated with BIGINT arithmetic only if it's used in a context where it's result are converted to an integer!

7.3.3 Bit functions

These have a maximum range of 64 bits because MySQL uses BIGINT (64-bit) arithmetic for bit operations.

|

Bitwise OR

mysql> select 29 | 15;
        -> 31

&

Bitwise AND

mysql> select 29 & 15;
        -> 13

<<

Shifts a longlong number to the left.

mysql> select 1 << 2
        -> 4

>>

Shifts a longlong number to the right.

mysql> select 4 >> 2
        -> 1

BIT_COUNT(N)

Returns the number of bits that are set in the argument N.

mysql> select BIT_COUNT(29);
        -> 4

7.3.4 Logical operations

All logical functions return 1 (TRUE) or 0 (FALSE).

NOT

!

Logical NOT. Returns 1 if the argument is 0, otherwise returns 0. Exception: NOT NULL returns NULL.

mysql> select NOT 1;
        -> 0
mysql> select NOT NULL;
        -> NULL
mysql> select ! (1+1);
        -> 0
mysql> select ! 1+1;
        -> 1

The last example returns 1 because the expression evaluates the same way as (!1)+1.

OR

||

Logical OR. Returns 1 if either argument is not 0 and not NULL.

mysql> select 1 || 0;
        -> 1
mysql> select 0 || 0;
        -> 0
mysql> select 1 || NULL;
        -> 1

AND

&&

Logical AND. Returns 0 if either argument is 0 or NULL, otherwise returns 1.

mysql> select 1 && NULL;
        -> 0
mysql> select 1 && 0;
        -> 0

7.3.5 Comparison operators

Comparison operations result in a value of 1 (TRUE), 0 (FALSE) or NULL. These functions work for both numbers and strings. Strings are automatically converted to numbers and numbers to strings as needed (as in Perl).

MySQL performs comparisons using the following rules:

If one or both arguments are NULL, the result of the comparison is NULL.
If both arguments in a comparison operation are strings, they are compared as strings.
If both arguments are integers, they are compared as integers.
If one of the arguments is a TIMESTAMP or DATETIME column and the other argument is a constant, the constant is converted to a timestamp before the comparison is performed. This is done to be more ODBC-friendly.
In all other cases, the arguments are compared as floating-point (real) numbers.

By default, string comparisons are done in case-independent fashion using the current character set (ISO-8859-1 Latin1 by default, which also works excellently for English).

The examples below illustrate conversion of strings to numbers for comparison operations:

mysql> SELECT 1 > '6x';
         -> 0
mysql> SELECT 7 > '6x';
         -> 1
mysql> SELECT 0 > 'x6';
         -> 0
mysql> SELECT 0 = 'x6';
         -> 1

=

Equal

mysql> select 1 = 0;
        -> 0
mysql> select '0' = 0;
        -> 1
mysql> select '0.0' = 0;
        -> 1
mysql> select '0.01' = 0;
        -> 0
mysql> select '.01' = 0.01;
        -> 1

<>

!=

Not equal

mysql> select '.01' <> '0.01';
        -> 1
mysql> select .01 <> '0.01';
        -> 0
mysql> select 'zapp' <> 'zappp';
        -> 1

<=

Less than or equal

mysql> select 0.1 <= 2;
        -> 1

<

Less than

mysql> select 2 <= 2;
        -> 1

>=

Greater than or equal

mysql> select 2 >= 2;
        -> 1

>

Greater than

mysql> select 2 > 2;
        -> 0

ISNULL(expr)

If expr is NULL, returns 1, otherwise returns 0.

mysql> select ISNULL(1+1);
        -> 0
mysql> select ISNULL(1/0);
        -> 1

expr BETWEEN min AND max

If expr is greater than or equal to min and expr is less than or equal to max, returns 1, otherwise returns 0. Does the same thing as the expression (min <= expr AND expr <= max) if all the arguments are of the same type. The first argument (expr) determines how the comparison is performed. If expr is a string expression, a case-insensitive string comparison is done. If expr is a binary string, a case-sensitive string comparison is done. If expr is an integer expression, an integer comparison is done. Otherwise, a floating-point (real) comparison is done.

mysql> select 1 BETWEEN 2 AND 3;
        -> 0
mysql> select 'b' BETWEEN 'a' AND 'c';
        -> 1
mysql> select 2 BETWEEN 2 AND '3';
        -> 1
mysql> select 2 BETWEEN 2 AND 'x-3';
        -> 0

expr IN (value,...)

Returns 1 if expr is any of the values in the IN list, else returns 0. If all values are constants, then all values are evaluated according to the type of expr and sorted. The search for the item is then done using a binary search. This means IN is very quick when used with constants in the IN value list. If expr is a case-sensitive string expression, the string comparison is done in case-sensitive fashion.

mysql> select 2 IN (0,3,5,'wefwf');
        -> 0
mysql> select 'wefwf' IN (0,3,5,'wefwf');
        -> 1

expr NOT IN (value,...)

Same as NOT (expr IN (value,...)).

INTERVAL(N,N1,N2,N3...)

Returns 0 if N < N1, 1 if N < N2 and so on. All arguments are treated as numbers. It is required that N1 < N2 < N3 < Nn for this function to work correctly. This is because a binary search is used (very fast).

mysql> select INTERVAL(23, 1, 15, 17, 30, 44, 200);
        -> 3
mysql> select INTERVAL(10, 1, 10, 100, 1000);
        -> 2
mysql> select INTERVAL(22, 23, 30, 44, 200);
        -> 0

7.3.6 String comparison functions

Normally, if one expression to be compared is not case sensitive, string comparisons are done in case-insensitive fashion.

expr1 LIKE expr2 [ESCAPE string-of-one-character]

SQL simple regular expression comparison. Returns 1 (TRUE) or 0 (FALSE). With LIKE you can use the following two wildcard characters:

`%`	Matches any number of characters, even zero characters
`_`	Matches exactly one character

If one doesn't specify the ESCAPE character '\' will be used. To test for literal instances of the wildcard characters, use the following sequences:

`\%`	Matches one `%` character
`\_`	Matches one `_` character

mysql> select 'David!' LIKE 'David_';
        -> 1
mysql> select 'David!' LIKE 'David\_';
        -> 0
mysql> select 'David_' LIKE 'David\_';
        -> 1
mysql> select 'David!' LIKE '%D%v%';
        -> 1
mysql> select 10 LIKE '1%';
        -> 1
mysql> select 'David_' LIKE 'David|_' ESCAPE '|'

LIKE is allowed on numeric expressions! (This is a MySQL extension to the ANSI SQL LIKE.)

expr1 NOT LIKE expr2 [ESCAPE 'string-of-one-character']

Same as NOT (expr1 LIKE expr2 [ESCAPE 'string-of-one-character']).

expr REGEXP pat

expr RLIKE pat

Performs a pattern match of a string expression expr against a pattern pat. The pattern can be an extended regular expression. See section H Description of MySQL regular expression syntax. Returns 1 if expr matches pat, otherwise returns 0. RLIKE is a synonym for REGEXP, provided for mSQL compatibility. NOTE: Because MySQL uses the C escape syntax in strings (\n), you must double any '\' that you use in your REGEXP strings.

mysql> select 'Monty!' REGEXP 'm%y%%';
        -> 0
mysql> select 'Monty!' REGEXP '.*';
        -> 1
mysql> select 'new*\n*line' REGEXP 'new\\*.\\*line';
        -> 1

REGEXP and RLIKE use the current character set (ISO-8859-1 Latin1 by default) when deciding the type of a character.

expr NOT REGEXP expr

Same as NOT (expr REGEXP expr).

STRCMP(expr1,expr2)

Returns 0 if the strings are the same. Returns -1 if the first argument is smaller than the second according to the current sort order. Otherwise returns 1.

mysql> select STRCMP('text', 'text2');
        -> -1
mysql> select STRCMP('text2', 'text');
        -> 1
mysql> select STRCMP('text', 'text');
        -> 0

7.3.7 Control flow functions

IFNULL(expr1,expr2)

If expr1 is not NULL, IFNULL() returns expr1, else returns expr2. IFNULL() returns a numeric or string value, depending on the context in which it are used.

mysql> select IFNULL(1,0);
        -> 1
mysql> select IFNULL(0,10);
        -> 0
mysql> select IFNULL(1/0,10);
        -> 10
mysql> select IFNULL(1/0,'yes');
        -> 'yes'

IF(expr1,expr2,expr3)

If expr1 is TRUE (expr1 <> 0 and expr1 <> NULL) then returns expr2, else returns expr3. IFNULL() returns a numeric or string value, depending on the context in which it are used. expr1 is evaluated as an INTEGER, which means that if you are testing floating-point values, you should do so using a comparison operation.

mysql> select IF(1>2,2,3);
        -> 3
mysql> select IF(1<2,'yes','no');
        -> 'yes'
mysql> select IF(strcmp('test','test1'),'yes','no');
        -> 'no'
mysql> select IF(0.1<>0,1,0);
        -> 1
mysql> select IF(0.1,1,0);
        -> 0

7.3.8 Mathematical functions

All mathematical functions return NULL in case of an error.

-

Sign. Changes the sign of the argument.

mysql> select - 2;
        -> -2

Note that if this function is used with a BIGINT, the return value is a BIGINT! This means that you should avoid using - on integers that may have the value of -2^63 !

ABS(X)

Returns the absolute value of X.

mysql> select ABS(2);
        -> 2
mysql> select ABS(-32);
        -> 32

This function is BIGINT safe.

SIGN(X)

Returns the sign of the argument (-1, 0 or 1, depending on whether X is negative, zero, or positive).

mysql> select SIGN(-32);
        -> -1
mysql> select SIGN(0);
        -> 0
mysql> select SIGN(234);
        -> 1

MOD(N,M)

%

Modulo (like % in C). Returns the remainder of N divided by M.

mysql> select MOD(234, 10);
        -> 4
mysql> select 253 % 7;
        -> 1
mysql> select MOD(29,9);
        -> 2

This function is BIGINT safe.

FLOOR(X)

Returns the largest integer value not greater than X.

mysql> select FLOOR(1.23);
        -> 1
mysql> select FLOOR(-1.23);
        -> -2

Note that the return value is converted to a BIGINT !

CEILING(X)

Returns the smallest integer value not less than X.

mysql> select CEILING(1.23);
        -> 2
mysql> select CEILING(-1.23);
        -> -1

Note that the return value is converted to a BIGINT !

ROUND(X)

Returns the argument X, rounded to an integer.

mysql> select ROUND(-1.23);
        -> -1
mysql> select ROUND(-1.58);
        -> -2
mysql> select ROUND(1.58);
        -> 2

Note that the return value is converted to a BIGINT !

ROUND(X,D)

Returns the argument X, rounded to a number with D decimals.

mysql> select ROUND(1.298, 1);
        -> 1.3

Note that the return value is converted to a BIGINT !

EXP(X)

Returns the value of e (the base of natural logarithms) raised to the power of X.

mysql> select EXP(2);
        -> 7.389056
mysql> select EXP(-2);
        -> 0.135335

LOG(X)

Returns the natural logarithm of X.

mysql> select LOG(2);
        -> 0.693147
mysql> select LOG(-2);
        -> NULL

If you want the log of a number X to some arbitary base B, use the formula LOG(X)/LOG(B).

LOG10(X)

Returns the base-10 logarithm of X.

mysql> select LOG10(2);
        -> 0.301030
mysql> select LOG10(100);
        -> 2.000000
mysql> select LOG10(-100);
        -> NULL

POW(X,Y)

POWER(X,Y)

Returns the value of X raised to the power of Y.

mysql> select POW(2,2);
        -> 4.000000
mysql> select POW(2,-2);
        -> 0.250000

SQRT(X)

Returns the non-negative square root of X.

mysql> select SQRT(4);
        -> 2.000000
mysql> select SQRT(20);
        -> 4.472136

PI()

Returns the value of PI.

mysql> select PI();
        -> 3.141593

COS(X)

Returns the cosine of X, where X is given in radians.

mysql> select COS(PI());
        -> -1.000000

SIN(X)

Returns the sine of X, where X is given in radians.

mysql> select SIN(PI());
        -> 0.000000

TAN(X)

Returns the tangent of X, where X is given in radians.

mysql> select TAN(PI()+1);
        -> 1.557408

ACOS(X)

Returns the arc cosine of X, that is, the value whose cosine is X. Returns NULL if X is not in the range -1 to 1.

mysql> select ACOS(1);
        -> 0.000000
mysql> select ACOS(1.0001);
        -> NULL
mysql> select ACOS(0);
        -> 1.570796

ASIN(X)

Returns the arc sine of X, that is, the value whose sine is X. Returns NULL if X is not in the range -1 to 1.

mysql> select ASIN(0.2);
        -> 0.201358
mysql> select ASIN('foo');
        -> 0.000000

ATAN(X)

Returns the arc tangent of X, that is, the value whose tangent is X.

mysql> select ATAN(2);
        -> 1.107149
mysql> select ATAN(-2);
        -> -1.107149

ATAN2(X,Y)

Returns the arc tangent of the two variables X and Y. It is similar to calculating the arc tangent of Y / X, except that the signs of both arguments are used to determine the quadrant of the result.

mysql> select ATAN(-2,2);
        -> -0.785398
mysql> select ATAN(PI(),0);
        -> 1.570796

COT(X)

Returns the cotangent of X.

mysql> select COT(12);
        -> -1.57267341
mysql> select COT(0);
        -> NULL

RAND()

RAND(N)

Returns a random floating-point value in the range 0 to 1.0. If an integer argument N is specified, it is used as the seed value.

mysql> select RAND();
        -> 0.5925
mysql> select RAND(20);
        -> 0.1811
mysql> select RAND(20);
        -> 0.1811
mysql> select RAND();
        -> 0.2079
mysql> select RAND();
        -> 0.7888

You can't do an ORDER BY on a column with RAND() values because ORDER BY would evaluate the column multiple times.

LEAST(X,Y...)

With two or more arguments, returns the smallest (minimum-valued) argument. The arguments are compared according to the following rules:

If the value is used as an INTEGER, or all arguments are integer-valued, then they are compared as integers.
If the value is used as a REAL, or all arguments are real-valued, then they are compared as reals.
If any argument is a case-sensitive string, then the arguments are compared as case-sensitive strings
In other cases, the arguments are compared as case-insensitive strings.

mysql> select LEAST(2,0);
        -> 0
mysql> select LEAST(34.0,3.0,5.0,767.0);
        -> 3.0
mysql> select LEAST("B","A","C");
        -> "A"

In MySQL versions prior to 3.22.5, you can use MIN() instead of LEAST.

GREATEST(X,Y...)

Returns the largest (maximum-valued) argument. The arguments are compared according to the same rules as for LEAST.

mysql> select GREATEST(2,0);
        -> 2
mysql> select GREATEST(34.0,3.0,5.0,767.0);
        -> 767.0
mysql> select GREATEST("B","A","C");
        -> "C"

In MySQL versions prior to 3.22.5, you can use MAX() instead of GREATEST.

DEGREES(X)

Returns the argument X, converted from radians to degrees.

mysql> select DEGREES(PI());
        -> 180.000000

RADIANS(X)

Returns the argument X, converted from degrees to radians.

mysql> select RADIANS(90);
        -> 1.570796

TRUNCATE(X,D)

Returns the number X, truncated to D decimals.

mysql> select TRUNCATE(1.223,1);
        -> 1.2
mysql> select TRUNCATE(1.999,1);
        -> 1.9
mysql> select TRUNCATE(1.999,0);
        -> 1

7.3.9 String functions

For functions that operate on string positions, the first position is numbered 1.

ASCII(str)

Returns the ASCII code value of the leftmost character of the string str. Returns 0 if str is the empty string. Returns NULL if str is NULL.

mysql> select ASCII(2);
        -> 50
mysql> select ASCII('dx');
        -> 100

CONV(N,FROM_BASE,TO_BASE)

Converts numbers between different number bases. Returns a string representation of the number N, converted from base FROM_BASE to base TO_BASE. Returns NULL if any argument is NULL. The argument N is interpreted as an integer, but may be specified as an integer or a string. The minimum base is 2 and the maximum base is 36. If TO_BASE is a negative number, N is regarded as a signed number. CONV works with 64-bit precision.

mysql> select CONV("a",16,2);
        -> '1010'
mysql> select CONV("6E",18,8);
        -> '172'
mysql> select CONV(-17,10,-18);
        -> '-H'
mysql> select CONV(10+"10"+'10'+0xa,10,10);
        -> '40'

BIN(N)

Returns a string representation of the binary value of N where N is a longlong number. This is the same as CONV(N,10,2). Returns NULL if N is NULL.

mysql> select BIN(12);
        -> '1100'

OCT(N)

Returns a string representation of the octal value of N where N is a longlong number. This is the same as CONV(N,10,8). Returns NULL if N is NULL.

mysql> select OCT(12);
        -> '14'

HEX(N)

Returns a string representation of the hexadecimal value of N where N is a longlong number. This is the same as CONV(N,10,16). Returns NULL if N is NULL.

mysql> select HEX(255);
        -> 'FF'

CHAR(N,...)

Returns a string consisting of the characters given by the ASCII code values of the arguments. NULL values are skipped.

mysql> select CHAR(77,121,83,81,'76');
        -> 'MySQL'

CONCAT(X,Y...)

Returns the string that results from concatenating the arguments. Returns NULL if any argument is NULL. May have more than 2 arguments.

mysql> select CONCAT('My', 'S', 'QL');
        -> 'MySQL'
mysql> select CONCAT('My', NULL, 'QL');
        -> NULL

LENGTH(str)

OCTET_LENGTH(str)

CHAR_LENGTH(str)

CHARACTER_LENGTH(str)

Returns the length of the string str.

mysql> select LENGTH('text');
        -> 4
mysql> select OCTET_LENGTH('text');
        -> 4

LOCATE(substr,str)

POSITION(substr IN str)

Returns the position of the first occurrence of substring substr in string str. Returns 0 if substr is not in str.

mysql> select LOCATE('bar', 'foobarbar');
        -> 4
mysql> select LOCATE('xbar', 'foobar');
        -> 0

LOCATE(substr,str,pos)

Returns the position of the first occurrence of substring substr in string str, starting at position pos. Returns 0 if substr is not in str.

mysql> select LOCATE('bar', 'foobarbar',5);
        -> 7

INSTR(str,substr)

Returns the position of the first occurrence of substring substr in string str. This is the same as the two-argument form of LOCATE, except that the arguments are swapped.

mysql> select INSTR('foobarbar', 'bar');
        -> 4
mysql> select INSTR('xbar', 'foobar');
        -> 0

LPAD(str,len,padstr)

Returns the string str, left-padded with the string padstr until str is len characters long.

mysql> select LPAD('hi',4,'??');
        -> '??hi'

RPAD(str,len,padstr)

Returns the string str, right-padded with the string padstr until str is len characters long.

mysql> select RPAD('hi',5,'?');
        -> 'hi???'

LEFT(str,len)

Returns the leftmost len characters from the string str.

mysql> select LEFT('foobarbar', 5);
        -> 'fooba'

RIGHT(str,len)

SUBSTRING(str FROM len)

Returns the rightmost len characters from the string str.

mysql> select RIGHT('foobarbar', 4);
        -> 'rbar'
mysql> select SUBSTRING('foobarbar' from 4);
        -> 'rbar'

SUBSTRING(str,pos,len)

SUBSTRING(str FROM pos FOR len)

MID(str,pos,len)

Returns a substring len characters long from string str, starting at position pos. The variant form that uses FROM is ANSI SQL 92 syntax.

mysql> select SUBSTRING('Quadratically',5,6);
        -> 'ratica'

SUBSTRING(str,pos)

Returns a substring from string str starting at position pos.

mysql> select SUBSTRING('Quadratically',5);
        -> 'ratically'

SUBSTRING_INDEX(str,delim,count)

Returns the substring from string str after count occurrences of the delimiter delim. If count is positive, everything to the left of the final delimiter (counting from the left) is returned. If count is negative, everything to the right of the final delimiter (counting from the right) is returned.

mysql> select SUBSTRING_INDEX('www.tcx.se', '.', 2);
        -> 'www.tcx'
mysql> select SUBSTRING_INDEX('www.tcx.se', '.', -2);
        -> 'tcx.se'

LTRIM(str)

Returns the string str with leading space characters removed.

mysql> select LTRIM('  barbar');
        -> 'barbar'

RTRIM(str)

Returns the string str with trailing space characters removed.

mysql> select RTRIM('barbar   ');
        -> 'barbar'

TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)

Returns the string str with all remstr prefixes and/or suffixes removed. If none of the specifiers BOTH, LEADING or TRAILING are given, BOTH is assumed. If remstr is not specified, spaces are removed.

mysql> select TRIM('  bar   ');
        -> 'bar'
mysql> select TRIM(leading 'x' from 'xxxbarxxx');
        -> 'barxxx'
mysql> select TRIM(both 'x' from 'xxxbarxxx');
        -> 'bar'
mysql> select TRIM(trailing 'xyz' from 'barxxyz');
        -> 'barx'

SOUNDEX(str)

Returns a soundex string from str. Two strings that sound "about the same" should have identical soundex strings. A "standard" soundex string is 4 characters long, but the SOUNDEX() function returns an arbitrarily long string. You can use SUBSTRING() on the result to get a "standard" soundex string. All non-alpha characters are ignored in the given string. All characters outside the A-Z range are treated as vowels.

mysql> select SOUNDEX('Hello');
        -> 'H400'
mysql> select SOUNDEX('Quadratically');
        -> 'Q36324'

SPACE(N)

Returns a string consisting of N space characters.

mysql> select SPACE(6);
        -> '      '

REPLACE(str,from,to)

Returns the string str with all all occurrences of the string from replaced by the string to.

mysql> select REPLACE('www.tcx.se', 'w', 'Ww');
        -> 'WwWwWw.tcx.se'

REPEAT(str,count)

Returns a string consisting of the string str repeated count times. If count <= 0, returns an empty string. Returns NULL if str or count are NULL or if LENGTH(str)*count > max_allowed_packet.

mysql> select REPEAT('MySQL', 3);
        -> 'MySQLMySQLMySQL'

REVERSE(str)

Returns the string str with the order of the characters reversed.

mysql> select REVERSE('abc');
        -> 'cba'

INSERT(str,start,len,newstr)

Returns the string str, with the substring beginning at position start and len characters long replaced by the string newstr.

mysql> select INSERT('Quadratic', 3, 4, 'What');
        -> 'QuWhattic'

ELT(N,str1,str2,str3...)

Returns str1 if N = 1, str2 if N = 2, and so on. Returns NULL if N is less than 1 or greater than the number of arguments. ELT() is the complement of FIELD().

mysql> select ELT(1, 'ej', 'Heja', 'hej', 'foo');
        -> 'ej'
mysql> select ELT(4, 'ej', 'Heja', 'hej', 'foo');
        -> 'foo'

FIELD(str,str1,str2,str3...)

Returns the index of str in the str1, str2, str3... list. Returns 0 if str is not found. FIELD() is the complement of ELT().

mysql> select FIELD('ej', 'Hej', 'ej', 'Heja', 'hej', 'foo');
        -> 2
mysql> select FIELD('fo', 'Hej', 'ej', 'Heja', 'hej', 'foo');
        -> 0

FIND_IN_SET(str,strlist)

Returns a value 1 to N if the string str is in the list strlist consisting of N substrings. A string list is itself a string with its individual substrings separated by ',' characters. If the first argument is a constant string and the second is a column of type SET, the FIND_IN_SET is optimized to use bit arithmetic! Returns 0 if strlist is the empty string. Returns NULL if either argument is NULL. This function will not work properly if the first argument contains a ','.

mysql> SELECT FIND_IN_SET('b','a,b,c,d');
        -> 2

LCASE(str)

LOWER(str)

Returns the string str with all characters changed to lowercase according to the current character set mapping (the default is Latin1).

mysql> select LCASE('QUADRATICALLY');
        -> 'quadratically'

UCASE(str)

UPPER(str)

Returns the string str with all characters changed to uppercase according to the current character set mapping (the default is Latin1).

mysql> select UCASE('Hej');
        -> 'HEJ'

7.3.10 Date and time functions

Here is an example that uses date functions. The query below selects all records with a date_field value from the last 30 days:

mysql> SELECT something FROM table
           WHERE TO_DAYS(NOW()) - TO_DAYS(date_field) <= 30;

See section 7.2.6 Date and time types for a description of the range of values each type has and the valid formats in which date and time values may be specified.

DAYOFWEEK(date)

Returns the weekday index for date (1 = Sunday, 2 = Monday, ... 7 = Saturday). These index values correspond to the ODBC standard.

mysql> select DAYOFWEEK('1998-02-03');
        -> 3

WEEKDAY(date)

Returns the weekday index for date (0 = Monday, 1 = Tuesday, ... 6 = Sunday).

mysql> select WEEKDAY('1997-10-04 22:23:00');
        -> 5
mysql> select WEEKDAY('1997-11-05');
        -> 2

DAYOFMONTH(date)

Returns the day of the month for date, in the range 1 to 31.

mysql> select DAYOFMONTH('1998-02-03');
        -> 3

DAYOFYEAR(date)

Returns the day of the year for date, in the range 1 to 366.

mysql> select DAYOFYEAR('1998-02-03');
        -> 34

MONTH(date)

Returns the month for date, in the range 1 to 12.

mysql> select MONTH('1998-02-03');
        -> 2

DAYNAME(date)

Returns the name of the weekday for date.

mysql> select DAYNAME("1998-02-05");
        -> Thursday

MONTHNAME(date)

Returns the name of the month for date.

mysql> select MONTHNAME("1998-02-05");
        -> February

QUARTER(date)

Returns the quarter of the year for date, in the range 1 to 4.

mysql> select QUARTER('98-04-01');
        -> 2

WEEK(date)

WEEK(date,first)

With a single argument, returns the week for date, in the range 0 to 52, for locations where Sunday is the first day of the week. The two-argument form of WEEK() allows you to specify whether the week starts on Sunday or Monday. The week starts on Sunday if the second argument is 0, on Monday if the second argument is 1.

mysql> select WEEK('1998-02-20');
        -> 7
mysql> select WEEK('1998-02-20',0);
        -> 7
mysql> select WEEK('1998-02-20',1);
        -> 8

YEAR(date)

Returns the year for date, in the range 1000 to 9999.

mysql> select YEAR('98-02-03');
        -> 1998

HOUR(time)

Returns the hour for time, in the range 0 to 23.

mysql> select HOUR('10:05:03');
        -> 10

MINUTE(time)

Returns the minute for time, in the range 0 to 59.

mysql> select MINUTE('98-02-03 10:05:03');
        -> 5

SECOND(time)

Returns the second for time, in the range 0 to 59.

mysql> select SECOND('10:05:03');
        -> 3

PERIOD_ADD(P,N)

Adds N months to period P (in the format YYMM or YYYYMM). Returns a value in the format YYYYMM.

mysql> select PERIOD_ADD(9801,2);
        -> 199803

PERIOD_DIFF(P1,P2)

Returns the number of months between periods P1 and P2. P1 and P2 should be in the format YYMM or YYYYMM.

mysql> select PERIOD_DIFF(9802,199703);
        -> 11

DATE_ADD(date,INTERVAL expr type)

DATE_SUB(date,INTERVAL expr type)

ADDDATE(date,INTERVAL expr type)

SUBDATE(date,INTERVAL expr type)

These functions perform date arithmetic. They are new for MySQL 3.22. ADDDATE() and SUBDATE() are synonyms for DATE_ADD() and DATE_SUB(). date is the starting date (a DATETIME or DATE value). expr is an expression specifying the interval value to be added or substracted from the starting date. expr is a string; it may start with a `-' for negative intervals. type is an interval type keyword indicating how the expression should be interpreted.

`type` value	Meaning	`expr` format
`SECOND`	Seconds	`SECONDS`
`MINUTE`	Minutes	`MINUTES`
`HOUR`	Hours	`HOURS`
`DAY`	Days	`DAYS`
`MONTH`	Months	`MINUTES`
`YEAR`	Years	`YEARS`
`MINUTE_SECOND`	Minutes and seconds	`"MINUTES:SECONDS"`
`HOUR_MINUTE`	Hours and minutes	`"HOURS:MINUTES"`
`DAY_HOUR`	Days and hours	`"DAYS HOURS"`
`YEAR_MONTH`	Years and months	`"YEARS-MONTHS"`
`HOUR_SECOND`	Hours, minutes, seconds	`"HOURS:MINUTES:SECONDS"`
`DAY_MINUTE`	Days, hours, minutes	`"DAYS HOURS:MINUTES"`
`DAY_SECOND`	Days, hours, minutes, seconds	`"DAYS HOURS:MINUTES:SECONDS"`

MySQL allows any non-numeric delimiter in the format. The ones shown in the table are the suggested delimiters. If the date is a DATE value and your calculations involve only YEAR, MONTH and DAY (that is, no time parts), the result is a DATE value. Otherwise the result is a DATETIME value.

mysql> select DATE_ADD("1997-12-31 23:59:59",INTERVAL 1 SECOND);
        -> 1998-01-01 00:00:00
mysql> select DATE_ADD("1997-12-31 23:59:59",INTERVAL "1:1" MINUTE_SECOND);
        -> 1998-01-01 00:01:00
mysql> select DATE_SUB("1998-01-01 00:00:00",INTERVAL "1 1:1:1" DAY_SECOND);
        -> 1997-12-30 22:58:59
mysql> select DATE_ADD("1997-12-31 23:59:59",INTERVAL 1 DAY);
        -> 1998-01-01 23:59:59
mysql> select DATE_ADD("1998-01-01 00:00:00",INTERVAL "-1 10" DAY_HOUR);
        -> 1997-12-30 14:00:00
mysql> select DATE_SUB("1998-01-02",INTERVAL 31 DAY);
        -> 1997-12-02

If you specify an interval value that is too short (does not include all the interval parts that would be expected from the interval type keyword), MySQL assumes you have left out the leftmost parts of the interval value. For example, if you specify a type of DAY_SECOND, the value of expr is expected to have day, hours, minutes and seconds parts. If you specify a value like "1:10", MySQL assumes that the day and hours parts are missing and the the value represents minutes and seconds. In other words, "1:10" DAY_SECOND is interpreted as "1:10" MINUTE_SECOND. If you use incorrect dates, the result is NULL.

TO_DAYS(date)

Given a date date, returns a daynumber (the number of days since year 0). TO_DAYS() is not intended for use with values that precede the advent of the Gregorian calendar (1582).

mysql> select TO_DAYS(950501);
        -> 728779
mysql> select TO_DAYS('1997-10-07);
        -> 729669

FROM_DAYS(N)

Given a daynumber N, returns a DATE value. FROM_DAYS() is not intended for use with values that precede the advent of the Gregorian calendar (1582).

mysql> select FROM_DAYS(729669);
        -> '1997-10-07'

DATE_FORMAT(date,format)

Formats the date value according to the format string. The following specifiers may be used in the format string:

`%M`	Month name (`January`..`December`)
`%W`	Weekday name (`Sunday`..`Saturday`)
`%D`	Day of the month with english suffix (`1st`, `2nd`, `3rd`, etc.)
`%Y`	Year, numeric, 4 digits
`%y`	Year, numeric, 2 digits
`%a`	Abbreviated weekday name (`Sun`..`Sat`)
`%d`	Day of the month, numeric (`00`..`31`)
`%m`	Month, numeric (`01`..`12`)
`%b`	Abbreviated month name (`Jan`..`Dec`)
`%j`	Day of year (`001`..`366`)
`%H`	Hour (`00`..`23`)
`%k`	Hour (`0`..`23`)
`%h`	Hour (`01`..`12`)
`%I`	Hour (`01`..`12`)
`%l`	Hour (`1`..`12`)
`%i`	Minutes, numeric (`00`..`59`)
`%r`	Time, 12-hour (`hh:mm:ss [AP]M`)
`%T`	Time, 24-hour (`hh:mm:ss`)
`%S`	Seconds (`00`..`59`)
`%s`	Seconds (`00`..`59`)
`%p`	`AM` or `PM`
`%w`	Day of the week (`0`=Sunday..`6`=Saturday)
`%U`	Week (`0`..`52`), where Sunday is the first day of the week.
`%u`	Week (`0`..`52`), where Monday is the first day of the week.
`%%`	Single `%' characters are ignored. Use `%%` to produce a literal `%' (for future extensions).

All other characters are just copied to the result.

mysql> select DATE_FORMAT('1997-10-04 22:23:00', '%W %M %Y');
        -> 'Saturday October 1997'
mysql> select DATE_FORMAT('1997-10-04 22:23:00', '%H:%i:%s');
        -> '22:23:00'
mysql> select DATE_FORMAT('1997-10-04 22:23:00', '%D %y %a %d %m %b %j');
        -> '4th 97 Sat 04 10 Oct 277'
mysql> select DATE_FORMAT('1997-10-04 22:23:00', '%H %k %I %r %T %S %w');
        -> '22 22 10 10:23:00 PM 22:23:00 00 6'

For the moment, % is optional. In future versions of MySQL, % will be required.

TIME_FORMAT(time,format)

This is used like the DATE_FORMAT() function above, but the format string may contain only those format specifiers that handle hours, minutes and seconds. Other specifiers produce a NULL value or 0.

CURDATE()

CURRENT_DATE

Returns today's date. The format is YYYYMMDD or 'YYYY-MM-DD', depending on whether the function is used in a numeric or string context.

mysql> select CURDATE();
        -> '1997-12-15'
mysql> select CURDATE()+0;
        -> 19971215

CURTIME()

CURRENT_TIME

Returns the current time. The format is HHMMSS or 'HH:MM:SS', depending on whether the function is used in a numeric or string context.

mysql> select CURTIME();
        -> '23:50:26'
mysql> select CURTIME()+0;
        -> 235026

NOW()

SYSDATE()

CURRENT_TIMESTAMP

Returns the current time, in the format YYYYMMDDHHMMSS or 'YYYY-MM-DD HH:MM:SS', depending on whether the function is used in a numeric or string context.

mysql> select NOW();
        -> '1997-12-15 23:50:26'
mysql> select NOW()+0;
        -> 19971215235026

UNIX_TIMESTAMP()

UNIX_TIMESTAMP(date)

If called with no argument, returns a Unix timestamp (seconds in GMT since '1970-01-01 00:00:00'). Normally, it is called with a TIMESTAMP-valued argument, in which case it returns the value of the argument in seconds. date may be a DATE string, a DATETIME string, a TIMESTAMP, or a number in the format YYMMDD or YYYYMMDD in local time.

mysql> select UNIX_TIMESTAMP();
        -> 882226357
mysql> select UNIX_TIMESTAMP('1997-10-04 22:23:00');
        -> 875996580

When UNIX_TIMESTAMP is used on a TIMESTAMP column, the function will get the value without an implicit `string-to-unix-timestamp' conversion.

FROM_UNIXTIME(Unix_timestamp)

Returns a representation of the timestamp value. The format is YYYYMMDDHHMMSS or 'YYYY-MM-DD HH:MM:SS', depending on whether the function is used in a numeric or string context.

mysql> select FROM_UNIXTIME(875996580);
        -> '1997-10-04 22:23:00'

FROM_UNIXTIME(Unix_timestamp,format)

Returns a string representation of the Unix timestamp, formatted according to the format string. format may contain the same specifiers as those listed in the entry for the DATE_FORMAT() function.

mysql> select FROM_UNIXTIME(UNIX_TIMESTAMP(), '%Y %D %M %h:%i:%s %x');
        -> '1997 23rd December 03:43:30 x'

SEC_TO_TIME(seconds)

Returns the seconds argument, converted to hours, minutes and seconds in the format HHMMSS or HH:MM:SS, depending on whether the function is used in a numeric or string context.

mysql> select SEC_TO_TIME(2378);
        -> '00:39:38'
mysql> select SEC_TO_TIME(2378)+0;
        -> 3938

TIME_TO_SEC(time)

Returns the time argument, converted to seconds.

mysql> select TIME_TO_SEC('22:23:00');
        -> 80580
mysql> select TIME_TO_SEC('00:39:38');
        -> 2378

7.3.11 Miscellaneous functions

DATABASE()

Returns the current database name.

mysql> select DATABASE();
        -> 'test'

USER()

SYSTEM_USER()

SESSION_USER()

Returns the current MySQL user name.

mysql> select USER();
        -> 'davida'

PASSWORD(str)

Calculates a password string from the plaintext password str. To store a password in the user grant table, this function must be used.

mysql> select PASSWORD('badpwd');
        -> '7f84554057dd964b'

PASSWORD() does not encrypt passwords the same way that Unix encrypts login passwords. See ENCRYPT().

ENCRYPT(str[,salt])

Encrypt str using the Unix crypt() system call. The salt argument should be a string with 2 characters. If crypt() is not available on your system, ENCRYPT() always returns NULL.

mysql> select ENCRYPT("hello");
        -> 'VxuFAJXVARROc'

LAST_INSERT_ID()

Returns the last automatically-generated value that was set in an AUTO_INCREMENT column. See section 18.4.49 How can I get the unique ID for the last inserted row?.

mysql> select LAST_INSERT_ID();
        -> 1

The last ID that was generated is maintained in the server on a per-connection basis. It will not be changed by another client. It will not even be changed if you update another AUTO_INCREMENT column with a non-magic value (that is, a value that is not NULL and not 0).

FORMAT(X,D)

Formats the number X to a format like '#,###,###.##' with D decimals.

mysql> select FORMAT(12332.33, 2);
        -> '12,332.33'

VERSION()

Returns a string indicating the MySQL server version.

mysql> select VERSION();
        -> '3.21.16-beta-log'

GET_LOCK(str,timeout)

Tries to obtain a lock with a name given by the string str, with a timeout of timeout seconds. Returns 1 if the lock was obtained successfully, 0 if the attempt timed out, or NULL if an error occurred (such as running out of memory or the thread was killed with mysqladmin kill). A lock is released when you execute RELEASE_LOCK(), execute a new GET_LOCK() or the thread terminates. This function can be used to implement application locks or to simulate record locks.

mysql> select GET_LOCK("automatically released",10);
        -> 1
mysql> select GET_LOCK("test",10);
        -> 1
mysql> select RELEASE_LOCK("test");
        -> 1
mysql> select RELEASE_LOCK("automatically released");
        -> NULL

RELEASE_LOCK(str)

Releases the lock named by the string str that was obtained with GET_LOCK(). Returns 1 if the lock was released, 0 if the lock wasn't locked by this thread and NULL if the named lock didn't exist.

7.3.12 Functions for use with `GROUP BY` clauses

COUNT(expr)

Returns a count of the number of non-NULL rows.

mysql> select COUNT(if(length(name)>3,1,NULL)) from student;

COUNT(*) is optimized to return very quickly if the SELECT retrieves from one table, no other columns are retrieved and there is no WHERE clause.

mysql> select COUNT(*) from student;

AVG(expr)

Returns the average value of expr.

MIN(expr)

MAX(expr)

Returns the minimum or maximum value of expr. MIN() and MAX() may take a string argument; in such cases they return the minimum or maximum string value.

SUM(expr)

Returns the sum of expr.

STD(expr)

STDDEV(expr)

Returns the standard deviation of expr. This is an extension to ANSI SQL. The STDDEV() form of this function is provided for Oracle compatability.

BIT_OR(expr)

Returns the bitwise OR of all bits in expr. The calculation is performed with 64-bit precision.

BIT_AND(expr)

Returns the bitwise AND of all bits in expr. The calculation is performed with 64-bit precision.

MySQL has extended the use of GROUP BY. You can use columns or calculations in the SELECT expressions which don't appear in the GROUP BY part. This stands for any possible value for this group. You can use this to get better performance by avoiding sorting and grouping on unnecessary items. For example, you don't need to group on b.name in the following query:

mysql> select a.id,b.name,count(*) from a,b where a.id=b.id GROUP BY a.id;

In ANSI SQL, you would have to add customer.name to the GROUP BY for the following query. In MySQL, the name is redundant.

mysql> select order.custid,customer.name,max(payments)
           from order,customer
           where order.custid = customer.custid
           GROUP BY order.custid;

Don't use this feature if the columns you omit from the GROUP BY part aren't unique in the group!

In some specific cases, you can use LEAST() and GREATEST() to get a specific column even if it isn't unique. The following gives the value from the row with the smallest "sort" value.

substr(LEAST(concat(sort,space(6-length(sort)),column),7,length(column)))

Note that you can't yet use expressions in GROUP BY or ORDER BY clauses. On the other hand, you can use an alias on an expression to solve the problem:

mysql> select id,floor(value/100) as val from tbl_name
           GROUP BY id,val ORDER BY val;

7.4 `CREATE DATABASE` syntax

CREATE DATABASE db_name

CREATE DATABASE creates a database with the given name. Rules for allowable database names are given in section 7.1.4 Database, table, index, column and alias names.

Databases in MySQL are implemented as directories containing files that correspond to tables in the database. Since there are no tables in a database when it is initially created, the CREATE DATABASE statement only creates a directory under the MySQL data directory.

You can also create databases with mysqladmin. See section 12.1 Overview of the different MySQL programs.

7.5 `DROP DATABASE` syntax

DROP DATABASE [IF EXISTS] db_name

DROP DATABASE drops all tables in the database and deletes the database. You must be VERY careful with this command! DROP DATABASE returns the number of files that were removed from the database directory. Normally, this is three times the number of tables, since each table corresponds to a `.ISD' file, a `.ISM' file and a `.frm' file.

In MySQL 3.22 or later, you can use the keywords IF EXISTS to prevent an error from occurring if the database doesn't exist.

You can also drop databases with mysqladmin. See section 12.1 Overview of the different MySQL programs.

7.6 `CREATE TABLE` syntax

CREATE TABLE tbl_name (create_definition,...)

create_definition:
  col_name type [NOT NULL | NULL] [DEFAULT default_value] [AUTO_INCREMENT]
            [PRIMARY KEY] [reference_definition]
  or    PRIMARY KEY (index_col_name,...)
  or    KEY [index_name] KEY(index_col_name,...)
  or    INDEX [index_name] (index_col_name,...)
  or    UNIQUE [index_name] (index_col_name,...)
  or    [CONSTRAINT symbol] FOREIGN KEY index_name (index_col_name,...)
            [reference_definition]
  or    CHECK (expr)

type:
        TINYINT[(length)] [UNSIGNED] [ZEROFILL]
  or    SMALLINT[(length)] [UNSIGNED] [ZEROFILL]
  or    MEDIUMINT[(length)] [UNSIGNED] [ZEROFILL]
  or    INT[(length)] [UNSIGNED] [ZEROFILL]
  or    INTEGER[(length)] [UNSIGNED] [ZEROFILL]
  or    BIGINT[(length)] [UNSIGNED] [ZEROFILL]
  or    REAL[(length,decimals)] [UNSIGNED] [ZEROFILL]
  or    DOUBLE[(length,decimals)] [UNSIGNED] [ZEROFILL]
  or    FLOAT[(length,decimals)] [UNSIGNED] [ZEROFILL]
  or    DECIMAL(length,decimals) [UNSIGNED] [ZEROFILL]
  or    NUMERIC(length,decimals) [UNSIGNED] [ZEROFILL]
  or    CHAR(length) [BINARY]
  or    VARCHAR(length) [BINARY]
  or    DATE
  or    TIME
  or    TIMESTAMP
  or    DATETIME
  or    TINYBLOB
  or    BLOB
  or    MEDIUMBLOB
  or    LONGBLOB
  or    TINYTEXT
  or    TEXT
  or    MEDIUMTEXT
  or    LONGTEXT
  or    ENUM(value1,value2,value3...)
  or    SET(value1,value2,value3...)

index_col_name:
        col_name [(length)]

reference_definition:
        REFERENCES tbl_name [(index_col_name,...)]
                   [MATCH FULL | MATCH PARTIAL]
                   [ON DELETE reference_option]
                   [ON UPDATE reference_option]

reference_option:
        RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT

CREATE TABLE creates a table with the given name in the current database. Rules for allowable table names are given in See section 7.1.4 Database, table, index, column and alias names.

Each table is represented by three files in the database directory:

File	Purpose
`tbl_name.frm'	Table definition (form) file
`tbl_name.ISD'	Data file
`tbl_name.ISM'	Index file

For more information on the properties of the various column types, see section 7.2 Column types.

If neither NULL nor NOT NULL is specified, the column is treated as though NULL had been specified.
An integer column may have the additional attribute AUTO_INCREMENT. When you insert a value of NULL or 0 into an AUTO_INCREMENT column, the column is set to value+1, where value is the largest value for the column currently in the table. See section 18.4.49 How can I get the unique ID for the last inserted row?. If you delete the row containing the maximum value for an AUTO_INCREMENT column, the value will be reused. If you delete all rows in the table, the sequence starts over. Note: there can be only one AUTO_INCREMENT column per table, and it must be indexed.
NULL values are handled differently for TIMESTAMP columns than for other column types. You cannot store a literal NULL in a TIMESTAMP column; setting it to NULL sets it to the current time. Because TIMESTAMP columns behave this way, the NULL and NOT NULL attributes do not apply in the normal way and are ignored if you specify them. On the other hand, to make it easier for MySQL clients to use TIMESTAMP columns, the server reports that the TIMESTAMP may take NULL values, even though TIMESTAMP never will actually hold a NULL value. You can see this when you use DESCRIBE tbl_name to get a description of your table. Note that setting a TIMESTAMP column to 0 is not the same as setting it to NULL, because 0 is a valid TIMESTAMP value.
If no DEFAULT value is specified for a column, and the column is not declared as NOT NULL, the default value is NULL.
If no DEFAULT value is specified for a column, and the column is declared as NOT NULL, MySQL automatically assigns a default value for the field. The default depends on the column type:
- For numeric types, the default is 0. Exception: for an AUTO_INCREMENT column, the default value is the next value in the sequence.
- For date and time types, the default is the appropriate "zero" value for the type. Exception: if the field is the first TIMESTAMP column in the table, the default value is the current time. See section 7.2.6 Date and time types.
- For string types, the default is the empty string.
KEY is a synonym for INDEX.
In MySQL, a UNIQUE key can have only distinct values. An error occurs if you try to add a new row with a key that matches an existing row.
A PRIMARY KEY is a unique KEY. A table can have only one PRIMARY KEY. MySQL marks the first UNIQUE key as the PRIMARY KEY if no PRIMARY KEY is specified explicitly.
A PRIMARY KEY can be a multiple-column index. However, you cannot create a multiple-column index using the PRIMARY KEY key attibute in a column specification; doing so will mark only that single column as primary. You must use the PRIMARY KEY(index_col_name...) syntax.
If you don't assign a name to an index, the index will be assigned the same name as the first index_col_name with an optional suffix (_2, _3, ...) to make it unique. You can see index names for a table using SHOW INDEX FROM tbl_name. See section 7.20 SHOW syntax (Get information about tables, columns...).
Columns that are indexed or part of an index cannot have NULL values. You must declare such columns NOT NULL or an error results.
With col_name(length) syntax, you can specify an index which uses only a part of a CHAR or VARCHAR column. This can make the index file much smaller. See section 7.2.9 Column indexes.
TEXT and BLOB columns cannot be indexed.
When you sort or group on a TEXT or BLOB column, only the first max_sort_length bytes are used. See section 5.5 Limitations of BLOB and TEXT types.
The FOREIGN KEY, CHECK and REFERENCES clauses don't actually do anything. The syntax for them is provided only for compatibility, to make it easier to port code from other SQL servers and to run applications that create tables with references. See section 5.2 Functionality missing from MySQL.
Each NULL column takes one bit extra, rounded up to the nearest byte.

The maximum record length can be calculated as follows:

row length = 1
             + (sum of column lengths)
             + (number of NULL columns + 7)/8
             + (number of variable-length columns)

7.6.1 Silent column specification changes

In some cases, MySQL silently changes a column specification from that given in the CREATE TABLE statement:

VARCHAR columns with a length less than four are changed to CHAR.
If any column in a table has a variable length, the entire row is variable-length as a result. Therefore, if a table contains any variable-length columns (VARCHAR, TEXT or BLOB), all CHAR columns longer than 3 are changed to VARCHARs. This doesn't affect how you use the columns in any way; in MySQL, VARCHAR is just a different way to store characters. MySQL does the conversion because it saves space and makes table operations faster. See section 10.14 What are the different row formats? Or, when should VARCHAR/CHAR be used?.
TIMESTAMP display sizes must be even and in the range from 2 to 14. If you specify a display size of 0 or greater than 14, the size is coerced to 14. Odd-valued sizes in the range from 1 to 13 are coerced to the next higher even number.

Certain other column type changes may occur if you compress a table using pack_isam. See section 10.14 What are the different row formats? Or, when should VARCHAR/CHAR be used?.

7.7 `ALTER TABLE` syntax

ALTER [IGNORE] TABLE tbl_name alter_spec [, alter_spec ...]

alter_specification:
        ADD [COLUMN] create_definition [FIRST | AFTER column_name ]
  or    ADD INDEX [index_name] (index_col_name,...)
  or    ADD UNIQUE [index_name] (index_col_name,...)
  or    ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT}
  or    CHANGE [COLUMN] old_col_name create_definition
  or    DROP [COLUMN] col_name
  or    DROP PRIMARY KEY
  or    DROP INDEX key_name
  or    RENAME [AS] new_tbl_name

ALTER TABLE allows you to change the structure of any existing table. For example, you can add or delete columns, create or destroy indexes, change the type of existing columns, or rename columns or the table itself.

ALTER TABLE works by making a temporary copy of the original table. The alteration is performed on the copy, then the original table is deleted and the new one is renamed. This is done in such a way that all updates are automatically redirected to the new table without any failed updates. While ALTER TABLE is executing, the original table is readable by other clients. Updates and writes to the table are stalled until the new table is ready.

To use ALTER TABLE, you need select, insert, delete, update, create and drop privileges on the table.
You can issue multiple ADD, ALTER, DROP and CHANGE clauses in a single ALTER TABLE statement. This is a MySQL extension to ANSI SQL92, which allows only one of each clause per ALTER TABLE statement.
IGNORE is a MySQL extension to ANSI SQL92. It controls how ALTER TABLE works if there are duplicates on unique keys in the new table. If IGNORE isn't specified, the copy is aborted and rolled back. If IGNORE is specified, then for rows with duplicates on a unique key, only the first row is used; the others are deleted.
CHANGE col_name, DROP col_name and DROP INDEX are MySQL extensions to ANSI SQL92.
The optional word COLUMN is a pure noise word and can be omitted.
If you use ALTER TABLE tbl_name RENAME AS new_name without any other options, MySQL simply renames the files that correspond to the table tbl_name. There is no need to create the temporary table.
create_definition uses the same syntax for ADD and CHANGE as for CREATE TABLE. See section 7.6 CREATE TABLE syntax.
You can rename a column using a CHANGE old_col_name create_definition clause. To do so, specify the old and new column names and the type that the column currently has. For example, to rename an INTEGER column from a to b, you can do this:
```
mysql> ALTER TABLE t1 CHANGE a b INTEGER;
```
If you want to change a column's type, but not the name, the syntax still requires two column names even if they are the same. For example:
```
mysql> ALTER TABLE t1 CHANGE b b BIGINT NOT NULL;
```
In MySQL 3.22 or later, you can use FIRST or ADD ... AFTER col_name to add a column at a specific position within a table row. The default is to add the column last (at the end of the row).
ALTER COLUMN specifies a new default value for a column or removes the old default value. If the old default is removed and the column can be NULL, the new default is NULL. If the column cannot be NULL, MySQL assigns a default value. Default value assignment is described in section 7.6 CREATE TABLE syntax.
DROP INDEX removes an index. This is a MySQL extension to ANSI SQL92.
If columns are dropped from a table, the columns are also removed from any index of which they are a part. If all columns that make up an index are dropped, the index is dropped as well.
DROP PRIMARY KEY drops the primary index. If no such index exists, it drops the first UNIQUE index in the table. (MySQL marks the first UNIQUE key as the PRIMARY KEY if no PRIMARY KEY was specified explicitly.)
When you change a column type using CHANGE, MySQL tries to convert data to the new type as well as possible.
With the C API function mysql_info(), you can find out how many records were copied, and (when IGNORE is used) how many records were deleted due to duplication of unique key values.
The FOREIGN KEY, CHECK and REFERENCES clauses don't actually do anything. The syntax for them is provided only for compatibility, to make it easier to port code from other SQL servers and to run applications that create tables with references. See section 5.2 Functionality missing from MySQL.

Here is an example that shows some of the uses of uses of ALTER TABLE. We begin with a table t1 that is created as shown below:

mysql> CREATE TABLE t1 (a INTEGER,b CHAR(10));

To rename the table from t1 to t2:

mysql> ALTER TABLE t1 RENAME t2;

To change column a from INTEGER to TINYINT NOT NULL (leaving the name the same), and change column b from CHAR(10) to CHAR(20) (and rename it from b to c):

mysql> ALTER TABLE t2 CHANGE a a TINYINT NOT NULL, CHANGE b c CHAR(20);

To add a new TIMESTAMP column named d:

mysql> ALTER TABLE t2 ADD d TIMESTAMP;

To add an index on column d, and make column a the primary key:

mysql> ALTER TABLE t2 ADD INDEX (d), ADD PRIMARY KEY (a);

To remove column c:

mysql> ALTER TABLE t2 DROP COLUMN c;

To add a new AUTO_INCREMENT integer column named c which cannot be NULL, and index it at the same time (since AUTO_INCREMENT columns must be indexed):

mysql> ALTER TABLE t2 ADD c INT UNSIGNED NOT NULL AUTO_INCREMENT,
           ADD INDEX (c);

7.8 `OPTIMIZE TABLE` syntax

OPTIMIZE TABLE tbl_name

OPTIMZE TABLE should be used if you have deleted a large part of the table or if you have made many changes to a table with variable-length rows (tables that have VARCHAR, BLOB or TEXT columns). Deleted records are maintained in a linked list and subsequent INSERT operations reuse old record positions. You can use OPTIMIZE TABLE to reclaim the unused space.

OPTIMIZE TABLE works by making a temporary copy of the original table. The old table is copied to the new table (without the unused rows), then the original table is deleted and the new one is renamed. This is done in such a way that all updates are automatically redirected to the new table without any failed updates. While OPTIMIZE TABLE is executing, the original table is readable by other clients. Updates and writes to the table are stalled until the new table is ready.

7.9 `DROP TABLE` syntax

DROP TABLE [IF EXISTS] tbl_name [, tbl_name...]

DROP TABLE removes one or more tables. All table data and the table definition are removed, so take it easy with this command!

In MySQL 3.22 or later, you can use the keywords IF EXISTS to prevent an error from occurring for tables that don't exist.

7.10 `DELETE` syntax

DELETE [LOW_PRIORITY] FROM tbl_name [WHERE where_definition]

DELETE deletes rows from tbl_name that satisfy the condition given by where_definition, and returns the number of records affected.

If you issue a DELETE with no WHERE clause, all rows are deleted. MySQL does this by recreating the table as an empty table, which is much faster than deleting each row. In this case, DELETE returns zero as the number of affected records. (MySQL can't return the number of rows that were actually deleted, since the recreate is done without opening the data files. As long as the table definition file `tbl_name.frm' is valid, the table can be recreated this way, even if the data or index files have become corrupted.)

If you specify the keyword LOW_PRIORITY, execution of the DELETE is delayed until no other clients are reading from the table.

Deleted records are maintained in a linked list and subsequent INSERT operations reuse old record positions. To get smaller files, use the OPTIMIZE TABLE statement or the isamchk utility to reorganize tables. OPTIMIZE TABLE is easier, but isamchk is faster. See section 13.5.3 Table optimization.

7.11 `SELECT` syntax

SELECT [STRAIGHT_JOIN] [DISTINCT | ALL] select_expression,...
    [INTO OUTFILE 'file_name' export_options]
    [FROM table_references
        [WHERE where_definition]
        [GROUP BY col_name,...]
        [HAVING where_definition]
        [ORDER BY {unsigned_integer | col_name} [ASC | DESC] ,...]
        [LIMIT [offset,] rows]
        [PROCEDURE procedure_name] ]

SELECT is usually used to retrieve rows selected from one or more tables. SELECT may also be used to retrieve rows computed without reference to any table. For example:

mysql> SELECT 1 + 1;
         -> 2

All keywords used must be given in exactly the order shown above. For example, a HAVING clause must come after any GROUP BY clause and before any ORDER BY clause.

A table reference may be aliased using tbl_name AS alias_name or tbl_name alias_name.
You can refer to a column as col_name, tbl_name.col_name or db_name.tbl_name.col_name. You need not specify a tbl_name or db_name.tbl_name prefix for a column reference in a SELECT statement unless the reference would be ambiguous. See section 7.1.4 Database, table, index, column and alias names, for examples of ambiguity that require the more explicit column reference forms.
A SELECT expression may be given an alias using AS. The alias is used as the expression's column name and can be used with SORT BY, ORDER BY or HAVING clauses. For example:
```
mysql> select concat(last_name,' ',first_name) AS full_name
    from mytable ORDER BY full_name;
```
The FROM table_references clause indicates a list of tables to join (i.e., one or more tables from which to select rows). This list may also contain LEFT OUTER JOIN references. See section 7.12 JOIN syntax.
In LIKE expressions, the wildcard characters `%' and `_' may be preceded with `\' to suppress their usual wildcard meaning and search for literal instances of `%' and `_'.
Columns selected for output may be referred to in ORDER BY and GROUP BY clauses using column names, column aliases or column numbers. Column numbers begin with 1.
The HAVING clause can refer to any column or alias named in the select_expression. It is applied last, just before items are sent to the client, with no optimization. Don't use HAVING for items that should be in the WHERE clause. For example, do not write this:
```
mysql> select col_name from tbl_name HAVING col_name > 0;
```
Write this instead:
```
mysql> select col_name from tbl_name WHERE col_name > 0;
```
In MySQL 3.22.5 or later, you can also write queries like this:
```
mysql> select user,max(salary) from users
           group by user HAVING max(salary)>10;
```
In older MySQL versions, you can write this instead:
```
mysql> select user,max(salary) AS sum from users
           group by user HAVING sum>10;
```
STRAIGHT_JOIN forces the optimizer to join the tables in the same order as that in which the tables are listed in the FROM clause. You can use this to speed up a query if the optimizer joins the tables in non-optimal order. See section 7.21 EXPLAIN syntax (Get information about a SELECT).
LIMIT takes one or two numeric arguments:
- If one argument is given, it indicates the maximum number of rows to return.
```
mysql> select * from table LIMIT 5;     # Retrieve first 5 rows
```
- If two arguments are given, the first specifies the offset of the first row to return, the second specifies the maximum number of rows to return. The offset of the initial row is 0 (not 1).
```
mysql> select * from table LIMIT 5,10;  # Retrieve rows 5-14
```
The SELECT ... INTO OUTFILE 'file_name' form of SELECT writes the selected rows to a file. The file is created on the server host, and cannot already exist (among other things, this prevents database tables and files such as `/etc/passwd' from being destroyed). SELECT ... INTO OUTFILE is the complement of LOAD DATA INFILE; the syntax for the export_options part of the statement consists of the same FIELDS and LINES clauses that are used with the LOAD DATA INFILE statement. See section 7.15 LOAD DATA INFILE syntax. Note that, by default, the escape character, ASCII 0 (nul) and all terminator characters will be escaped when you use INTO OUTFILE.

7.12 `JOIN` syntax

MySQL supports the following JOIN syntaxes for use in SELECT statements:

table_reference, table_reference
table_reference [CROSS] JOIN table_reference
table_reference STRAIGHT_JOIN table_reference
table_reference LEFT [OUTER] JOIN table_reference ON conditional_expr
table_reference LEFT [OUTER] JOIN table_reference USING (column_list)
table_reference NATURAL LEFT [OUTER] JOIN table_reference
{ oj table_reference LEFT OUTER JOIN table_reference ON conditional_expr }

The last LEFT OUTER JOIN syntax shown above exists only for compatibility with ODBC.

A table reference may be aliased using tbl_name AS alias_name or tbl_name alias_name.
JOIN and , (comma) are semantically identical. Both do a full join between the tables used. Normally, you specify how the tables should be linked in the WHERE condition.
The ON conditional is any conditional of the form that may be used in a WHERE clause. If there is no matching record for the right table in a LEFT JOIN, a row with all columns set to NULL is used for the right table. You can use this fact to find records in a table that have no counterpart in another table:
```
mysql> select table1.* from table1
           LEFT JOIN table2 ON table1.id=table2.id
           where table2.id is NULL;
```
This example finds all rows in table1 with an id value that is not present in table2 (i.e., all rows in table1 with no corresponding row in table2). This assumes that table2.id is declared NOT NULL, of course.
The USING column_list clause names a list of columns that must exist in both tables. A USING clause such as:
```
A LEFT JOIN B USING (C1,C2,C3...)
```
is defined to be semantically identical to an ON expression like this:
```
A.C1=B.C1 AND A.C2=B.C2 AND A.C3=B.C3...
```
The NATURAL LEFT JOIN of two tables is defined to be semantically identical to a USING with all column names that exist in both tables.
STRAIGHT_JOIN is identical to JOIN, except that the left table is always read before the right table. This can be used in the few cases where the join optimizer puts the tables in the wrong order.

Some examples:

mysql> select * from table1,table2 where table1.id=table2.id;
mysql> select * from table1 LEFT JOIN table2 ON table1.id=table2.id;
mysql> select * from table1 LEFT JOIN table2 USING (id);
mysql> select * from table1 LEFT JOIN table2 ON table1.id=table2.id
           LEFT JOIN table3 ON table3.id=table2.id;

7.13 `INSERT` syntax

    INSERT [LOW_PRIORITY] [IGNORE] [INTO] tbl_name [(col_name,...)]
        VALUES (expression,...),(...),...
or  INSERT [LOW_PRIORITY] [INTO] tbl_name [(col_name,...)]
        SELECT ...
or  INSERT [LOW_PRIORITY] [INTO] tbl_name SET col_name=expression,
        col_name=expression,...

INSERT inserts new rows into an existing table. The INSERT ... VALUES form inserts rows based on explicitly-specified values. The INSERT ... SELECT form inserts rows selected from another table or tables. The INSERT ... VALUES form with multiple value lists is supported in MySQL 3.22.5 or later. The col_name=expression syntax is supported in MySQL 3.22.10 and later.

tbl_name is the table to insert rows into. The column name list indicates which columns the rest of the statement specifies values for.

If you specify no column list, values for all columns must be given. If you don't know the order of the columns in the table, use DESCRIBE tbl_name to find out.
If you specify a column list that doesn't name all the columns in the table, columns not named are set to their default values. Default value assignment is described in section 7.6 CREATE TABLE syntax.
If MySQL was configured using the DONT_USE_DEFAULT_FIELDS option, INSERT statements generate an error unless you explicitly specify values for all columns that require a non-NULL value. See section 4.7.3 Typical configure options.
If you insert NULL into a TIMESTAMP column, the column is set to the current time. If you insert other values, the column is simply set to the value specified.

An expression may refer to any column that was set earlier in a value list. For example, you can say this:

mysql> INSERT INTO tbl_name (colA,colB) VALUES(15,colA*2);

But not this:

mysql> INSERT INTO tbl_name (colA,colB) VALUES(colB*2,15);

If you specify the keyword LOW_PRIORITY, execution of the INSERT is delayed until no other clients are reading from the table.
If you don't specify the keyword IGNORE to an INSERT with many value rows, the insert will be aborted if there is any row with a similar PRIMARY or UNIQUE key in the table. If you specify IGNORE, the rows with a duplicate key value will be not be inserted. You can check with the mysql_info() how many rows was inserted into the table.
The following conditions hold for a INSERT INTO ... SELECT statement:
- The query cannot contain an ORDER BY clause.
- The target table of the INSERT statement cannot appear in the FROM clause of the SELECT part of the query, because it's forbidden in ANSI SQL to SELECT from the same table into which you are INSERTing. (The problem is that the SELECT possibly would find records that were inserted earlier during the same run. When using sub-select clauses, the situation could easily be very confusing!)
- AUTO_INCREMENT columns work as usual.

If you use INSERT INTO ... SELECT ... or a INSERT INTO ... VALUES() statement with multiple value lists, you can use the C API function mysql_info() to get information about the query. The format of the information string is shown below:

Records: 100 Duplicates: 0 Warnings: 0

Duplicates indicates the number of rows which couldn't be inserted because some unique index value in existing rows would be duplicated. Warnings indicates the number of attempts to insert column values that were problematic in some way. Warnings can occur under any of the following conditions:

Inserting NULL into a column that has been declared NOT NULL. The column is set to its default value.
Setting a numeric column to a value that lies outside the column's range. The value is clipped to the appropriate endpoint of the range.
Setting a numeric column to a value such as `10.34 a'. The trailing garbage is stripped and the remaining numeric part is inserted. If the value doesn't make sense as a number at all, the column is set to 0.
Inserting a string into a CHAR, VARCHAR, TEXT or BLOB column that exceeds the column's maximum length. The value is truncated to the column's maximum length.
Inserting a value into a date or time column that is illegal for the column type. The column is set to the appropriate "zero" value for the type.

7.14 `REPLACE` syntax

    REPLACE [LOW_PRIORITY] [INTO] tbl_name [(col_name,...)]
        VALUES (expression,...)
or  REPLACE [LOW_PRIORITY] [INTO] tbl_name [(col_name,...)]
        SELECT ...
or  REPLACE [LOW_PRIORITY] [INTO] tbl_name SET col_name=expression,
        col_name=expression,...

REPLACE works exactly like INSERT, except that if an old record in the table has the same value on a unique index as a new record, the old record is deleted before the new record is inserted. See section 7.13 INSERT syntax.

7.15 `LOAD DATA INFILE` syntax

LOAD DATA [LOCAL] INFILE 'file_name.txt' [REPLACE | IGNORE]
    INTO TABLE tbl_name
    [FIELDS
        [TERMINATED BY '\t']
        [OPTIONALLY] ENCLOSED BY "]
        [ESCAPED BY '\\' ]]
    [LINES TERMINATED BY '\n']
    [(col_name,...)]

The LOAD DATA INFILE statement reads rows from a text file into a table at a very high speed. If the LOCAL keyword is specified, the file is read from the client host. If LOCAL is not specified, the file must be located on the server. (LOCAL is available in MySQL 3.22.6 or later.) Using LOCAL will be a bit slower than letting the server access the files directly, since the contents of the file must travel from the client host to the server host.

The mysqlimport utility can be used to read data files; it operates by sending a LOAD DATA INFILE command to the server. The --local option causes mysqlimport to read data files from the client host. You can specify the --compress option to get better performance over slow networks if the client and server support the compressed protocol.

When locating files on the server host, the server uses the following rules:

If an absolute pathname is given, the server uses the pathname as is.
If a relative pathname with one or more leading components is given, the server searches for the file relative to the server's data directory.
If a filename with no leading components is given, the server looks for the file in the database directory.

Note that these rules mean a file given as `myfile.txt' is read from the database directory, whereas a file given as `./myfile.txt' is read from the server's data directory.

For security reasons, when reading text files from the server, the files must either reside in the database directory or be readable by all. Also, to use LOAD DATA INFILE on server files, you must have the file privilege for the database. See section 6.4 How the privilege system works.

LOAD DATA INFILE is the complement of SELECT ... INTO OUTFILE. See section 7.11 SELECT syntax. To write data from a database to a file, use SELECT ... INTO OUTFILE. To read the file back into the database, use LOAD DATA INFILE. The syntax of the FIELDS and LINES clauses is the same for both commands. Both clauses are optional, but FIELDS must precede LINES if both are specified.

If you specify a FIELDS clause, each of its subclauses (TERMINATED BY, [OPTIONALLY] ENCLOSED BY and ESCAPED BY) is also optional, except that you must specify at least one of them.

If you don't specify a FIELDS clause, the defaults are the same as if you had written this:

FIELDS TERMINATED BY '\t' ENCLOSED BY " ESCAPED BY '\\'

If you don't specify a LINES clause, the default is the same as if you had written this:

LINES TERMINATED BY '\n'

In other words, the defaults cause SELECT ... INTO OUTFILE to act as follows when writing output:

Write tabs between fields
Do not enclose fields within any quoting characters
Use `\' to escape instances of tab, newline or `\' that occur within field values
Write newlines at the ends of lines

Conversely, the defaults cause LOAD DATA INFILE to act as follows when reading input:

Look for line boundaries at newlines
Break lines into fields at tabs
Do not expect fields to be enclosed within any quoting characters
Interpret occurrences of tab, newline or `\' preceded by `\' as literal characters that are part of field values

Note that to write FIELDS ESCAPED BY '\\', you must specify two backslashes for the value to be read as a single backslash.

When you use SELECT ... INTO OUTFILE in tandem with LOAD DATA INFILE to write data from a database into a file and then read the file back into the database later, the field and line handling options for both commands must match. Otherwise, LOAD DATA INFILE will not interpret the contents of the file properly. Suppose you use SELECT ... INTO OUTFILE to write a file with fields delimited by commas:

mysql> SELECT * FROM table1 INTO OUTFILE 'data.txt'
           FIELDS TERMINATED BY ','
           FROM ...

To read the comma-delimited file back in, the correct statement would be:

mysql> LOAD DATA INFILE 'data.txt' INTO TABLE table2
           FIELDS TERMINATED BY ',';

If instead you tried to read in the file with the statement shown below, it wouldn't work because it instructs LOAD DATA INFILE to look for tabs between fields:

mysql> LOAD DATA INFILE 'data.txt' INTO TABLE table2
           FIELDS TERMINATED BY '\t';

The likely result is that each input line would be interpreted as a single field.

LOAD DATA INFILE can be used to read files obtained from external sources, too. For example, a file in DBASE format will have fields separated by commas and enclosed in double quotes. If lines in the file are terminated by newlines, the command shown below illustrates the field and line handling options you would use to load the file:

mysql> LOAD DATA INFILE 'file_name.txt' INTO TABLE tbl_name
           FIELDS TERMINATED BY ',' ENCLOSED BY '"'
           LINES TERMINATED BY '\n';

Any of the field or line handling options may specify an empty string ("). If not empty, the FIELDS [OPTIONALLY] ENCLOSED BY and FIELDS ESCAPED BY values must be a single character. The FIELDS TERMINATED BY and LINES TERMINATED BY values may be more than one character. For example, to write lines that are terminated by carriage return-linefeed pairs, or to read a file containing such lines, specify a LINES TERMINATED BY '\r\n' clause.

FIELDS [OPTIONALLY] ENCLOSED BY controls quoting of fields. For output (SELECT ... INTO OUTFILE), if you omit the word OPTIONALLY, all fields are enclosed by the ENCLOSED BY character. An example of such output (using a comma as the field delimiter) is shown below:

"1","a string","100.20"
"2","a string containing a , comma","102.20"
"3","a string containing a \" quote","102.20"
"4","a string containing a \", quote and comma","102.20"

If you specify OPTIONALLY, the ENCLOSED BY character is used only to enclose CHAR and VARCHAR fields:

1,"a string",100.20
2,"a string containing a , comma",102.20
3,"a string containing a \" quote",102.20
4,"a string containing a \", quote and comma",102.20

Note that occurrences of the ENCLOSED BY character within a field value are escaped by prefixing them with the ESCAPED BY character. Also note that if you specify an empty ESCAPED BY value, you may generate output that cannot be read properly by LOAD DATA INFILE. For example, the output just shown above would appear as shown below if the escape character is empty. Observe that the second field in the fourth line contains a comma following the quote, which (erroneously) appears to terminate the field:

1,"a string",100.20
2,"a string containing a , comma",102.20
3,"a string containing a " quote",102.20
4,"a string containing a ", quote and comma",102.20

For input, the ENCLOSED BY character, if present, is stripped from the ends of field values. (This is true whether or not OPTIONALLY is specified; OPTIONALLY has no effect on input interpretation.) Occurrences of the ENCLOSED BY character preceded by the ESCAPED BY character are interpreted as part of the current field value. In addition, duplicated ENCLOSED BY characters occurring within fields are interpreted as single ENCLOSED BY characters if the field itself starts with that character. For example, if ENCLOSED BY '"' is specified, quotes are handled as shown below:

"The ""BIG"" boss"  -> The "BIG" boss
The "BIG" boss      -> The "BIG" boss
The ""BIG"" boss    -> The ""BIG"" boss

FIELDS ESCAPED BY controls how to write or read special characters. If the FIELDS ESCAPED BY character is not empty, it is used to prefix the following characters on output:

The FIELDS ESCAPED BY character
The FIELDS [OPTIONALLY] ENCLOSED BY character
The first character of the FIELDS TERMINATED BY and LINES TERMINATED BY values
ASCII 0 (what is actually written following the escape character is ASCII '0', not a zero-valued byte)

If the FIELDS ESCAPED BY character is empty, no characters are escaped. It is probably not a good idea to specify an empty escape character, particularly if field values in your data contain any of the characters in the list just given.

For input, if the FIELDS ESCAPED BY character is not empty, occurrences of that character are stripped and the following character is taken literally as part of a field value. The exceptions are an escaped `0' or `N' (e.g., \0 or \N if the escape character is `\'). These sequences are interpreted as ASCII 0 (a zero-valued byte) and NULL. See below for the rules on NULL handling.

For more information about `\'-escape syntax, see section 7.1 Literals: how to write strings and numbers.

In certain cases, field and line handling options interact:

If LINES TERMINATED BY is an empty string and FIELDS TERMINATED BY is non-empty, lines are also terminated with FIELDS TERMINATED BY.
If the FIELDS TERMINATED BY and FIELDS ENCLOSED BY values are both empty ("), a fixed-row (non-delimited) format is used. With fixed-row format, no delimiters are used between fields. Instead, column values are written and read using the "display" widths of the columns. For example, if a column is declared as INT(7), values for the column are written using 7-character fields. On input, values for the column are obtained by reading 7 characters. Fixed-row format also affects handling of NULL values; see below.

Handling of NULL values varies, depending on the FIELDS and LINES options you use:

For the default FIELDS and LINES values, NULL is written as \N for output and \N is read as NULL for input (assuming the ESCAPED BY character is `\').
If FIELDS ENCLOSED BY is not empty, a field value of the literal word NULL is read as a NULL value (this differs from the word NULL enclosed within FIELDS ENCLOSED BY characters, which is read as the string 'NULL').
If FIELDS ESCAPED BY is empty, NULL is written as the word NULL.
With fixed-row format (which happens when FIELDS TERMINATED BY and FIELDS ENCLOSED BY are both empty), NULL is written as a blank string. Note that this makes NULL values and blank values indistinguishable in the file. If you need to be able to tell the two apart when reading the file back in, you should not use fixed-row format.

The REPLACE and IGNORE keywords control handling of input records that duplicate existing records on unique key values. If you specify REPLACE, new rows replace existing rows that have the same unique key value. If you specify IGNORE, input rows that duplicate an existing row on a unique key value are skipped. If you don't specify either option, an error occurs when a duplicate key value is found, and the rest of the text file is ignored.

Some cases are not supported by LOAD DATA INFILE:

Fixed-size rows (FIELDS TERMINATED BY and FIELDS ENCLOSED BY both empty) and BLOB columns.
If you specify one separator that is the same as or a prefix of another, LOAD DATA INFILE won't be able to interpret the input properly. For example, the following FIELDS clause would cause problems:
```
FIELDS TERMINATED BY '"' ENCLOSED BY '"'
```
If FIELDS ESCAPED BY is empty, a field value that contains an occurrence of FIELDS ENCLOSED BY or LINES TERMINATED BY followed by the FIELDS TERMINATED BY value will cause LOAD DATA INFILE to stop reading a field or line too early. This happens because LOAD DATA INFILE cannot properly determine where the field or line value ends.

The following example loads all columns of the persondata table:

mysql> LOAD DATA INFILE 'persondata.txt' INTO TABLE persondata;

No field list is specified, so LOAD DATA INFILE expects input rows to contain a field for each table column. The default FIELDS and LINES values are used.

If you wish to load only some of a table's columns, specify a field list:

mysql> LOAD DATA INFILE 'persondata.txt'
           INTO TABLE persondata (col1,col2,...);

You must also specify a field list if the order of the fields in the input file differs from the order of the columns in the table, so that MySQL can tell how to match up input fields with table columns.

If a row has too few fields, the columns for which no input field is present are set to default values. TIMESTAMP columns are only set to the current time if there is a NULL value for the column, or (for the first TIMESTAMP column only) if the TIMESTAMP column is left out from the field list when a field list is specified. Default value assignment is described in section 7.6 CREATE TABLE syntax.

If an input row has too many fields, the extra fields are ignored and a warning is generated.

LOAD DATA INFILE regards all input as strings, so you can't use numeric values for ENUM or SET columns the way you can with INSERT statements. All ENUM and SET values must be given as strings!

When the LOAD DATA INFILE query finishes, you can use the C API function mysql_info() to get information about the query. The format of the information string is shown below:

Records: 1  Deleted: 0  Skipped: 0  Warnings: 0

Warnings occur under the same circumstances as when values are inserted via the INSERT statement (see section 7.13 INSERT syntax), except that LOAD DATA INFILE also generates warnings when there are too few or too many fields in the input row.

For more information about the efficiency of INSERT versus LOAD DATA INFILE and speeding up LOAD DATA INFILE, see section 10.10 How to arrange a table to be as fast/small as possible.

7.16 `UPDATE` syntax

UPDATE [LOW_PRIORITY] tbl_name SET col_name1=expr1,col_name2=expr2,...
    [WHERE where_definition]

UPDATE updates columns in existing table rows with new values. The WHERE clause, if given, specifies which rows should be updated. Otherwise all rows are updated.

If you specify the keyword LOW_PRIORITY, execution of the UPDATE is delayed until no other clients are reading from the table.

If you access a tbl_name column in an expression, UPDATE uses the current value of the column. For example, the following statement sets the age column to one more than its current value:

mysql> UPDATE persondata SET age=age+1;

UPDATE assignments are evaluated from left to right. For example, the following statement doubles the age column, then increments it:

mysql> UPDATE persondata SET age=age*2,age=age+1;

If you set a column to the value it currently has, MySQL notices this and doesn't update it.

UPDATE returns the number of rows that were actually changed. In MySQL 3.22 or later, the C API function mysql_info() returns the number of rows that were matched and updated and the number of warnings that occurred during the UPDATE.

7.17 `USE` syntax

USE db_name

The USE db_name statement tells MySQL to use the db_name database as the default database for subsequent queries. The database remains current until the end of the session, or until another USE statement is issued:

mysql> USE db1;
mysql> SELECT count(*) FROM mytable;      ; selects from db1.mytable
mysql> USE db2;
mysql> SELECT count(*) FROM mytable;      ; selects from db2.mytable

Making a particular database current by means of the USE statement does not preclude you from accessing tables in other databases. The example below accesses the author table from the db1 database and the editor table from the db2 database:

mysql> USE db1;
mysql> SELECT author_name,editor_name FROM author,db2.editor
           WHERE author.editor_id = db2.editor.editor_id;

The USE statement is provided for Sybase compatibility.

7.18 `FLUSH` syntax (clearing caches)

You should use the FLUSH command if you want to clear some of the internal caches MySQL uses.

FLUSH flush_option [,flush_option]

Where flush_option is one of the following:

HOSTS	Empties the host cache tables. You should use this if some of your hosts change IP or if you get the error message `Host ... is blocked`
LOGS	Will close and reopen the standard and the update log file. If you have specified the update log file without an extension, the extension number will be incremented by 1.
PRIVILEGES	Reloads the privileges from the privilege tables in the `mysql` database.
TABLES	Closes all open tables.

You can also access all of the above commands with the mysqladmin command line command.

7.19 `KILL` syntax.

KILL thread_id

All connections to mysqld runs in a separate thread. With the SHOW PROCESSLIST command, you can see which threads are running and kill a thread with the KILL thread_id command.

If you are not a MySQL user with the process privilege, you can only see and kill your own threads.

You can also use mysqladmin processlist and mysqladmin kill to examine and kill threads.

7.20 `SHOW` syntax (Get information about tables, columns...)

   SHOW DATABASES [LIKE wild]
or SHOW TABLES [FROM db_name] [LIKE wild]
or SHOW COLUMNS FROM tbl_name [FROM db_name] [LIKE wild]
or SHOW INDEX FROM tbl_name [FROM db_name]
or SHOW STATUS
or SHOW VARIABLES [LIKE wild]
or SHOW PROCESSLIST

SHOW provides information about databases, tables, columns or the server. If the LIKE wild part is used, the wild string should be a normal SQL wildcard string that uses the `%' and `_' wildcard characters.

SHOW FIELDS is a synonym for SHOW COLUMNS and SHOW KEYS is a synonym for SHOW INDEX.

Instead of using tbl_name FROM db_name, you can also use db_name.tbl_name. These two statements are equivalent:

mysql> SHOW INDEX FROM mytable FROM mydb;
mysql> SHOW INDEX FROM mydb.mytable;

SHOW STATUS provides server status information (like mysqladmin status). The output resembles that shown below, though the format may differ somewhat:

Uptime  Running_threads  Questions  Reloads  Open_tables
   119                1          4        1            3

SHOW VARIABLES shows the values of the some of MySQL system variables. If the default values are unsuitable, you can set most of these variables using command-line options when mysqld starts up.

SHOW PROCESSLIST shows you which threads are running. If your are not a MySQL user with the Process_priv privilege, you can only see your own threads. See section 7.19 KILL syntax.

7.21 `EXPLAIN` syntax (Get information about a `SELECT`)

EXPLAIN SELECT select_options

When you precede a SELECT statement with the keyword EXPLAIN, MySQL explains how it would process the SELECT, providing information about how tables are joined and in which order.

With the help of EXPLAIN, you can see when you must add indexes to tables to get a faster SELECT that uses indexes to find the records. You can also see if the optimizer joins the tables in an optimal order. To force the optimizer to use a specific join order for a SELECT statement, add a STRAIGHT_JOIN clause.

For non-simple joins, EXPLAIN returns a row of information for each table used in the SELECT statement. The tables are listed in the order they would be read. MySQL resolves all joins using a one-sweep multi-join method. This means that MySQL reads a row from the first table, then finds a matching row in the second table, then in the third table and so on. When all tables are processed, it outputs the selected columns and the table list is back-tracked until a table is found for which there are more matching rows. The next row is read from this table and the process continues with the next table.

Output from EXPLAIN includes the following columns:

table: The table to which the row of output refers.
type: The join type. Information about the various types is given below.
possible_keys: The possible_keys column indicates which indexes MySQL could use to find the rows in the table. If this column is empty, there are no relevant indexes. In this case, you may be able to improve the performance of your query by examining the WHERE clause to see if it refers to some column or columns that would be suitable for indexing. If so, create an appropriate index and check the query with EXPLAIN again. To see what indexes a table has, use SHOW INDEX FROM tbl_name.
key: The key column indicates the key that MySQL actually decided to use. The key is NULL if no index was chosen.
key_len: The key_len column indicates the length of the key that MySQL decided to use. The length is NULL if the key is NULL.
ref: The ref column shows which columns or constants are used with the key to select rows from the table.
rows: The rows column indicates the number of rows MySQL must examine to execute the query.
Extra: If the Extra column includes the text Only index, this means that only information from the index tree is used to retrieve information from the table (which should be much faster than scanning the entire table). If the Extra column includes the text where used, it means that a WHERE clause will be used to restrict which rows will be matched against the next table or sent to the client.

The different join types are listed below, ordered from best to worst type:

system: The table has only one row (= system table). This is a special case of the const join type.
const: The table has at most one matching row, which will be read at the start of the query. Since there is only one row, values from the column in this row can be regarded as constants by the rest of the optimizer. const tables are very fast as they are read only once!
eq_ref: One row will be read from this table for each combination of rows from the previous tables. This the best possible join type, other than the const types. It is used when all parts of an index are used by the join and the index is UNIQUE or a PRIMARY KEY.
ref: All rows with matching index values will be read from this table for each combination of rows from the previous tables. ref is used if the join uses only a leftmost prefix of the key, or if the key is not UNIQUE or a PRIMARY KEY (in other words, if the join cannot select a single row based on the key value). If the key that is used matches only a few rows, this join type is good.
range: Only rows that are in a given range will be retrieved, using an index to select the rows. The ref column indicates which index is used.
index: This is the same as ALL, except that only the index tree is scanned. This is usually faster than ALL, as the index file is usually smaller than the data file.
ALL: A full table scan will be done for each combination of rows from the previous tables. This is normally not good if the table is the first table not marked const, and usually very bad in all other cases. You normally can avoid ALL by adding more indexes, so that the row can be retrieved based on constant values or column values from earlier tables.

You can get a good indication of how good a join is by multiplying all values in the rows column of the EXPLAIN output. This should tell you roughly how many rows MySQL must examine to execute the query. This number is also used when you restrict queries with the max_join_size variable. See section 10.1 Changing the size of MySQL buffers

The following example shows how a JOIN can be optimized progressively using the information provided by EXPLAIN.

Suppose you have the SELECT statement shown below, that you examine using EXPLAIN:

EXPLAIN SELECT tt.TicketNumber, tt.TimeIn,
            tt.ProjectReference, tt.EstimatedShipDate,
            tt.ActualShipDate, tt.ClientID,
            tt.ServiceCodes, tt.RepetitiveID,
            tt.CurrentProcess, tt.CurrentDPPerson,
            tt.RecordVolume, tt.DPPrinted, et.COUNTRY,
            et_1.COUNTRY, do.CUSTNAME
        FROM tt, et, et AS et_1, do
        WHERE tt.SubmitTime IS NULL
            AND tt.ActualPC = et.EMPLOYID
            AND tt.AssignedPC = et_1.EMPLOYID
            AND tt.ClientID = do.CUSTNMBR;

For this example, assume that:

The columns being compared have been declared as follows:

Table Column Column type

tt ActualPC CHAR(10)

tt AssignedPC CHAR(10)

tt ClientID CHAR(10)

et EMPLOYID CHAR(15)

do CUSTNMBR CHAR(15)
The tables have the indexes shown below:

Table Index

tt ActualPC

tt AssignedPC

tt ClientID

et EMPLOYID (primary key)

do CUSTNMBR (primary key)
The tt.ActualPC values aren't evenly distributed.

Initially, before any optimizations have been performed, the EXPLAIN statement produces the following information:

table type possible_keys                key  key_len ref  rows  Extra
et    ALL  PRIMARY                      NULL NULL    NULL 74
do    ALL  PRIMARY                      NULL NULL    NULL 2135
et_1  ALL  PRIMARY                      NULL NULL    NULL 74
tt    ALL  AssignedPC,ClientID,ActualPC NULL NULL    NULL 3872
      range checked for each record (key map: 35)

This output indicates that MySQL is doing a full join for all tables---type is ALL for each table! This will take quite a long time, as the product of the number of rows in each table must be examined! For the case at hand, this is 74 * 2135 * 74 * 3872 = 45,268,558,720 rows. If the tables were bigger, you can only imagine how long it would take...

One problem here is that MySQL can't (yet) use indexes on columns efficiently if they are declared differently. VARCHAR and CHAR are not different in this context, unless they are not declared to be the same length. Since tt.ActualPC is declared as CHAR(10) and et.EMPLOYID is declared as CHAR(15), there is a length mismatch.

To fix this disparity between column lengths, use ALTER TABLE to lengthen ActualPC from 10 characters to 15 characters:

mysql> ALTER TABLE tt CHANGE ActualPC ActualPC VARCHAR(15);

Now tt.ActualPC and et.EMPLOYID are both VARCHAR(15). Executing the EXPLAIN statement again produces this result:

table type   possible_keys   key     key_len ref         rows    Extra
tt    ALL    AssignedPC,ClientID,ActualPC NULL NULL NULL 3872    where used
do    ALL    PRIMARY         NULL    NULL    NULL        2135
      range checked for each record (key map: 1)
et_1  ALL    PRIMARY         NULL    NULL    NULL        74
      range checked for each record (key map: 1)
et    eq_ref PRIMARY         PRIMARY 15      tt.ActualPC 1

This is not perfect, but is much better (the product of the rows values is now less by a factor of 74). This version is executed in a couple of seconds.

A second alteration can be made to eliminate the column length mismatches in the tt.AssignedPC = et_1.EMPLOYID and tt.ClientID = do.CUSTNMBR comparisons:

mysql> ALTER TABLE tt CHANGE AssignedPC AssignedPC VARCHAR(15),
                      CHANGE ClientID   ClientID   VARCHAR(15);

Now EXPLAIN produces the output shown below:

table type   possible_keys   key     key_len ref            rows     Extra
et    ALL    PRIMARY         NULL    NULL    NULL           74
tt    ref    AssignedPC,ClientID,ActualPC ActualPC 15 et.EMPLOYID 52 where used
et_1  eq_ref PRIMARY         PRIMARY 15      tt.AssignedPC  1
do    eq_ref PRIMARY         PRIMARY 15      tt.ClientID    1

This is "almost" as good as it can get.

The remaining problem is that, by default, MySQL assumes that values in the tt.ActualPC column are evenly distributed, and that isn't the case for the tt table. Fortunately, it is easy to tell MySQL about this:

shell> isamchk --analyze PATH_TO_MYSQL_DATABASE/tt
shell> mysqladmin refresh

Now the join is "perfect", and EXPLAIN produces this result:

table type   possible_keys   key     key_len ref            rows    Extra
tt    ALL    AssignedPC,ClientID,ActualPC NULL NULL NULL    3872    where used
et    eq_ref PRIMARY         PRIMARY 15      tt.ActualPC    1
et_1  eq_ref PRIMARY         PRIMARY 15      tt.AssignedPC  1
do    eq_ref PRIMARY         PRIMARY 15      tt.ClientID    1

7.22 `DESCRIBE` syntax (Get information about columns)

{DESCRIBE | DESC} tbl_name {col_name | wild}

DESCRIBE provides information about a table's columns. col_name may be a column name or a string containing the SQL `%' and `_' wildcard characters.

This statement is provided for Oracle compatibility.

The SHOW statement provides similar information. See section 7.20 SHOW syntax (Get information about tables, columns...).

7.23 `LOCK TABLES/UNLOCK TABLES` syntax

LOCK TABLES tbl_name [AS alias] {READ | [LOW_PRIORITY] WRITE}
            [, tbl_name {READ | [LOW_PRIORITY] WRITE} ...]
...
UNLOCK TABLES

LOCK TABLES locks tables for the current thread. UNLOCK TABLES releases any locks held by the current thread. All tables that are locked by the current thread are automatically unlocked when the thread issues another LOCK TABLES, or when the connection to the server is closed.

If a thread obtains a READ lock on a table, that thread (and all other threads) can only read from the table. If a thread obtains a WRITE lock on a table, then only the thread holding the lock can READ from or WRITE to the table. Other threads are blocked.

Each thread waits (without timing out) until it obtains all the locks it has requested.

WRITE locks normally have higher priority than READ locks, to ensure that updates are processed as soon as possible. This means that if one thread obtains a READ lock and then another thread requests a WRITE lock, subsequent READ lock requests will wait until the WRITE thread has gotten the lock and released it. You can use LOW_PRIORITY WRITE locks to allow other threads to obtain READ locks while the thread is waiting for the WRITE lock. You should only use LOW_PRIORITY WRITE locks if you are sure that there will eventually be time when there are no threads that have a READ lock.

When you use LOCK TABLES, you must lock all tables that you are going to use! This policy ensures that table locking is deadlock free.

Normally, you don't have to lock tables, as all single UPDATE statements are atomic; no other thread can interfere with any other currently executing SQL statement. There are a few cases when you would like to lock tables anyway:

If you are going to run many operations on a bunch of tables, it's much faster to lock the tables you are going to use. The downside is, of course, that no other thread can update a READ-locked table and no other thread can read a WRITE-locked table.
As MySQL doesn't support a transaction environment, you must use LOCK TABLES if you want to ensure that no other thread comes between a SELECT and an UPDATE. For example, the example shown below requires LOCK TABLES in order to execute safely! Without LOCK TABLES, there is a chance that another thread might insert a new row in the trans table between execution of the SELECT and UPDATE statements:
```
mysql> LOCK TABLES trans READ, customer WRITE;
mysql> SELECT SUM(value) FROM trans WHERE customer_id= some_id;
mysql> UPDATE customer SET total_value=sum_from_previous_statement
           WHERE customer_id=some_id;
mysql> UNLOCK TABLES;
```

By using incremental updates (UPDATE customer set value=value+new_value) or the LAST_INSERT_ID() function you can avoid using LOCK TABLES in many cases.

You can also solve some cases by using the user-level lock functions GET_LOCK() and RELEASE_LOCK(). These locks are saved in a hash table in the server and implemented with pthread_mutex_lock() and pthread_mutex_unlock() for high speed. See section 7.3.11 Miscellaneous functions.

See section 10.9 How MySQL locks tables, for more information on locking policy.

7.24 `SET OPTION` syntax

SET [OPTION] SQL_VALUE_OPTION= value, ...

SET OPTION sets various options that affect the operation of the server or your client. Any option you set remains in effect until the current session ends, or until you set the option to a different value.

The options are:

SQL_SELECT_LIMIT= value | DEFAULT: The maximum number of records to return from SELECT statements. If a SELECT has a LIMIT clause, the LIMIT takes precedence over the value of SQL_SELECT_LIMIT. The default value for a new connection is "unlimited". If you have changed the limit, the default value can be restored by using a SQL_SELECT_LIMIT value of DEFAULT.
SQL_BIG_TABLES= 0 | 1: If set to 1, all temporary tables are stored on disk rather than in memory. This will be a little slower, but you will not get the error The table tbl_name is full for big SELECT operations that require a large temporary table. The default value for a new connection is 0 (i.e., use in-memory temporary tables).
SQL_BIG_SELECTS= 0 | 1: If set to 1, MySQL will abort if a SELECT is attempted that probably will take a very long time. This is useful when an inadvisable WHERE statement has been issued. A big query is defined as a SELECT that probably will have to examine more than max_join_size rows. The default value for a new connection is 0 (which will allow all SELECT statements).
SQL_LOW_PRIORITY_UPDATES= 0 | 1: If set to 1, all INSERT, UPDATE and DELETE statements wait until there is no pending SELECT on the affected table.
CHARACTER SET character_set_name | DEFAULT: This maps all strings from and to the client with the given mapping. Currently the only option for character_set_name is cp1251_koi8, but you can easily add new mappings by editing the `sql/convert.cc' file in the MySQL source distribution. The default mapping can be restored by using a character_set_name value of DEFAULT. Note that the syntax for setting the CHARACTER SET options differs from the syntax for setting the other options.
SQL_LOG_OFF= 0 | 1: If set to 1, no logging will be done to the standard log for this client, if the client has the process privilege. This does not affect the update log!
SQL_UPDATE_LOG= 0 | 1: If set to 0, no logging will be done to the update log for the client, if the client has the process privilege.
TIMESTAMP= timestamp_value | DEFAULT: Set the time for this client. This is used to get the original timestamp if you use the update log to restore rows.
LAST_INSERT_ID= #: Set the value to be returned from LAST_INSERT_ID(). This is stored in the update log when you use LAST_INSERT_ID() in a command that updates a table.
INSERT_ID= #: Set the value to be used by the following INSERT command when inserting an AUTO_INCREMENT value. This is mainly used with the update log.

7.25 `GRANT` syntax (Compatibility function)

GRANT (ALL PRIVILEGES | (SELECT, INSERT, UPDATE, DELETE,
    REFERENCES (column_list), USAGE))
    ON tbl_name TO user_name,... [WITH GRANT OPTION]

The GRANT statement doesn't do anything. It is provided in MySQL only for compatibility reasons, to make it easier to port code from other SQL servers. Privileges in MySQL are handled with the MySQL grant tables. See section 6.4 How the privilege system works.

7.26 `CREATE INDEX` syntax (Compatibility function)

CREATE [UNIQUE] INDEX index_name ON tbl_name (col_name[(length]),... )

The CREATE INDEX statement doesn't do anything in MySQL prior to version 3.22. In 3.22 or later, CREATE INDEX is mapped to an ALTER TABLE call to create indexes. See section 7.7 ALTER TABLE syntax.

Normally, you create all indexes on a table at the time the table itself is created with CREATE TABLE. See section 7.6 CREATE TABLE syntax. CREATE INDEX allows you to add indexes to existing tables.

A column list of the form (col1,col2,...) creates a multiple-column index. Index values are formed by concatenating the values of the given columns.

For CHAR and VARCHAR columns, indexes can be created that use only part of a column, using col_name(length) syntax. The statement shown below creates an index using the first 10 characters of the name column:

mysql> CREATE INDEX part_of_name ON customer (name(10));

Use of partial columns for indexes can make the index file much smaller. Since most names usually differ in the first 10 characters, this index should not be much slower than an index created from the entire name column, it could save a lot of disk space and might also speed up INSERT operations!

For more information about how MySQL uses indexes, see section 10.4 How MySQL uses indexes.

7.27 `DROP INDEX` syntax (Compatibility function)

DROP INDEX index_name

DROP INDEX doesn't do anything in MySQL prior to version 3.22. In 3.22 or later, DROP INDEX is mapped to an ALTER TABLE call to drop the INDEX or UNIQUE definition. See section 7.7 ALTER TABLE syntax.

7.28 Comment syntax

MySQL supports the # to end of line and /* in-line or multiple-line */ comment styles:

mysql> select 1+1;     # This comment continues to the end of line
mysql> select 1 /* this is an in-line comment */ + 1;
mysql> select 1+
/*
this is a
multiple-line comment
*/
1;

MySQL doesn't support the `--' ANSI SQL comment style. See section 5.2.7 `--' as the start of a comment.

7.29 `CREATE FUNCTION/DROP FUNCTION` syntax

CREATE FUNCTION function_name RETURNS {STRING|REAL|INTEGER}
       SONAME shared_library_name

DROP FUNCTION function_name

A user-definable function (UDF) is a way to extend MySQL with a new function that works like native (built in) MySQL functions such as ABS() and CONCAT().

CREATE FUNCTION saves the function's name, type and shared library name in the system table func in the mysql database. You must have the insert and delete privileges for the mysql database to create and drop functions.

All active functions are reloaded each time the server starts, unless you start mysqld with the --skip-grant-tables option. In this case, UDF initialization is skipped and UDFs are unavailable. (An active function is one that has been loaded with CREATE FUNCTION and not removed with DROP FUNCTION.)

For the UDF mechanism to work, functions must be written in C or C++ and your operating system must support dynamic loading.

For information on how to write user-definable functions, see section 14 Adding new functions to MySQL.

7.30 Is MySQL picky about reserved words?

A common problem stems from trying to create a table with column names like TIMESTAMP or GROUP, the names of datatypes and functions built into MySQL. You're allowed to do it (for example, ABS is an allowed column name), but whitespace is not allowed between a function name and the `(' when using functions whose names are also column names.

The following words are explicitly reserved in MySQL. Most of them are forbidden by ANSI SQL92 as column and/or table names (for example, group). A few are reserved because MySQL needs them and is (currently) using a yacc parser:

`action`	`add`	`all`	`alter`
`after`	`and`	`as`	`asc`
`auto_increment`	`between`	`bigint`	`bit`
`binary`	`blob`	`bool`	`both`
`by`	`cascade`	`char`	`character`
`change`	`check`	`column`	`columns`
`constraint`	`create`	`cross`	`current_date`
`current_time`	`current_timestamp`	`data`	`database`
`databases`	`date`	`datetime`	`day`
`day_hour`	`day_minute`	`day_second`	`dayofmonth`
`dayofweek`	`dayofyear`	`dec`	`decimal`
`default`	`delete`	`desc`	`describe`
`distinct`	`distinctrow`	`double`	`drop`
`escaped`	`enclosed`	`enum`	`explain`
`exists`	`fields`	`first`	`float`
`float4`	`float8`	`foreign`	`from`
`for`	`full`	`function`	`grant`
`group`	`having`	`hour`	`hour_minute`
`hour_second`	`ignore`	`in`	`index`
`infile`	`insert`	`int`	`integer`
`interval`	`int1`	`int2`	`int3`
`int4`	`int8`	`into`	`if`
`is`	`join`	`key`	`keys`
`last_insert_id`	`leading`	`left`	`like`
`lines`	`limit`	`load`	`lock`
`long`	`longblob`	`longtext`	`low_priority`
`match`	`mediumblob`	`mediumtext`	`mediumint`
`middleint`	`minute`	`minute_second`	`month`
`monthname`	`natural`	`numeric`	`no`
`not`	`null`	`on`	`option`
`optionally`	`or`	`order`	`outer`
`outfile`	`partial`	`password`	`precision`
`primary`	`procedure`	`processlist`	`privileges`
`quarter`	`read`	`real`	`references`
`rename`	`regexp`	`reverse`	`repeat`
`replace`	`restrict`	`returns`	`rlike`
`second`	`select`	`set`	`show`
`smallint`	`soname`	`sql_big_tables`	`sql_big_selects`
`sql_select_limit`	`sql_low_priority_updates`	`sql_log_off`	`sql_log_update`
`straight_join`	`starting`	`status`	`string`
`table`	`tables`	`terminated`	`text`
`time`	`timestamp`	`tinyblob`	`tinytext`
`tinyint`	`trailing`	`to`	`use`
`using`	`unique`	`unlock`	`unsigned`
`update`	`usage`	`values`	`varchar`
`variables`	`varying`	`varbinary`	`with`
`write`	`where`	`year`	`year_month`
`zerofill`

The following symbols (from the table above) are disallowed by ANSI SQL but allowed by MySQL as column/table names. This is because some of these names are very natural names and a lot of people have already used them.

ACTION
BIT
DATE
ENUM
NO
TEXT
TIME
TIMESTAMP

8 Example SQL queries

8.1 Queries from twin project

At Analytikerna and Lentus, we have been doing the systems and field work for a big research project. This project is a collaboration between the Institute of Environmental Medicine at Karolinska Institutet Stockholm and the Section on Clinical Research in Aging and Psychology at the University of Southern California.

The project involves a screening part where all twins in Sweden older than 65 years are interviewed by telephone. Twins who meet certain criteria are passed on to the next stage. In this latter stage, twins who want to participate are visited by a doctor/nurse team. Some of the examinations include physical and neuropsychological examination, laboratory testing, neuroimaging, psychological status assessment, and family history collection. In addition, data are collected on medical and environmental risk factors.

More information about Twin studies can be found at http://www.imm.ki.se/TWIN/TWINUKW.HTM.

The latter part of the project is administered with a web interface written using Perl and MySQL.

Each night all data from the interviews are moved into a MySQL database.

8.1.1 Find all non-distributed twins

The following query is used to determine who goes into the second part of the project:

select
        concat(p1.id, p1.tvab)+0 as tvid,
        concat(p1.christian_name, " ", p1.surname) as Name,
        p1.postal_code as Code,
        p1.city as City,
        pg.abrev as Area,
        if(td.participation = "Aborted", "A", " ") as A,
        p1.dead as dead1,
        l.event as event1,
        td.suspect as tsuspect1,
        id.suspect as isuspect1,
        td.severe as tsevere1,
        id.severe as isevere1,
        p2.dead as dead2,
        l2.event as event2,
        h2.nurse as nurse2,
        h2.doctor as doctor2,
        td2.suspect as tsuspect2,
        id2.suspect as isuspect2,
        td2.severe as tsevere2,
        id2.severe as isevere2,
        l.finish_date
from
        twin_project as tp
        /* For Twin 1 */
        left join twin_data as td on tp.id = td.id and tp.tvab = td.tvab
        left join informant_data as id on tp.id = id.id and tp.tvab = id.tvab
        left join harmony as h on tp.id = h.id and tp.tvab = h.tvab
        left join lentus as l on tp.id = l.id and tp.tvab = l.tvab
        /* For Twin 2 */
        left join twin_data as td2 on p2.id = td2.id and p2.tvab = td2.tvab
        left join informant_data as id2 on p2.id = id2.id and p2.tvab = id2.tvab
        left join harmony as h2 on p2.id = h2.id and p2.tvab = h2.tvab
        left join lentus as l2 on p2.id = l2.id and p2.tvab = l2.tvab,
        person_data as p1,
        person_data as p2,
        postal_groups as pg
where   
        /* p1 gets main twin and p2 gets his/her twin. */
        /* ptvab is a field inverted from tvab */
        p1.id = tp.id and p1.tvab = tp.tvab and
        p2.id = p1.id and p2.ptvab = p1.tvab and
        /* Just the sceening survey */
        tp.survey_no = 5 and    
        /* Skip if partner died before 65 but allow emigration (dead=9) */
        (p2.dead = 0 or p2.dead = 9 or
         (p2.dead = 1 and
          (p2.death_date = 0 or
           (((to_days(p2.death_date) - to_days(p2.birthday)) / 365)
            >= 65))))
        and       
        (
        /* Twin is suspect */
        (td.future_contact = 'Yes' and td.suspect = 2) or
        /* Twin is suspect - Informant is Blessed */
        (td.future_contact = 'Yes' and td.suspect = 1 and id.suspect = 1) or
        /* No twin - Informant is Blessed */
        (ISNULL(td.suspect) and id.suspect = 1 and id.future_contact = 'Yes') or
        /* Twin broken off - Informant is Blessed */
        (td.participation = 'Aborted' 
         and id.suspect = 1 and id.future_contact = 'Yes') or
        /* Twin broken off - No inform - Have partner */
        (td.participation = 'Aborted' and ISNULL(id.suspect) and p2.dead = 0))
        and
        l.event = 'Finished'
        /* Get at area code */
        and substring(p1.postal_code, 1, 2) = pg.code
        /* Not already distributed */
        and (h.nurse is NULL or h.nurse=00 or h.doctor=00)
        /* Has not refused or been aborted */
        and not (h.status = 'Refused' or h.status = 'Aborted'
        or h.status = 'Died' or h.status = 'Other')
order by
        tvid;

Some explanations:

concat(p1.id, p1.tvab)+0 as tvid: We want to sort on the concatenated id and tvab in numerical order. Adding 0 to the result causes MySQL to treat the result as a number.
column id: This identifies a pair of twins. It is a key in all tables.
column tvab: This identifies a twin in a pair. It has a value of 1 or 2.
column ptvab: This is an inverse of tvab. When tvab is 1 this is 2, and vice versa. It exists to save typing and to make it easier for MySQL to optimize the query.

This query demonstrates, among other things, how to do lookups on a table from the same table with a join (p1 and p2). In the example, this is used to check whether a twin's partner died before the age of 65. If so, the row is not returned.

All of the above exist in all tables with twin-related information. We have a key on both id,tvab (all tables) and id,ptvab (person_data) to make queries faster.

On our production machine (A 200MHz UltraSparc), this query returns about 150-200 rows and takes less than one second.

The current number of records in the tables used above:

Table Rows

person_data 71074

lentus 5291

twin_project 5286

twin_data 2012

informant_data 663

harmony 381

postal_groups 100

8.1.2 Show a table on twin pair status

Each interview ends with a status code called event. The query shown below is used to display a table over all twin pairs combined by event. This indicates in how many pairs both twins are finished, in how many pairs one twin is finished and the other refused, and so on.

select
        t1.event,
        t2.event,
        count(*)
from
        lentus as t1,
        lentus as t2,
        twin_project as tp 
where
        /* We are looking at one pair at a time */
        t1.id = tp.id
        and t1.tvab=tp.tvab
        and t1.id = t2.id
        /* Just the sceening survey */
        and tp.survey_no = 5
        /* This makes each pair only appear once */
        and t1.tvab='1' and t2.tvab='2'
group by
        t1.event, t2.event;

9 MySQL Server functions

9.1 What languages are supported by MySQL?

mysqld can issue error messages in the following languages: Czech, Dutch, English (the default), French, German, Hungarian, Italian, Norwegian, Norwegian-ny, Polish, Portuguese, Spanish and Swedish.

To start mysqld with a particular language, use either the --language=lang or -L lang options. For example:

shell> mysqld --language=swedish

or:

shell> mysqld --language=/usr/local/share/swedish

Note that all language names are specified in lowercase.

The language files are located (by default) in `mysql_base_dir/share/LANGUAGE/'.

To update the error message file, you should edit the `errmsg.txt' file and execute the following command to generate the `errmsg.sys' file:

shell> comp_err errmsg.txt errmsg.sys

If you upgrade to a newer version of MySQL, remember to repeat your changes with the new `errmsg.txt' file.

9.1.1 The character set used for data and sorting

By default, MySQL uses the ISO-8859-1 (Latin1) character set. This is the character set used in the USA and western Europe.

The character set determines what characters are allowed in names and how things are sorted by the ORDER BY and GROUP BY clauses of the SELECT statement.

You can change the character set at compile time by using the --with-charset=charset option to configure. See section 4.7.1 Quick installation overview.

To add another character set to MySQL, use the following procedure:

9.1.2 Adding a new character set

Choose a name for the character set, denoted MYSET below.
Create the file `strings/ctype-MYSET.c' in the MySQL source distribution.
Look at one of the existing `ctype-*.c' files to see what needs to be defined. Note that the arrays in your file must have names like ctype_MYSET, to_lower_MYSET and so on. to_lower[] and to_upper[] are simple arrays that hold the lowercase and uppercase characters corresponding to each member of the character set. For example:
```
to_lower['A'] should contain 'a'
to_upper['a'] should contain 'A'
```
sort_order[] is a map indicating how characters should be ordered for comparison and sorting purposes. For many character sets, this is the same as to_upper[] (which means sorting will be case insensitive). MySQL will sort characters based on the value of sort_order[character]. ctype[] is an array of bit values, with one element for one character. (Note that to_lower[], to_upper[] and sort_order[] are indexed by character value, but ctype[] is indexed by character value + 1. This is an old legacy to be able to handle EOF.) You can find the following bitmask definitions in `m_ctype.h':
```
#define _U      01      /* Upper case */
#define _L      02      /* Lower case */
#define _N      04      /* Numeral (digit) */
#define _S      010     /* Spacing character */
#define _P      020     /* Punctuation */
#define _C      040     /* Control character */
#define _B      0100    /* Blank */
#define _X      0200    /* heXadecimal digit */
```
The ctype[] entry for each character should be the union of the applicable bitmask values that describe the character. For example, 'A' is an uppercase character (_U) as well as a hexidecimal digit (_X), so ctype['A'+1] should contain the value:
```
_U + _X = 01 + 0200 = 0201
```
Add a unique number for your character set to `include/m_ctype.h.in'.
Add the character set name to the CHARSETS_AVAILABLE list in configure.in.
Reconfigure, recompile and test.

9.1.3 Multi-byte character support

If you are creating a multi-byte character set, you can use the _MB macros. In `include/m_ctype.h.in', add:

#define MY_CHARSET_MYSET  X
#if MY_CHARSET_CURRENT == MY_CHARSET_MYSET
#define USE_MB
#define USE_MB_IDENT
#define ismbchar(p, end)  (...)
#define ismbhead(c)       (...)
#define mbcharlen(c)      (...)
#define MBMAXLEN          N
#endif

Where:

`MY_CHARSET_MYSET`	A unique character set value.
`USE_MB`	This character set has multi-byte characters, handled by `ismbhead()` and `mbcharlen()`
`USE_MB_IDENT`	(optional) If defined, you can use table and column names that use multi-byte characters
`ismbchar(p, e)`	return 0 if `p` is not a multi-byte character string, or the size of the character (in bytes) if it is. `p` and `e` point to the beginning and end of the string. Check from `(char)p` to `(char)e-1`.
`ismbhead(c)`	True if `c` is the first character of a multi-byte character string
`mbcharlen(c)`	Size of a multi-byte character string if `c` is the first character of such a string
`MBMAXLEN`	Size in bytes of the largest character in the set

9.2 The update log

When started with the --log-update=file_name option, mysqld writes a log file containing all SQL commands that update data. The file is written in the data directory and has a name of file_name.#, where # is a number that is incremented each time you execute mysqladmin refresh or mysqladmin flush-logs, the FLUSH LOGS statement, or restart the server.

If you use the --log or -l options, the filename is `hostname.log', and restarts and refreshes do not cause a new log file to be generated. By default, the mysql.server script starts the MySQL server with the -l option. If you need better performance when you start using MySQL in a production environment, you can remove the -l option from mysql.server.

Update logging is smart since it logs only statements that really update data. So an UPDATE or a DELETE with a WHERE that finds no rows is not written to the log. It even skips UPDATE statements that set a column to the value it already has.

If you want to update a database from update log files, you could do the following (assuming your log files have names of the form `file_name.#'):

shell> ls -1 -t -r file_name.[0-9]* | xargs cat | mysql

ls is used to get all the log files in the right order.

This can be useful if you have to revert to backup files after a crash and you want to redo the updates that occurred between the time of the backup and the crash.

You can also use the update logs when you have a mirrored database on another host and you want to replicate the changes that have been made to the master database.

9.3 How big MySQL tables can be

MySQL itself has a 4G limit on table size, and operating systems have their own file size limits. On Linux, the current limit is 2G; on Solaris 2.5.1, the limit is 4G; on Solaris 2.6, the limit is going to be 1000G. Currently, table sizes are limited to either 4G (the MySQL limit) or the operating system limit, whichever is smaller. To get more than 4G requires some changes to MySQL that are on the TODO. See section F List of things we want to add to MySQL in the future (The TODO).

If your big table is going to be read-only, you could use pack_isam to merge and compress many tables to one. pack_isam usually compresses a table by at least 50%, so you can have, in effect, much bigger tables. See section 12.3 The MySQL compressed read-only table generator.

Another solution can be the included MERGE library, which allows you to handle a collection of identical tables as one. (Identical in this case means that all tables are created with identical column information.) Currently MERGE can only be used to scan a collection of tables because it doesn't support indexes. We will add indexes to this in the near future.

10 Getting maximum performance from MySQL

10.1 Changing the size of MySQL buffers

You can get the default buffer sizes used by the mysqld server with this command:

shell> mysqld --help

This command produces a list of all mysqld options and configurable variables. The output includes the default values and looks something like this:

Possible variables for option --set-variable (-O) are:
back_log              current value: 5
join_buffer           current value: 131072
key_buffer            current value: 1048568
long_query_time       current value: 10
max_allowed_packet    current value: 1048576
max_connections       current value: 90
max_connect_errors    current value: 10
max_join_size         current value: 4294967295
max_sort_length       current value: 1024
net_buffer_length     current value: 16384
record_buffer         current value: 131072
sort_buffer           current value: 2097144
table_cache           current value: 64
tmp_table_size        current value: 1048576
thread_stack          current value: 65536
wait_timeout          current value: 28800

If there is a mysqld server currently running, you can see what values it actually is using for the variables by executing this command:

shell> mysqladmin variables

Each option is described below. Values for buffer sizes, lengths and stack sizes are given in bytes. You can specify values with a suffix of `K' or `M' to indicate kilobytes or megabytes. For example, 16M indicates 16 megabytes. Case of suffix letters does not matter; 16M and 16m are equivalent.

back_log: The number of outstanding connection requests MySQL can have. This comes into play when the main MySQL thread gets VERY many connection requests in a very short time. It then takes some time (but very short) for the main thread to check the connection and start a new thread. The back_log value indicates how many requests can be stacked during this short time before MySQL momentarily stops answering new requests. You need to increase this only if you expect a large number of connections in a short period of time. In other words, this value is the size of the listen queue for incoming TCP/IP connections. Your operating system has its own limit on the size of this queue. The manual page for the Unix system call listen(2) should have more details. Check your OS documentation for the maximum value for this variable. Attempting to set back_log higher than this maximum will be ineffective.
join_buffer: The size of the buffer that is used for full joins (joins that do not use indexes). The buffer is allocated one time for each full join between two tables. Increase this value to get a faster full join when adding indexes is not possible. (Normally the best way to get fast joins is to add indexes.)
key_buffer: Index blocks are buffered and are shared by all threads. key_buffer is the size of the buffer used for index blocks. You might want to increase this value when doing many DELETE or INSERT operations on a table with lots of indexes. To get even more speed, use LOCK TABLES. See section 7.23 LOCK TABLES/UNLOCK TABLES syntax.
max_allowed_packet: The maximum size of one packet. The message buffer is initialized to net_buffer_length bytes, but can grow up to max_allowed_packet bytes when needed. This value by default is small to catch big (possibly wrong) packets. You must increase this value if you are using big BLOB columns. It should be as big as the biggest BLOB you want to use.
max_connections: The number of simultaneous clients allowed. Increasing this value increases the number of file descriptors that mysqld requires. See below for comments on file descriptor limits.
max_join_size: Joins that are probably going to read more than max_join_size records return an error. Set this value if your users tend to perform joins without a WHERE clause that take a long time and return millions of rows.
max_sort_length: The number of bytes to use when sorting BLOB or TEXT values (only the first max_sort_length bytes of each value are used; the rest are ignored).
net_buffer_length: The communication buffer is reset to this size between queries. This should not normally be changed, but if you have very little memory, you can set it to the expected size of a query. (That is, the expected length of SQL statements sent by clients. If statements exceed this length, the buffer is automatically enlarged, up to max_allowed_packet bytes.)
record_buffer: Each thread that does a sequential scan allocates a buffer of this size for each table it scans. If you do many sequential scans, you may want to increase this value.
sort_buffer: Each thread that needs to do a sort allocates a buffer of this size. Increase this value for faster ORDER BY or GROUP BY operations. See section 16.4 Where MySQL stores temporary files.
table_cache: The number of open tables for all threads. Increasing this value increases the number of file descriptors that mysqld requires. MySQL needs two file descriptors for each unique open table. See below for comments on file descriptor limits. For information about how the table cache works, see section 10.6 How MySQL opens and closes tables.
tmp_table_size: If a temporary table exceeds this size, MySQL generates an error of the form The table tbl_name is full. Increase the value of tmp_table_size if you do many advanced GROUP BY queries.
thread_stack: The stack size for each thread. Many of the limits detected by the crash-me test are dependent on this value. The default is normally large enough. See section 11 The MySQL benchmark suite.
wait_timeout: The number of seconds the server waits for activity on a connection before closing it.

table_cache and max_connections affect the maximum number of files the server keeps open. If you increase one or both of these values, you may run up against a limit imposed by your operating system on the per-process number of open file descriptors. However, you can increase the limit on many systems. Consult your OS documentation to find out how to do this, because the method for changing the limit varies widely from system to system.

table_cache is related to max_connections. For example, for 200 open connections, you should have a table cache of at least 200 * n, where n is the maximum number of tables in a join.

MySQL uses algorithms that are very scalable, so you can usually run with very little memory or give MySQL more memory to get better performance.

If you have much memory and many tables and want maximum performance with a moderate number of clients, you should use something like this:

shell> safe_mysqld -O key_buffer=16M -O table_cache=128 \
           -O sort_buffer=4M -O record_buffer=1M &

If you have little memory and lots of connections, use something like this:

shell> safe_mysqld -O key_buffer=512k -O sort_buffer=100k \
           -O record_buffer=100k &

or even:

shell> safe_mysqld -O key_buffer=512k -O sort_buffer=16k \
           -O table_cache=32 -O record_buffer=8k -O net_buffer=1K &

If there are very many connections, "swapping problems" may occur unless mysqld has been configured to use very little memory for each connection. mysqld performs better if you have enough memory for all connections, of course.

Note that if you change an option to mysqld, it remains in effect only for that instance of the server.

To see the effects of a parameter change, do something like this:

shell> mysqld -O key_buffer=32m --help

Make sure that the --help option is last; otherwise, the effect of any options listed after it on the command line will not be reflected in the output.

10.2 How MySQL uses memory

The list below indicates some of the ways that the mysqld server uses memory. Where applicable, the name of the server variable relevant to the memory use is given.

The key buffer (variable key_buffer) is shared by all threads; Other buffers used by the server are allocated as needed.
Each connection uses some thread space, a stack (64K, variable thread_stack) and a connection buffer (variable net_buffer_length).
All threads share the same base memory.
Nothing is memory-mapped yet (except compressed tables, but that's another story). This is because the 32-bit memory space of 4GB is not large enough for most large tables. When we get a system with a 64-bit address space, we may add general support for memory-mapping.
Each request doing a sequential scan over a table allocates a read buffer (variable record_buffer).
All joins are done in one pass and most joins can be done without even using a temporary table. Most temporary tables are memory-based (HEAP) tables. Temporary tables with a big record length (calculated as the sum of all column lengths) or that contain BLOB columns are stored on disk. One current problem is that if a HEAP table exceeds the size of tmp_table_size, you get the error The table tbl_name is full. In the future, we will fix this by automatically changing the in-memory (HEAP) table to a disk-based (NISAM) table as necessary. To work around this problem, you can increase the temporary table size by setting the tmp_table_size option to mysqld, or by setting the SQL option SQL_BIG_TABLES in the client program. See section 7.24 SET OPTION syntax. In MySQL 3.20, the maximum size of the temporary table was record_buffer*16, so if you are using this version, you have to increase the value of record_buffer. You can also start mysqld with the --big-tables option to always store temporary tables on disk, however, this will affect the speed of all complicated queries.
Most requests doing a sort allocate a sort buffer and one or two temporary files. See section 16.4 Where MySQL stores temporary files.
Almost all parsing and calculating is done in a local memory store. No memory overhead is needed for small items and the normal slow memory allocation and freeing is avoided. Memory is allocated only for unexpectedly large strings (this is done with malloc() and free()).
Each index file is opened once and the data file is opened once for each concurrently-running thread. For each concurrent thread, a table structure, column structures for each column, and a buffer of size 3 * n is allocated (where n is the maximum row length, not counting BLOB columns). A BLOB uses 5 to 8 bytes plus the length of the BLOB data.
For each table having BLOB columns, a buffer is enlarged dynamically to read in larger BLOB values. If you scan a table, a buffer as large as the largest BLOB value is allocated.
Table handlers for all in-use tables are saved in a cache and managed as a FIFO. Normally the cache has 64 entries. If a table has been used by two running threads at the same time, the cache contains two entries for the table. See section 10.6 How MySQL opens and closes tables.
A mysqladmin flush-tables command closes all tables that are not in use and marks all in-use tables to be closed when the currently executing thread finishes. This will effectively free most in-use memory.

ps and other system status programs may report that mysqld uses a lot of memory. This may be caused by thread-stacks on different memory addresses. For example, the Solaris version of ps counts the unused memory between stacks as used memory. You can verify this by checking available swap with swap -s. We have tested mysqld with commercial memory-leakage detectors, so there should be no memory leaks.

10.3 How compiling and linking affects the speed of MySQL

Most of the following tests are done on Linux and with the MySQL benchmarks, but they should give some indication for other operating systems.

You get the fastest executable when you link with -static. Using Unix sockets rather than TCP/IP to connect to a database also gives better performance.

On Linux, you will get the fastest code when compiling with pgcc and -O6. To compile `sql_yacc.cc' with these options, you need 180M memory because gcc/pgcc needs a lot of memory to make all functions inline. You should also set CXX=gcc when configuring MySQL to avoid inclusion of the libstdc++ library.

If you use pgcc and compile everything with -O6, the mysqld server is 11% faster than with gcc.
If you link dynamically (without -static), the result is 13% slower.
If you connect using TCP/IP rather than Unix sockets, the result is 7.5% slower.
On a Sun sparcstation 10, gcc 2.7.3 is 13% faster than Sun Pro C++ 4.2.
On Solaris 2.5.1, MIT-pthreads is 8-12% slower than Solaris native threads.

The MySQL-Linux distribution provided by TcX is compiled with pgcc and linked statically.

10.4 How MySQL uses indexes

All indexes (PRIMARY, UNIQUE and INDEX()) are stored in B-trees. Strings are automatically prefix- and end-space compressed. See section 7.26 CREATE INDEX syntax (Compatibility function).

Indexes are used to:

Quickly find the rows that match a WHERE clause.
Retrieve rows from other tables when performing joins.
Find the MAX() or MIN() value for a specific key.
Sort or group a table if the sorting or grouping is done on a leftmost prefix of a usable key (e.g., ORDER BY key_part_1,key_part_2 ). The key is read in reverse order if all key parts are followed by DESC.
Retrieve values without consulting the data file, in some cases. If all used columns for some table are numeric and form a leftmost prefix for some key, the values may be retrieved from the index tree for greater speed.

Suppose you issue the following SELECT statement:

mysql> SELECT * FROM tbl_name WHERE col1=val1 AND col2=val2;

If a multiple-column index exists on col1 and col2, the appropriate rows can be fetched directly. If separate single-column indexes exist on col1 and col2, the optimizer decides which index will find fewer rows and uses that index to fetch the rows.

If the table has a multiple-column index, any leftmost prefix of the index can be used by the optimizer to find rows. For example, if you have a three-column index on (col1,col2,col3), you have indexed search capabilities on (col1), (col1,col2) and (col1,col2,col3).

MySQL can't use a partial index if the columns don't form a leftmost prefix of the index. Suppose you have the SELECT statements shown below:

mysql> SELECT * FROM tbl_name WHERE col1=val1;
mysql> SELECT * FROM tbl_name WHERE col2=val2;
mysql> SELECT * FROM tbl_name WHERE col2=val2 AND col3=val3;

If an index exists on (col1,col2,col3), only the first query shown above uses the index. The second and third queries do involve indexed columns, but (col2) and (col2,col3) are not leftmost prefixes of (col1,col2,col3).

MySQL also uses indexes for LIKE comparisons if the argument to LIKE is a constant string that doesn't start with a wildcard character. For example, the following SELECT statements use indexes:

mysql> select * from tbl_name where key_col LIKE "Patrick%";
mysql> select * from tbl_name where key_col LIKE "Pat%_ck%";

In the first statement, only rows with "Patrick" <= key_col < "Patricl" are considered. In the second statement, only rows with "Pat" <= key_col < "Pau" are considered.

The following SELECT statements will not use indexes:

mysql> select * from tbl_name where key_col LIKE "%Patrick%";
mysql> select * from tbl_name where key_col LIKE other_col;

In the first statement, the LIKE value begins with a wildcard character. In the second statement, the LIKE value is not a constant.

10.5 How MySQL optimizes `WHERE` clauses

(This section is incomplete; MySQL does many optimizations.)

In general, when you want to make a slow SELECT ... WHERE faster, the first thing to check is whether or not you can add an index. All references between different tables should usually be done with indexes. You can use the EXPLAIN command to determine which indexes are used for a SELECT. See section 7.21 EXPLAIN syntax (Get information about a SELECT).

Some of the optimizations performed by MySQL are listed below:

Removal of unnecessary parentheses:

   ((a AND b) AND c OR (((a AND b) AND (c AND d))))
-> (a AND b AND c) OR (a AND b AND c AND d)

Constant folding:

   (a<b AND b=c) AND a=5
-> b>5 AND b=c AND a=5

Constant condition removal (needed because of constant folding):

   (B>=5 AND B=5) OR (B=6 AND 5=5) OR (B=7 AND 5=6)
-> B=5 OR B=6

Constant expressions used by indexes are evaluated only once.
COUNT(*) on a single table without a WHERE is retrieved directly from the table information. This is also done for any NOT NULL expression when used with only one table.
Early detection of invalid constant expressions. MySQL quickly detects that some SELECT statements are impossible and returns no rows.
HAVING is merged with WHERE if you don't use GROUP BY or group functions (COUNT(), MIN()...)
For each sub join, a simpler WHERE is constructed to get a fast WHERE evaluation for each sub join and also to skip records as soon as possible.
MySQL normally uses the index that finds least number of rows. An index is used for columns that you compare with the following operators: =, >, >=, <, <=, BETWEEN and a LIKE with a non-wildcard prefix like 'something%'.

Any index that doesn't span all AND levels in the WHERE clause is not used to optimize the query. The following WHERE clauses use indexes:

... WHERE index_part1=1 AND index_part2=2
... WHERE index=1 OR A=10 AND index=2      /* index = 1 OR index = 2 */
... WHERE index_part1='hello' AND index_part_3=5
          /* optimized like "index_part1='hello'" */

These WHERE clauses do NOT use indexes:

... WHERE index_part2=1 AND index_part3=2  /* index_part_1 is not used */
... WHERE index=1 OR A=10                  /* No index */
... WHERE index_part1=1 OR index_part2=10  /* No index spans all rows */

All constant tables are read first, before any other tables in the query. A constant table is:
- An empty table or a table with 1 row.
- A table that is used with a WHERE clause on a UNIQUE index or a PRIMARY KEY, where all index parts are used with constant expressions.
All the following tables are used as constant tables:
```
mysql> SELECT * FROM t WHERE primary_key=1;
mysql> SELECT * FROM t1,t2
           WHERE t1.primary_key=1 AND t2.primary_key=t1.id;
```
The best join combination to join the tables is found by trying all possibilities :(. If all columns in ORDER BY and in GROUP BY come from the same table, then this table is preferred first when joining.
If there is an ORDER BY clause and a different GROUP BY clause, or if the ORDER BY or GROUP BY contains columns from tables other than the first table in the join queue, a temporary table is created.
Each table index is queried and the best index that spans less than 30% of the rows is used. If no such index can be found, a quick table scan is used.
In some cases, MySQL can read rows from the index without even consulting the data file. If all columns used from the index are numeric, then only the index tree is used to resolve the query.
Before each record is output, those that do not match the HAVING clause are skipped.

Some examples of queries that are very fast:

mysql> SELECT COUNT(*) FROM tbl_name;
mysql> SELECT MIN(key_part1),MAX(key_part1) FROM tbl_name;
mysql> SELECT MAX(key_part2) FROM tbl_name
           WHERE key_part_1=constant;
mysql> SELECT ... FROM tbl_name
           ORDER BY key_part1,key_part2,... LIMIT 10;
mysql> SELECT ... FROM tbl_name
           ORDER BY key_part1 DESC,key_part2 DESC,... LIMIT 10;

The following queries are resolved using only the index tree (assuming the indexed columns are numeric):

mysql> SELECT key_part1,key_part2 FROM tbl_name WHERE key_part1=val;
mysql> SELECT COUNT(*) FROM tbl_name
           WHERE key_part1=val1 and key_part2=val2;
mysql> SELECT key_part2 FROM tbl_name GROUP BY key_part1;

The following queries use indexing to retrieve the rows in sorted order without a separate sorting pass:

mysql> SELECT ... FROM tbl_name ORDER BY key_part1,key_part2,...
mysql> SELECT ... FROM tbl_name ORDER BY key_part1 DESC,key_part2 DESC,...

10.6 How MySQL opens and closes tables

The cache of open tables can grow to a maximum of table_cache (default 64; this can be changed with with the -O table_cache=# option to mysqld). A table is never closed, except when the cache is full and another thread tries to open a table or if you use mysqladmin refresh or mysqladmin flush-tables.

When the table cache fills up, the server uses the following procedure to locate a cache entry to use:

Tables that are not currently in use are released, in least-recently-used order.
If the cache is full and no tables can be released, but a new table needs to be opened, the cache is temporarily extended as necessary.
If the cache is in a temporarily-extended state and a table goes from in-use to not-in-use state, it is closed and released from the cache.

A table is opened for each concurrent access. This means that if you have two threads accessing the same table or access the table twice in the same query (with AS) the table needs to be opened twice. The first open of any table takes two file descriptors; each additional use of the table takes only one file descriptor. The extra descriptor for the first open is used for the index file; this descriptor is shared among all threads.

10.6.1 Drawbacks of creating large numbers of tables in a database

If you have many files in a directory, open, close and create operations will be slow. If you execute SELECT statements on many different tables, there will be a little overhead when the table cache is full, because for every table that has to be opened, another must be closed. You can reduce this overhead by making the table cache larger.

10.7 Why so many open tables?

When you run mysqladmin status, you'll see something like this:

Uptime: 426 Running threads: 1 Questions: 11082 Reloads: 1 Open tables: 12

This can be somewhat perplexing if you only have 6 tables.

MySQL is multithreaded, so it may have many queries on the same table at once. To minimize the problem with two threads having different states on the same file, the table is opened independently by each concurrent thread. This takes some memory and one extra file descriptor for the data file. The index file descriptor is shared between all threads.

10.8 Using symbolic links for databases and tables

You can move tables and databases from the database directory to other locations and replace them with symbolic links to the new locations. You might want to do this, for example, to move a database to a file system with more free space.

If MySQL notices that a table is a symbolically-linked, it will resolve the symlink and use the table it points to instead. This works on all systems that support the realpath() call (at least Linux and Solaris support realpath())! On systems that don't support realpath(), you should not access the table through the real path and through the symlink at the same time! If you do, the table will be inconsistent after any update.

MySQL doesn't support linking of databases by default. Things will work fine as long as you don't make a symbolic link between databases. Suppose you have a database db1 under the MySQL data directory, and then make a symlink db2 that points to db1:

shell> cd /path/to/datadir
shell> ln -s db1 db2

Now, for any table tbl_a in db1, there also appears to be a table tbl_a in db2. If one thread updates db1.tbl_a and another thread updates db2.tbl_a, there will be problems.

If you really need this, you must change the following code in `mysys/mf_format.c':

if (!lstat(to,&stat_buff))  /* Check if it's a symbolic link */
    if (S_ISLNK(stat_buff.st_mode) && realpath(to,buff))

Change the code to this:

if (realpath(to,buff))

10.9 How MySQL locks tables

All locking in MySQL is deadlock-free. This is managed by always requesting all needed locks at once at the beginning of a query and always locking the tables in the same order.

The locking method MySQL uses for WRITE locks works as follows:

If there are no locks on the table, put a write lock on it.
Otherwise, put the lock request in the write lock queue.

The locking method MySQL uses for READ locks works as follows:

If there are no write locks on the table, put a read lock on it.
Otherwise, put the lock request in the read lock queue.

When a lock is released, the lock is made available to the threads in the write lock queue, then to the threads in the read lock queue.

This means that if you have many updates on a table, SELECT statements will wait until there are no more updates.

To work around this for the case where you want to do many INSERT and SELECT operations on a table, you can insert rows in a temporary table and update the real table with the records from the temporary table once in a while.

This can be done with the following code:

mysql> LOCK TABLES real_table WRITE, insert_table WRITE;
mysql> insert into real_table select * from insert_table;
mysql> delete from insert_table;
mysql> UNLOCK TABLES;

You can use the LOW_PRIORITY or HIGH_PRIORITY options with INSERT if you want to prioritize retrieval in some specific cases. See section 7.13 INSERT syntax

You could also change the locking code in `mysys/thr_lock.c' to use a single queue. In this case, write locks and read locks would have the same priority, which might help some applications.

10.10 How to arrange a table to be as fast/small as possible

You can get better performance on a table and minimize storage space using the techniques listed below:

Declare columns to be NOT NULL if possible. It makes everything faster and you save one bit per column.
Take advantage of the fact that all columns have default values. Insert values explicitly only when the value to be inserted differs from the default. You don't have to insert a value into the first TIMESTAMP column or into an AUTO_INCREMENT column in an INSERT statement. See section 18.4.49 How can I get the unique ID for the last inserted row?.
Use the smaller integer types if possible to get smaller tables. For example, MEDIUMINT is often better than INT.
If you don't have any variable-length columns (VARCHAR, TEXT or BLOB columns), a fixed-size record format is used. This is much faster but unfortunately may waste some space. See section 10.14 What are the different row formats? Or, when should VARCHAR/CHAR be used?.
To help MySQL optimize queries better, run isamchk --analyze on a table after it has been loaded with relevant data. This updates a value for each index that indicates the average number of rows that have the same value. (For unique indexes, this is always 1, of course.)
To sort an index and data according to an index, use isamchk --sort-index --sort-records=1 (if you want to sort on index 1). If you have a unique index from which you want to read all records in order according to that index, this is a good way to make that faster.
For INSERT statements, use multiple value lists if possible. This is much faster than using separate INSERT statements.
When loading a table with data, use LOAD DATA INFILE. This is usually 20 times faster than using a lot of INSERT statements. See section 7.15 LOAD DATA INFILE syntax. You can even get more speed when loading data into a table with many indexes using the following procedure:
1. Create the table in mysql or Perl with CREATE TABLE.
2. Execute mysqladmin flush-tables.
3. Use isamchk --keys-used=0 /path/to/db/tbl_name. This will remove all usage of all indexes from the table.
4. Insert data into the table with LOAD DATA INFILE.
5. If you have pack_isam and want to compress the table, run pack_isam on it.
6. Recreate the indexes with isamchk -r -q /path/to/db/tbl_name.
7. Execute mysqladmin flush-tables.
To get some more speed for both LOAD DATA INFILE and INSERT, enlarge the key buffer. This can be done with the -O key_buffer=# option to mysqld or safe_mysqld. For example, 16M should be a good value if you have much RAM. :)
When dumping data as text files for use by other programs, use SELECT ... INTO OUTFILE. See section 7.15 LOAD DATA INFILE syntax.
When doing many successive inserts or updates, you can get more speed by locking your tables using LOCK TABLES. LOAD DATA INFILE and SELECT ...INTO OUTFILE are atomic, so you don't have to use LOCK TABLES when using them. See section 7.23 LOCK TABLES/UNLOCK TABLES syntax.

To check how fragmented your tables are, run isamchk -evi on the `.ISM' file. See section 13 Using isamchk for table maintenance and crash recovery.

10.11 Factors affecting the speed of `INSERT` statements

The time to insert a record consists of:

Connect: (3)
Sending query to server: (2)
Parsing query: (2)
Inserting record: (1 x size of record)
Inserting indexes: (1 x indexes)
Close: (1)

Where (number) is proportional time. This does not take into consideration the initial overhead to open tables (which is done once for each concurrently-running query).

The size of the table slows down the insertion of indexes by N log N (B-trees).

You can speed up insertions by locking your table and/or using multiple value lists with INSERT statements. Using multiple value lists can be up to 5 times faster than using separate inserts.

mysql> LOCK TABLES a WRITE;
mysql> INSERT INTO a VALUES (1,23),(2,34),(4,33);
mysql> INSERT INTO a VALUES (8,26),(6,29);
mysql> UNLOCK TABLES;

The main speed difference is that the index buffer is flushed to disk only once, after all INSERT statements have completed. Normally there would be as many index buffer flushes as there are different INSERT statements. Locking is not needed if you can insert all rows with a single statement.

Locking will also lower the total time of multi-connection tests, but the maximum wait time for some threads will go up (because they wait for locks). For example:

thread 1 does 1000 inserts
thread 2, 3, and 4 does 1 insert
thread 5 does 1000 inserts

If you don't use locking, 2, 3 and 4 will finish before 1 and 5. If you use locking, 2, 3 and 4 probably will not finish before 1 or 5, but the total time should be about 40% faster.

As INSERT, UPDATE and DELETE operations are very fast in MySQL, you will obtain better overall performance by adding locks around everything that does more than about 5 inserts or updates in a row. If you do very many inserts in a row, you could do a LOCK TABLES followed by a UNLOCK TABLES once in a while (about each 1000 rows) to allow other threads access to the table. This would still result in a nice performance gain.

Of course, LOAD DATA INFILE is much faster still.

10.12 Factors affecting the speed of `DELETE` statements

The time to delete a record is exactly proportional to the number of indexes. To delete records more quickly, you can increase the size of the index cache. The default index cache is 1M; to get faster deletes, it should be increased by several factors (try 16M if you have enough memory).

10.13 How do I get MySQL to run at full speed?

Start by benchmarking your problem! You can take any program from the MySQL benchmark suite (normally found in the `sql-bench' directory) and modify it for your needs. By doing this, you can try different solutions to your problem and test which is really the fastest solution for you.

Start mysqld with the correct options. More memory gives more speed if you have it. See section 10.1 Changing the size of MySQL buffers.
Create indexes to make your SELECT statements faster. See section 10.4 How MySQL uses indexes.
Optimize your column types to be as efficient as possible. For example, declere columns to be NOT NULL if possible. See section 10.10 How to arrange a table to be as fast/small as possible.
The --skip-locking option disables file locking between SQL requests. This gives greater speed but has the following consequences:
- You MUST flush all tables with mysqladmin flush-tables before you try to check or repair tables with isamchk. (isamchk -d tbl_name is always allowed, since that simply displays table information.)
- You can't run two MySQL servers on the same data files, if both are going to update the same tables.
The --skip-locking option is on by default when compiling with MIT-pthreads, because flock() isn't fully supported by MIT-pthreads on all platforms.
If updates are a problem, you can delay updates and then do many updates in a row later. Doing many updates in a row is much quicker than doing one at a time.
On FreeBSD systems, if the problem is with MIT-pthreads, upgrading to FreeBSD 3.0 (or higher) should help. This makes it possible to use Unix sockets (with FreeBSD, this is quicker than connecting using TCP/IP with MIT-pthreads) and the threads package is much more integrated.

10.14 What are the different row formats? Or, when should `VARCHAR/CHAR` be used?

MySQL dosen't have true SQL VARCHAR types.

Instead, MySQL has three different ways to store records and uses these to emulate VARCHAR.

If a table doesn't have any VARCHAR, BLOB or TEXT columns, a fixed row size is used. Otherwise a dynamic row size is used. CHAR and VARCHAR columns are treated identically from the application's point of view; both have trailing spaces removed when the columns are retrieved.

You can check the format used in a table with isamchk -d (-d means "describe the table").

MySQL has three different table formats: fixed-length, dynamic and compressed. These are compared below.

Fixed-length tables

This is the default format. It's used when the table contains no VARCHAR, BLOB or TEXT columns.
All CHAR, NUMERIC and DECIMAL columns are space-padded to the column width.
Very quick.
Easy to cache.
Easy to reconstruct after a crash, because records are located in fixed positions.
Doesn't have to be reorganized (with isamchk) unless a huge number of records are deleted and you want to return free disk space to the operating system.
Usually requires more disk space than dynamic tables.

Dynamic tables

This format is used if the table contains any VARCHAR, BLOB or TEXT columns.
All string columns are dynamic (except those with a length less than 4).
Each record is preceded by a bitmap indicating which columns are empty (") for string columns, or zero for numeric columns (this isn't the same as columns containing NULL values). If a string column has a length of zero after removal of trailing spaces, or a numeric column has a value of zero, it is marked in the bit map and not saved to disk. Non-empty strings are saved as a length byte plus the string contents.
Usually takes much less disk space than fixed-length tables.
Each record uses only as much space as is required. If a record becomes larger, it is split into as many pieces as required. This results in record fragmentation.
If you update a row with information that extends the row length, the row will be fragmented. In this case, you may have to run isamchk -r from time to time to get better performance. Use isamchk -ei tbl_name for some statistics.
Not as easy to reconstruct after a crash, because a record may be fragmented into many pieces and a link (fragment) may be missing.
The expected row length for dynamic sized records is:
```
3
+ (number of columns + 7) / 8
+ (number of char columns)
+ packed size of numeric columns
+ length of strings
+ (number of NULL columns + 7) / 8
```
There is a penalty of 6 bytes for each link. A dynamic record is linked whenever an update causes an enlargement of the record. Each new link will be at least 20 bytes, so the next enlargement will probably go in the same link. If not, there will be another link. You may check how many links there are with isamchk -ed. All links may be removed with isamchk -r.

Compressed tables

A read-only table made with the pack_isam utility. All customers with extended MySQL email support are entitled to a copy of pack_isam for their internal usage.
The uncompress code exists in all MySQL distributions so that even customers who don't have pack_isam can read tables that were compressed with pack_isam (as long as the table was compressed on the same platform).
Takes very little disk space. Minimises disk usage.
Each record is compressed separately (very little access overhead). The header for a record is fixed (1-3 bytes) depending on the biggest record in the table. Each column is compressed differently. Some of the compression types are:
- There is usually a different Huffman table for each column.
- Suffix space compression.
- Prefix space compression.
- Numbers with value 0 are stored using 1 bit.
- If values in an integer column have a small range, the column is stored using the smallest possible type. For example, a BIGINT column (8 bytes) may be stored as a TINYINT column (1 byte) if all values are in the range 0 to 255.
- If a column has only a small set of possible values, the column type is converted to ENUM.
- A column may use a combination of the above compressions.
Can handle fixed or dynamic length records, but not BLOB or TEXT columns.
Can be uncompressed with isamchk.

MySQL can support different index types, but the normal type is NISAM. This is a B-tree index and you can roughly calculate the size for the index file as (key_length+4)*0.67, summed over all keys. (This is for the worst case when all keys are inserted in sorted order.)

String indexes are space compressed. If the first index part is a string, it will also be prefix compressed. Space compression makes the index file smaller if the string column has a lot of trailing space or is a VARCHAR column that is not always used to the full length. Prefix compression helps if there are many strings with an identical prefix.

11 The MySQL benchmark suite

This should contain a technical description of the MySQL benchmark suite (and crash-me) but that description is not written yet. Currently, you should look at the code and results in the `bench' directory in the distribution (and of course on the web page at http://www.tcx.se/crash-me-choose.htmy).

It is meant to be a benchmark that will tell any user what things a given SQL implementation performs well or poorly at.

crash-me tries to determine what features a database supports and what its capabilities and limitations are by actually running queries. For example, it determines:

What column types are supported
How many indexes are supported
What functions are supported
How big a query can be
How big a VARCHAR column can be

12 MySQL Utilites

12.1 Overview of the different MySQL programs

All MySQL clients that communicate with the server using the mysqlclient library use the following environment variables:

Name Description

MYSQL_UNIX_PORT The default socket; used for connections to localhost

MYSQL_TCP_PORT The default TCP/IP port

MYSQL_PWD The default password

MYSQL_DEBUG Debug-trace options when debugging

TMPDIR The directory where temporary tables/files are created

Use of MYSQL_PWD is insecure. See section 6.2 Connecting to the MySQL server.

The `mysql' client uses the file named in the MYSQL_HISTFILE environment variable to save the command line history. The default value for the history file is `$HOME/.mysql_history', where $HOME is the value of the HOME environment variable.

All MySQL programs take many different options. However, every MySQL program provides a --help option that you can use to get a full description of the program's different options. For example, try mysql --help.

The list below briefly describes the MySQL programs:

isamchk

Utility to describe, check, optimize and repair MySQL tables. Because isamchk has many functions, it is described in its own chapter. See section 13 Using isamchk for table maintenance and crash recovery.

make_binary_release

Makes a binary release of a compiled MySQL. This could be sent by FTP to `/pub/mysql/Incoming' on ftp.tcx.se for the convenience of other MySQL users.

msql2mysql

A shell script that converts mSQL programs to MySQL. It doesn't handle all cases, but it gives a good start when converting.

mysql

mysql is a simple SQL shell (with GNU readline capabilities). It supports interactive and non-interactive use. When used interactively, query results are presented in an ASCII-table format. When used non-interactively (e.g., as a filter), the result is presented in tab-separated format. (The output format can be changed using command-line options.) You can run scripts simply like this:

shell> mysql database < script.sql > output.tab

If you have problems due to insufficient memory in the client, use the --quick option! This forces mysql to use mysql_use_result() rather than mysql_store_result() to retrieve the result set.

mysqlaccess

A script that checks the access privileges for a host, user and database combination.

mysqladmin

Utility for performing administrative operations, such as creating or dropping databases, reloading the grant tables, flushing tables to disk and reopening log files. mysqladmin can also be used to retrieve version, process and status information from the server.

mysqlbug

The MySQL bug report script. This script should always be used when filing a bug report to the MySQL list.

mysqld

The SQL daemon. This should always be running.

mysqldump

Dumps a MySQL database into a file as SQL statements or as tab-separated text files. Enhanced freeware originally by Igor Romanenko.

mysqlimport

Imports text files into their respective tables using

LOAD DATA
INFILE

. See section 12.2 Importing data from text files.

mysqlshow

Displays information about databases, tables, columns and indexes.

mysql_install_db

Creates the MySQL grant tables with default privileges. This is usually executed only once, when first installing MySQL on a system.

replace

A utility program that is used by msql2mysql, but that has more general applicability as well. replace changes strings in place in files or on the standard input. Uses a finite state machine to match longer strings first. Can be used to swap strings. For example, this command swaps a and b in the given files:

shell> replace a b b a -- file1 file2 ...

safe_mysqld

A script that starts the mysqld daemon with some safety features, such as restarting the server when an error occurs and logging runtime information to a log file.

12.2 Importing data from text files

mysqlimport provides a command line interface to the LOAD DATA INFILE SQL statement. Most options to mysqlimport correspond directly to the same options to LOAD DATA INFILE. See section 7.15 LOAD DATA INFILE syntax.

mysqlimport is invoked like this:

shell> mysqlimport [options] filename ...

For each text file named on the command line, mysqlimport strips any extension from the filename and uses the result to determine which table to import the file's contents into. For example, files named `patient.txt', `patient.text' and `patient' would all be imported into a table named patient.

mysqlimport supports the following options:

-C, --compress: Compress all information between the client and the server if both support compression.
-#, --debug[=option_string]: Trace usage of the program (for debugging).
-d, --delete: Empty the table before importing the text file.
--fields-terminated-by=...
--fields-enclosed-by=...
--fields-optionally-enclosed-by=...
--fields-escaped-by=...
--fields-terminated-by=...: These options have the same meaning as the corresponding clauses for LOAD DATA INFILE.
-f, --force: Ignore errors. For example, if a table for a text file doesn't exist, continue processing any remaining files. Without --force, mysqlimport exits if a table doesn't exist.
--help: Display a help message and exit.
-h host_name, --host=host_name: Import data to the MySQL server on the named host. The default host is localhost.
-i, --ignore: See the description for the --replace option.
-l, --lock-tables: Lock ALL tables for writing before processing any text files. This ensures that all tables are synchronized on the server.
-L, --local: Read input files from the client. By default, text files are assumed to be on the server if you connect to localhost (which is the default host).
-pyour_pass, --password[=your_pass]: The password to use when connecting to the server. If you specify no `=your_pass' part, mysqlimport solicits the password from the terminal.
-P port_num, --port=port_num: The TCP/IP port number to use for connecting to a host. (This is used for connections to hosts other than localhost, for which Unix sockets are used.)
-r, --replace: The --replace and --ignore options control handling of input records that duplicate existing records on unique key values. If you specify --replace, new rows replace existing rows that have the same unique key value. If you specify --ignore, input rows that duplicate an existing row on a unique key value are skipped. If you don't specify either option, an error occurs when a duplicate key value is found, and the rest of the text file is ignored.
-s, --silent: Silent mode. Write output only when errors occur.
-S /path/to/socket, --socket=/path/to/socket: The socket file to use when connecting to localhost (which is the default host).
-u user_name, --user=user_name: The MySQL user name to use when connecting to the server. The default value is your Unix login name.
-v, --verbose: Verbose mode. Print out more information what the program does.
-V, --version: Print version information and exit.

12.3 The MySQL compressed read-only table generator

pack_isam is an extra utility that you get when you order more than 10 licenses or extended support. Since pack_isam is distributed only in binary form, pack_isam is available only on some platforms.

Of course, all future updates to pack_isam are included in the price. pack_isam may at some time be included as standard when we get some kind of turnover for MySQL.

pack_isam works by compressing each column in the table separately. The information needed to decompress columns is read into memory when the table is opened. This results in much better performance when accessing individual records, since you only have to uncompress exactly one record, not a much larger disk block like when using Stacker on MS-DOS. Usually, pack_isam packs the data file 40%-70%.

MySQL uses memory mapping (mmap()) on compressed tables and falls back to normal read/write file usage if mmap() doesn't work.

There are currently two limitations with pack_isam:

After packing, the table is read only.
It can't pack BLOB columns, yet.

Fixing these limitations is on our TODO list but with low priority.

pack_isam is invoked like this:

shell> pack_isam [options] filename ...

Each filename should be the name of an index (`.ISM') file. If you are not in the database directory, you should specify the pathname to the file. It is permissible to omit the `.ISM' extension.

pack_isam supports the following options:

-b, --backup: Make a backup of the table as tbl_name.OLD.
-#, --debug=debug_options: Output debug log. The debug_options string often is 'd:t:o,filename'.
-f, --force: Force packing of the table even if it becomes bigger or if the temporary file exists. (pack_isam creates a temporary file named `tbl_name.TMD' while it compresses the table. If you kill pack_isam, the `.TMD' file may not be deleted. Normally, pack_isam exits with an error if it finds that `tbl_name.TMD' exists. With --force, pack_isam packs the table anyway.
-?, --help: Display a help message and exit.
-j big_tbl_name, --join=big_tbl_name: Join all tables named on the command line into a single table big_tbl_name. All tables that are to be combined MUST be identical (same column names and types, same indexes, etc.)
-p #, --packlength=#: Specify the record length storage size, in bytes. The value should be 1, 2 or 3. (pack_isam stores all rows with length pointers of 1, 2 or 3 bytes. In most normal cases, pack_isam can determine the right length value before it begins packing the file, but it may notice during the packing process that it could have used a shorter length. In this case, pack_isam will print a note that the next time you pack the same file, you could use a shorter record length.)
-s, --silent: Silent mode. Write output only when errors occur.
-t, --test: Don't pack table, only test packing it.
-T dir_name, --tmp_dir=dir_name: Use the named directory as the location in which to write the temporary table.
-v, --verbose: Verbose mode. Write info about progress and packing result.
-V, --version: Display version information and exit.
-w, --wait: Wait and retry if table is in use. If the mysqld server was invoked with the --skip-locking option, it is not a good idea to invoke pack_isam if the table might be updated during the packing process.

The sequence of commands shown below illustrates a typical table compression session:

shell> ls -l station.*
-rw-rw-r--   1 monty    my         994128 Apr 17 19:00 station.ISD
-rw-rw-r--   1 monty    my          53248 Apr 17 19:00 station.ISM
-rw-rw-r--   1 monty    my           5767 Apr 17 19:00 station.frm

shell> isamchk -dvv station

ISAM file:     station
Isam-version:  2
Creation time: 1996-03-13 10:08:58
Recover time:  1997-02-02  3:06:43
Data records:              1192  Deleted blocks:              0
Datafile: Parts:           1192  Deleted data:                0
Datafile pointer (bytes):     2  Keyfile pointer (bytes):     2
Max datafile length:   54657023  Max keyfile length:   33554431
Recordlength:               834
Record format: Fixed length

table description:
Key Start Len Index   Type                       Root  Blocksize    Rec/key
1   2     4   unique  unsigned long              1024       1024          1
2   32    30  multip. text                      10240       1024          1

Field Start Length Type
1     1     1
2     2     4
3     6     4
4     10    1
5     11    20
6     31    1
7     32    30
8     62    35
9     97    35
10    132   35
11    167   4
12    171   16
13    187   35
14    222   4
15    226   16
16    242   20
17    262   20
18    282   20
19    302   30
20    332   4
21    336   4
22    340   1
23    341   8
24    349   8
25    357   8
26    365   2
27    367   2
28    369   4
29    373   4
30    377   1
31    378   2
32    380   8
33    388   4
34    392   4
35    396   4
36    400   4
37    404   1
38    405   4
39    409   4
40    413   4
41    417   4
42    421   4
43    425   4
44    429   20
45    449   30
46    479   1
47    480   1
48    481   79
49    560   79
50    639   79
51    718   79
52    797   8
53    805   1
54    806   1
55    807   20
56    827   4
57    831   4

shell> pack_isam station.ISM
Compressing station.ISM: (1192 records)
- Calculating statistics

normal:     20  empty-space:      16  empty-zero:        12  empty-fill:  11
pre-space:   0  end-space:        12  table-lookups:      5  zero:         7
Original trees:  57  After join: 17
- Compressing file
87.14%

shell> ls -l station.*
-rw-rw-r--   1 monty    my         127874 Apr 17 19:00 station.ISD
-rw-rw-r--   1 monty    my          55296 Apr 17 19:04 station.ISM
-rw-rw-r--   1 monty    my           5767 Apr 17 19:00 station.frm

shell> isamchk -dvv station

ISAM file:     station
Isam-version:  2
Creation time: 1996-03-13 10:08:58
Recover time:  1997-04-17 19:04:26
Data records:              1192  Deleted blocks:              0
Datafile: Parts:           1192  Deleted data:                0
Datafilepointer (bytes):      3  Keyfile pointer (bytes):     1
Max datafile length:   16777215  Max keyfile length:     131071
Recordlength:               834
Record format: Compressed

table description:
Key Start Len Index   Type                       Root  Blocksize    Rec/key
1   2     4   unique  unsigned long             10240       1024          1
2   32    30  multip. text                      54272       1024          1

Field Start Length Type                         Huff tree  Bits
1     1     1      constant                             1     0
2     2     4      zerofill(1)                          2     9
3     6     4      no zeros, zerofill(1)                2     9
4     10    1                                           3     9
5     11    20     table-lookup                         4     0
6     31    1                                           3     9
7     32    30     no endspace, not_always              5     9
8     62    35     no endspace, not_always, no empty    6     9
9     97    35     no empty                             7     9
10    132   35     no endspace, not_always, no empty    6     9
11    167   4      zerofill(1)                          2     9
12    171   16     no endspace, not_always, no empty    5     9
13    187   35     no endspace, not_always, no empty    6     9
14    222   4      zerofill(1)                          2     9
15    226   16     no endspace, not_always, no empty    5     9
16    242   20     no endspace, not_always              8     9
17    262   20     no endspace, no empty                8     9
18    282   20     no endspace, no empty                5     9
19    302   30     no endspace, no empty                6     9
20    332   4      always zero                          2     9
21    336   4      always zero                          2     9
22    340   1                                           3     9
23    341   8      table-lookup                         9     0
24    349   8      table-lookup                        10     0
25    357   8      always zero                          2     9
26    365   2                                           2     9
27    367   2      no zeros, zerofill(1)                2     9
28    369   4      no zeros, zerofill(1)                2     9
29    373   4      table-lookup                        11     0
30    377   1                                           3     9
31    378   2      no zeros, zerofill(1)                2     9
32    380   8      no zeros                             2     9
33    388   4      always zero                          2     9
34    392   4      table-lookup                        12     0
35    396   4      no zeros, zerofill(1)               13     9
36    400   4      no zeros, zerofill(1)                2     9
37    404   1                                           2     9
38    405   4      no zeros                             2     9
39    409   4      always zero                          2     9
40    413   4      no zeros                             2     9
41    417   4      always zero                          2     9
42    421   4      no zeros                             2     9
43    425   4      always zero                          2     9
44    429   20     no empty                             3     9
45    449   30     no empty                             3     9
46    479   1                                          14     4
47    480   1                                          14     4
48    481   79     no endspace, no empty               15     9
49    560   79     no empty                             2     9
50    639   79     no empty                             2     9
51    718   79     no endspace                         16     9
52    797   8      no empty                             2     9
53    805   1                                          17     1
54    806   1                                           3     9
55    807   20     no empty                             3     9
56    827   4      no zeros, zerofill(2)                2     9
57    831   4      no zeros, zerofill(1)                2     9

The information printed by pack_isam is described below:

normal: The number of columns for which no extra packing is used.
empty-space: The number of columns containing values that are only spaces; these will occupy 1 bit.
empty-zero: The number of columns containing values that are only binary 0's; these will occupy 1 bit.
empty-fill: The number of integer columns that don't occupy the full byte range of their type; these are changed to a smaller type (for example, an INTEGER column may be changed to MEDIUMINT).
pre-space: The number of decimal columns that are stored with leading space. In this case, each value will contain a count for the number of leading spaces.
end-space: The number of columns that have a lot of trailing space. In this case, each value will contain a count for the number of trailing spaces.
table-lookup: The column had only a small number of different values, and that were converted to an ENUM before Huffman compression.
zero: The number of columns for which all values are zero.
Original trees: The initial number of Huffman trees.
After join: The number of distinct Huffman trees left after joining trees to save some header space.

After a table has been compressed, isamchk -dvv prints additional information about each field:

Type

The field type may contain the following descriptors:

constant: All rows have the same value.
no endspace: Don't store endspace.
no endspace, not_always: Don't store endspace and don't do end space compression for all values.
no endspace, no empty: Don't store endspace. Don't store empty values.
table-lookup: The column was converted to an ENUM.
zerofill(n): The most significant n bytes in the value are always 0 and are not stored.
no zeros: Don't store zeros.
always zero: 0 values are stored in 1 bit.

Huff tree

The Huffman tree associated with the field

Bits

The number of bits used in the Huffman tree.

13 Using `isamchk` for table maintenance and crash recovery

You can use the isamchk utility to get information about your database tables, check and repair them or optimize them. The following sections describe how to invoke isamchk (including a description of its options), how to set up a table maintenance schedule, and how to use isamchk to perform its various functions.

13.1 `isamchk` invocation syntax

isamchk is invoked like this:

shell> isamchk [options] tbl_name

The options specify what you want isamchk to do. They are described below. (You can also get a list of options by invoking isamchk --help.) With no options, isamchk simply checks your table. To get more information or to tell isamchk to take corrective action, specify options as described below and in the following sections.

tbl_name is the database table you want to check. If you run isamchk somewhere other than in the database directory, you must specify the path to the file, since isamchk has no idea where your database is located. Actually, isamchk doesn't care whether or not the files you are working on are located in a database directory; you can copy the files that correspond to a database table into another location and perform recovery operations on them there.

You can name several tables on the isamchk command line if you wish. You can also specify a name as an index file name (with the `.ISM' suffix), which allows you to specify all tables in a directory by using the pattern `*.ISM'. For example, if you are in a database directory, you can check all the tables in the directory like this:

shell> isamchk *.ISM

If you are not in the database directory, you can check all the tables there by specifying the path to the directory:

shell> isamchk /path/to/database_dir/*.ISM

You can even check all tables in all databases by specifying a wildcard with the path to the MySQL data directory:

shell> isamchk /path/to/datadir/*/*.ISM

isamchk supports the following options:

-a, --analyze: Analyze the distribution of keys. This will make some joins in MySQL faster.
-#, --debug=debug_options: Output debug log. The debug_options string often is 'd:t:o,filename'.
-d, --description: Prints some information about the table.
-e, --extend-check: Check the table VERY thoroughly. This is necessary only in extreme cases. Normally, isamchk should find all errors even without this option.
-f, --force: Overwrite old temporary files. If you use -f when checking tables (running isamchk without -r), isamchk will automatically restart with -r on any table for which an error occurs during checking.
--help: Display a help message and exit.
-i, --information: Print informational statistics about the table that is checked.
-k #, --keys-used=#: Used with -r. Tell the NISAM table handler to update only the first # indexes. This can be used to get faster inserts!
-l, --no-symlinks: Do not follow symbolic links when repairing. Normally isamchk repairs the table a symlink points at.
-q, --quick: Used with -r to get a faster repair. Normally, the original data file isn't touched; you can specify a second -q to force the original data file to be used.
-r, --recover: Recovery mode. Can fix almost anything except unique keys that aren't unique.
-o, --safe-recover: Recovery mode. Uses an old recovery method; this is slower than -r, but can handle a couple of cases that -r cannot handle.
-O var=option, --set-variable var=option: Set the value of a variable. The possible variables are listed below.
-s, --silent: Silent mode. Write output only when errors occur. You can use -s twice (-ss) to make isamchk very silent.
-S, --sort-index: Sort index blocks. This speeds up "read-next" in applications.
-R #, --sort-records=#: Sort records according to an index. This makes your data much more localized and may speed up ranged SELECT and ORDER BY operations on this index. (It may be VERY slow to do a sort the first time!) To find out a table's index numbers, use SHOW INDEX, which shows a table's indexes in the same order that isamchk sees them. Indexes are numbered beginning with 1.
-u, --unpack: Unpack a table that was packed with pack_isam.
-v, --verbose: Verbose mode. Print more information. This can be used with -d and -e. Use -v multiple times (-vv, -vvv) for more verbosity!
-V, --version: Print the isamchk version and exit.
-w, --wait: Wait if the table is locked.

Possible variables for the --set-variable (-O) option are:

keybuffer             default value: 520192
readbuffer            default value: 262136
writebuffer           default value: 262136
sortbuffer            default value: 2097144
sort_key_blocks       default value: 16
decode_bits           default value: 9

13.2 `isamchk` memory usage

Memory allocation is important when you run isamchk. isamchk uses no more memory than you specify with the -O options. If you are going to use isamchk on very large files, you should first decide how much memory you want it to use. The default is to use only about 3M to fix things. By using larger values, you can get isamchk to operate faster. For example, if you have more than 32M RAM, you could use options such as these (in addition to any other options you might specify):

shell> isamchk -O sortbuffer=16M -O keybuffer=16M \
           -O readbuffer=1M -O writebuffer=1M ...

Using -O sortbuffer=16M should probably be enough for most cases.

Be aware that isamchk uses temporary files in TMPDIR. If TMPDIR points to a memory file system, you may easily get out of memory errors.

13.3 Setting up a table maintenance regime

It is a good idea to perform table checks on a regular basis rather than waiting for problems to occur. For maintenance purposes, you can use isamchk -s to check tables. The -s option causes isamchk to run in silent mode, printing messages only when errors occur.

It's a good idea to check tables when the server starts up. For example, whenever the machine has done a reboot in the middle of an update, you usually need to check all the tables that could have been affected. (This is an "expected crashed table".) You could add a test to safe_mysqld that runs isamchk to check all tables that have been modified during the last 24 hours if there is an old `.pid' (process ID) file left after a reboot. (The `.pid' file is created by mysqld when it starts up and removed when it terminates normally. The presence of a `.pid' file at system startup time indicates that mysqld terminated abnormally.)

An even better test would be to check any table whose last-modified time is more recent than that of the `.pid' file.

You should also check your tables regularly during normal system operation. At TcX, we run a cron job to check all our important tables once a week, using a line like this in a `crontab' file:

35 0 * * 0 /path/to/isamchk -s /path/to/datadir/*/*.ISM

This prints out information about crashed tables so we can examine and repair them when needed.

As we haven't had any unexpectedly crashed tables (tables that become corrupted for reasons other than hardware trouble) for a couple of years now (this is really true), once a week is more than enough for us.

We recommend that to start with, you execute isamchk -s each night on all tables that have been updated during the last 24 hours, until you come to trust MySQL as much as we do.

13.4 Getting information about a table

To get a description of a table or statistics about it, use the commands shown below. We explain some of the information in more detail later.

isamchk -d tbl_name: Runs isamchk in "describe mode" to produce a description of your table. If you start the MySQL server using the --skip-locking option, isamchk may report an error for a table that is updated while it runs. However, since isamchk doesn't change the table in describe mode, there isn't any risk of destroying data.
isamchk -d -v tbl_name: To produce more information about what isamchk is doing, add -v to tell it to run in verbose mode.
isamchk -eis tbl_name: Shows only the most important information from a table. It is slow since it must read the whole table.
isamchk -eiv tbl_name: This is like -eis, but tells you what is being done.

Example of isamchk -d output:

ISAM file:     company.ISM
Data records:           1403698  Deleted blocks:         0
Recordlength:               226
Record format: Fixed length

table description:
Key Start Len Index   Type
1   2     8   unique  double
2   15    10  multip. text packed stripped
3   219   8   multip. double
4   63    10  multip. text packed stripped
5   167   2   multip. unsigned short
6   177   4   multip. unsigned long
7   155   4   multip. text
8   138   4   multip. unsigned long
9   177   4   multip. unsigned long
    193   1           text

Example of isamchk -d -v output:

ISAM file:     company.ISM
Isam-version:  2
Creation time: 1996-08-28 11:44:22
Recover time:  1997-01-12 18:35:29
Data records:           1403698  Deleted blocks:              0
Datafile: Parts:        1403698  Deleted data:                0
Datafilepointer (bytes):      3  Keyfile pointer (bytes):     3
Max datafile length: 3791650815  Max keyfile length: 4294967294
Recordlength:               226
Record format: Fixed length

table description:
Key Start Len Index   Type                      Root Blocksize Rec/key
1   2     8   unique  double                15845376      1024       1
2   15    10  multip. text packed stripped  25062400      1024       2
3   219   8   multip. double                40907776      1024      73
4   63    10  multip. text packed stripped  48097280      1024       5
5   167   2   multip. unsigned short        55200768      1024    4840
6   177   4   multip. unsigned long         65145856      1024    1346
7   155   4   multip. text                  75090944      1024    4995
8   138   4   multip. unsigned long         85036032      1024      87
9   177   4   multip. unsigned long         96481280      1024     178
    193   1           text

Example of isamchk -eis output:

Checking ISAM file: company.ISM
Key:  1:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
Key:  2:  Keyblocks used:  98%  Packed:   50%  Max levels:  4
Key:  3:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
Key:  4:  Keyblocks used:  99%  Packed:   60%  Max levels:  3
Key:  5:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  6:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  7:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  8:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  9:  Keyblocks used:  98%  Packed:    0%  Max levels:  4
Total:    Keyblocks used:  98%  Packed:   17%

Records:          1403698    M.recordlength:     226   Packed:             0%
Recordspace used:     100%   Empty space:          0%  Blocks/Record:   1.00
Recordblocks:     1403698    Deleteblocks:         0
Recorddata:     317235748    Deleted data:         0
Lost space:             0    Linkdata:             0

User time 1626.51, System time 232.36
Maximum resident set size 0, Integral resident set size 0
Non physical pagefaults 0, Physical pagefaults 627, Swaps 0
Blocks in 0 out 0, Messages in 0 out 0, Signals 0
Voluntary context switches 639, Involuntary context switches 28966

Example of isamchk -eiv output:

Checking ISAM file: company.ISM
Data records: 1403698   Deleted blocks:       0
- check file-size
- check delete-chain
index  1:
index  2:
index  3:
index  4:
index  5:
index  6:
index  7:
index  8:
index  9:
No recordlinks
- check index reference
- check data record references index: 1
Key:  1:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
- check data record references index: 2
Key:  2:  Keyblocks used:  98%  Packed:   50%  Max levels:  4
- check data record references index: 3
Key:  3:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
- check data record references index: 4
Key:  4:  Keyblocks used:  99%  Packed:   60%  Max levels:  3
- check data record references index: 5
Key:  5:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 6
Key:  6:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 7
Key:  7:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 8
Key:  8:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 9
Key:  9:  Keyblocks used:  98%  Packed:    0%  Max levels:  4
Total:    Keyblocks used:   9%  Packed:   17%

- check records and index references
[LOTS OF ROW NUMBERS DELETED]

Records:          1403698    M.recordlength:     226   Packed:             0%
Recordspace used:     100%   Empty space:          0%  Blocks/Record:   1.00
Recordblocks:     1403698    Deleteblocks:         0
Recorddata:     317235748    Deleted data:         0
Lost space:             0    Linkdata:             0

User time 1639.63, System time 251.61
Maximum resident set size 0, Integral resident set size 0
Non physical pagefaults 0, Physical pagefaults 10580, Swaps 0
Blocks in 4 out 0, Messages in 0 out 0, Signals 0
Voluntary context switches 10604, Involuntary context switches 122798

Here are the sizes of the data and index files for the table used in the preceding examples:

-rw-rw-r--   1 monty    tcx     317235748 Jan 12 17:30 company.ISD
-rw-rw-r--   1 davida   tcx      96482304 Jan 12 18:35 company.ISM

Explanations for the types of information isamchk produces are given below. The "keyfile" is the index file. "Record" and "row" are synonymous.

ISAM file

Name of the ISAM (index) file.

Isam-version

Version of ISAM format. Currently always 2.

Creation time

When the data file was created.

Recover time

When the index/data file was last reconstructed.

Data records

How many records are in the table.

Deleted blocks

How many deleted blocks still have reserved space. You can optimize your table to minimize this space. See section 13.5.3 Table optimization.

Datafile: Parts

For dynamic record format, this indicates how many data blocks there are. For an optimized table without fragmented records, this is the same as

Data
records

Deleted data

How many bytes of non-reclaimed deleted data there are. You can optimize your table to minimize this space. See section 13.5.3 Table optimization.

Datafile pointer

The size of the data file pointer, in bytes. It is usually 2, 3, 4 or 5 bytes. Most tables manage with 2 bytes, but this cannot be controlled from MySQL yet. For fixed tables, this is a record address. For dynamic tables, this is a byte address.

Keyfile pointer

The size of the index file pointer, in bytes. It is usually 1, 2 or 3 bytes. Most tables manage with 2 bytes, but this is calculated automatically by MySQL. It is always a block address.

Max datafile length

How long the table's data file (.ISD file) can become, in bytes.

Max keyfile length

How long the table's key file (.ISM file) can become, in bytes.

Recordlength

How much space each record takes, in bytes.

Record format

The format used to store table rows. The examples shown above use Fixed length. Other possible values are Compressed and Packed.

table description

A list of all keys in the table. For each key, some low-level information is presented:

Key: This key's number.
Start: Where in the record this index part starts.
Len: How long this index part is. For packed numbers, this should always be the full length of the column. For strings, it may be shorter than the full length of the indexed column, because you can index a prefix of a string column.
Index: unique or multip. (multiple). Indicates whether or not one value can exist multiple times in this index.
Type: What data-type this index part has. This is an NISAM data-type with the options packed, stripped or empty.
Root: Address of the root index block.
Blocksize: The size of each index block. By default this is 1024, but the value may be changed at compile time.
Rec/key: This is a statistical value used by the optimizer. It tells how many records there are per value for this key. A unique key always has a value of 1. This may be updated after a table is loaded (or greatly changed) with isamchk -a. If this is not updated at all, a default value of 30 is given.

In the first example above, the 9th key is a multi-part key with two parts.

Keyblocks used

What percentage of the keyblocks are used. Since the table used in the examples had just been reorganized with isamchk, the values are very high (very near the theoretical maximum).

Packed

MySQL tries to pack keys with a common suffix. This can only be used for CHAR/VARCHAR/DECIMAL keys. For long strings like names, this can significantly reduce the space used. In the third example above, the 4th key is 10 characters long and a 60% reduction in space is achieved.

Max levels

How deep the B-tree for this key is. Large tables with long keys get high values.

Records

How many rows are in the table.

M.recordlength

The average record length. For tables with fixed-length records, this is the exact record length.

Packed

MySQL strips spaces from the end of strings. The Packed value indicates the percentage savings achieved by doing this.

Recordspace used

What percentage of the data file is used.

Empty space

What percentage of the data file is unused.

Blocks/Record

Average number of blocks per record (i.e., how many links a fragmented record is composed of). This is always 1 for fixed-format tables. This value should stay as close to 1.0 as possible. If it gets too big, you can reorganize the table with isamchk. See section 13.5.3 Table optimization.

Recordblocks

How many blocks (links) are used. For fixed format, this is the same as the number of records.

Deleteblocks

How many blocks (links) are deleted.

Recorddata

How many bytes in the data file are used.

Deleted data

How many bytes in the data file are deleted (unused).

Lost space

If a record is updated to a shorter length, some space is lost. This is the sum of all such losses, in bytes.

Linkdata

When the dynamic table format is used, record fragments are linked with pointers (4 to 7 bytes each). Linkdata is the sum of the amount of storage used by all such pointers.

If a table has been compressed with pack_isam, isamchk -d prints additional information about each table column. See section 12.3 The MySQL compressed read-only table generator, for an example of this information and a description of what it means.

13.5 Using `isamchk` for crash recovery

The file format that MySQL uses to store data has been extensively tested, but there are always external circumstances that may cause database tables to become corrupted:

The mysqld process being killed in the middle of a write
Unexpected shutdown of the computer (for example, if the computer is turned off)
A hardware error

This chapter describes how to check for and deal with data corruption in MySQL databases.

When performing crash recovery, it is important to understand that each table tbl_name in a database corresponds to three files in the database directory:

File	Purpose
`tbl_name.frm'	Table definition (form) file
`tbl_name.ISD'	Data file
`tbl_name.ISM'	Index file

Each of these three file types is subject to corruption in various ways, but problems occur most often in data files and index files.

isamchk works by creating a copy of the `.ISD' (data) file row by row. It ends the repair stage by removing the old `.ISD' file and renaming the new file to the original file name. If you use --quick, isamchk does not create a temporary `.ISD' file, but instead assumes that the `.ISD' file is correct and only generates a new index file without touching the `.ISD' file. This is safe, because isamchk automatically detects if the `.ISD' file is corrupt and aborts the repair in this case. You can also give two --quick options to isamchk. In this case, isamchk does not abort on some errors (like duplicate key) but instead tries to resolve them by modifying the `.ISD' file. Normally the use of two --quick options is useful only if you have too little free disk space to perform a normal repair. In this case you should at least make a backup before running isamchk.

13.5.1 How to check tables for errors

To check a table, use the following commands:

isamchk tbl_name: This finds 99.99% of all errors. What it can't find is corruption that involves ONLY the data file (which is very unusual). If you want to check a table, you should normally run isamchk without options or with either the -s or --silent option.
isamchk -e tbl_name: This does a complete and thorough check of all data (-e means "extended check"). It does a check-read of every key for each row to verify that they indeed point to the correct row. This may take a LONG time on a big table with many keys. isamchk will normally stop after the first error it finds. If you want to obtain more information, you can add the --verbose (-v) option. This causes isamchk to keep going, up through a maximum of 20 errors. In normal usage, a simple isamchk (with no arguments other than the table name) is sufficient.
isamchk -e -i tbl_name: Like the previous command, but the -i option tells isamchk to print some informational statistics, too.

13.5.2 How to repair tables

The symptoms of a corrupted table are usually that queries abort unexpectedly and that you observe errors such as these:

`tbl_name.frm' is locked against change
Can't find file `tbl_name.ISM' (Errcode: ###)
Got error ### from table handler (Error 135 is an exception in this case)
Unexpected end of file
Record file is crashed

In these cases, you must repair your tables. isamchk can usually detect and fix most things that go wrong.

The repair process involves up to four stages, described below. Before you begin, you should cd to the database directory and check the permissions of the table files. Make sure they are readable by the Unix user that mysqld runs as (and to you, since you need to access the files you are checking). If it turns out you need to modify files, they must also be writable by you.

Stage 1: Checking your tables

Run isamchk *.ISM or (isamchk -e *.ISM if you have more time). Use the -s (silent) option to suppress unnecessary information.

You have to repair only those tables for which isamchk announces an error. For such tables, proceed to Stage 2.

If you get weird errors when checking (such as out of memory errors), or if isamchk crashes, go to Stage 3.

Stage 2: Easy safe repair

First, try isamchk -r -q tbl_name (-r -q means "quick recovery mode"). This will attempt to repair the index file without touching the data file. If the data file contains everything that it should and the delete links point at the correct locations within the data file, this should work and the table is fixed. Start repairing the next table. Otherwise, use the following procedure:

Make a backup of the data file before continuing.
Use isamchk -r tbl_name (-r means "recovery mode"). This will remove incorrect records and deleted records from the data file and reconstruct the index file.
If the preceding step fails, use isamchk --safe-recover tbl_name. Safe recovery mode uses an old recovery method that handles a few cases that regular recovery mode doesn't (but is slower).

If you get weird errors when repairing (such as out of memory errors), or if isamchk crashes, go to Stage 3.

Stage 3: Difficult repair

You should only reach this stage if the first 16K block in the index file is destroyed or contains incorrect information, or if the index file is missing. In this case, it's necessary to create a new index file. Do so as follows:

Move the data file to some safe place.
Use the table description file to create new (empty) data and index files:
```
shell> mysql db_name
mysql> DELETE FROM tbl_name;
mysql> quit
```
Copy the old data file back onto the newly created data file. (Don't just move the old file back onto the new file; you want to retain a copy in case something goes wrong.)

Go back to Stage 2. isamchk -r -q should work now. (This shouldn't be an endless loop).

Stage 4: Very difficult repair

You should reach this stage only if the description file has also crashed. That should never happen, because the description file isn't changed after the table is created.

Restore the description file from a backup and go back to Stage 3. You can also restore the index file and go back to Stage 2. In the latter case, you should start with isamchk -r.
If you don't have a backup but know exactly how the table was created, create a copy of the table in another database. Remove the new data file, then move the description and index files from the other database to your crashed database. This gives you new description and index files, but leaves the data file alone. Go back to Stage 2 and attempt to reconstruct the index file.

13.5.3 Table optimization

To coalesce fragmented records and eliminate wasted space resulting from deleting or updating records, run isamchk in recovery mode:

shell> isamchk -r tbl_name

This is the same as the command you might run during crash recovery, but the assumption when you are optimizing a table is that you begin with a non-corrupted table.

You can optimize a table in the same way using the SQL OPTIMIZE TABLE statement. OPTIMIZE TABLE is easier, but isamchk is faster.

isamchk also has a number of other options you can use to enhance the performance of a table:

isamchk --sort-index: Sort the index tree blocks in high-low order. This will optimize seeks and will make table scanning by key faster.
isamchk --sort-records
isamchk --analyze: Analyzes the distribution of keys in a table. This improves join performance when you retrieve records from the table later.

14 Adding new functions to MySQL

There are two ways to add new functions to MySQL:

You can add the function through the user-definable function (UDF) interface. User-definable functions are added and removed dynamically using the CREATE FUNCTION and DROP FUNCTION statements. See section 7.29 CREATE FUNCTION/DROP FUNCTION syntax.
You can add the function as a native (built in) MySQL function. Native functions are compiled into the mysqld server and become available on a permanent basis.

Each method has advantages and disadvantages:

If you write a user-definable function, you must install the object file in addition to the server itself. If you compile your function into the server, you don't need to do that.
You can add UDFs to a binary MySQL distribution. Native functions require you to modify a source distribution.
If you upgrade your MySQL distribution, you can continue to use your previously-installed UDFs. For native functions, you must repeat your modifications each time you upgrade.

Whichever method you use to add new functions, they may be used just like native functions such as ABS() or SOUNDEX().

14.1 Adding a new user-definable function

For the UDF mechanism to work, functions must be written in C or C++ and your operating system must support dynamic loading. The MySQL source distribution includes a file `sql/udf_example.cc' that defines 5 new functions. Consult this file to see how UDF calling conventions work.

For each function that you want to use in SQL statements, you should define corresponding C (or C++) functions. In the discussion below, the name "xxx" is used for an example function name. To distinquish between SQL and C/C++ usage, XXX() (uppercase) indicates a SQL function call, and xxx() (lowercase) indicates a C/C++ function call.

The C/C++ functions that you write to implement the inferface for XXX() are:

xxx() (required)

The main function. This is where the function result is computed. The correspondence between the SQL type and return type of your C/C++ function is shown below:

SQL type	C/C++ type
`STRING`	`char *`
`INTEGER`	`long long`
`REAL`	`double`

xxx_init() (optional)

The initialization function for xxx(). It can be used to:

Check the number of arguments to XXX()
Check that the arguments are of a required type, or, alternatively, tell MySQL to coerce arguments to the types you want when the main function is called
Allocate any memory required by the main function
Specify the maximum length of the result
Specify (for REAL functions) the maximum number of decimals
Specify whether or not the result can be NULL

xxx_deinit() (optional)

The deinitialization function for xxx(). It should deallocate any memory allocated by the initialization function.

When a SQL statement invokes XXX(), MySQL calls the initialization function xxx_init() to let it perform any required setup, such as argument checking or memory allocation. If xxx_init() returns an error, the SQL statement is aborted with an error message and the main and deinitialization functions are not called. Otherwise, the main function xxx() is called once for each row. After all rows have been processed, the deinitialization function xxx_deinit() is called so it can perform any required cleanup.

All functions must be thread-safe (not just the main function, but the initialization and deinitialization functions as well). This means that you are not allowed to allocate any global or static variables that change! If you need memory, you should allocate it in xxx_init() and free it in xxx_deinit().

14.1.1 UDF calling sequences

The main function should be declared as shown below. Note that the return type and parameters differ, depending on whether you will declare the SQL function XXX() to return STRING, INTEGER or REAL in the CREATE FUNCTION statement:

For STRING functions:

char *xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *result, unsigned long *length,
              char *is_null, char *error);

For INTEGER functions:

long long xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *is_null, char *error);

For REAL functions:

double xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *is_null, char *error);

The initialization and deinitialization functions are declared like this:

my_bool xxx_init(UDF_INIT *initid, UDF_ARGS *args, char *message);

void xxx_deinit(UDF_INIT *initid);

The initid parameter is passed to all three functions. It points to a UDF_INIT structure that is used to communicate information between functions. The UDF_INIT structure members are listed below. The initialization function should fill in any members that it wishes to change. (To use the default for a member, leave it unchanged.)

my_bool maybe_null

xxx_init() should set maybe_null to 1 if xxx() can return NULL. The default value is 1 if any of the arguments are declared maybe_null.

unsigned int decimals

Number of decimals. The default value is the maximum number of decimals in the arguments passed to the main function. (For example, if the function is passed 1.34, 1.345 and 1.3, the default would be 3, since 1.345 has 3 decimals.

unsigned int max_length

The maximum length of the string result. The default value differs depending on the result type of the function. For string functions, the default is the length of the longest argument. For integer functions, the default is 21 digits. For real functions, the default is 13 plus the number of decimals indicated by initid->decimals. (For numeric functions, the length includes any sign or decimal point characters.)

char *ptr

A pointer that the function can use for its own purposes. For example, functions can use initid->ptr to communicate allocated memory between functions. In xxx_init(), allocate the memory and assign it to this pointer:

initid->ptr = allocated_memory;

In xxx() and xxx_deinit(), refer to initid->ptr to use or deallocate the memory.

14.1.2 Argument processing

The args parameter points to a UDF_ARGS structure which has the members listed below:

unsigned int arg_count

The number of arguments. Check this value in the initialization function if you want your function to be called with a particular number of arguments. For example:

if (args->arg_count != 2)
{
    strcpy(message,"XXX() requires two arguments");
    return 1;
}

enum Item_result *arg_type

The types for each argument. The possible type values are STRING_RESULT, INT_RESULT and REAL_RESULT. To make sure that arguments are of a given type and return an error if they are not, check the arg_type array in the initialization function. For example:

if (args->arg_type[0] != STRING_RESULT
      && args->arg_type[1] != INT_RESULT)
{
    strcpy(message,"XXX() requires a string and an integer");
    return 1;
}

As an alternative to requiring your function's arguments to be of particular types, you can use the initialization function to set the arg_type elements to the types you want. This causes MySQL to coerce arguments to those types for each call to xxx(). For example, to specify coercion of the first two arguments to string and integer, do this in xxx_init():

args->arg_type[0] = STRING_RESULT;
args->arg_type[1] = INT_RESULT;

char **args

args->args communicates information to the initialization function about the general nature of the arguments your function was called with. For a constant argument i, args->args[i] points to the argument value. (See below for instructions on how to access the value properly.) For a non-constant argument, args->args[i] is 0. A constant argument is an expression that uses only constants, such as 3 or 4*7-2 or SIN(3.14). A non-constant argument is an expression that refers to values that may change from row to row, such as column names or functions that are called with non-constant arguments. For each invocation of the main function, args->args contains the actual arguments that are passed for the row currently being processed. Functions can refer to an argument i as follows:

An argument of type STRING_RESULT is given as a string pointer plus a length, to allow handling of binary data or data of arbitrary length. The string contents are available as args->args[i] and the string length is args->lengths[i]. You should not assume that strings are null-terminated.
For an argument of type INT_RESULT, you must cast args->args[i] to a long long value:
```
long long int_val;
int_val = *((long long*) args->args[i]);
```
For an argument of type REAL_RESULT, you must cast args->args[i] to a double value:
```
double    real_val;
real_val = *((double*) args->args[i]);
```

unsigned long *lengths

For the initialization function, the lengths array indicates the maximum string length for each argument. For each invocation of the main function, lengths contains the actual lengths of any string arguments that are passed for the row currently being processed. For arguments of types INT_RESULT or REAL_RESULT, lengths still contains the maximum length of the argument (as for the initialization function).

14.1.3 Return values and error handling

The initialization function should return 0 if no error occurred and 1 otherwise. If an error occurs, xxx_init() should store a null-terminated error message in the message parameter. The message will be returned to the client. The message buffer is MYSQL_ERRMSG_SIZE characters long, but you should try to keep the message to less than 80 characters so that it fits the width of a standard terminal screen.

The return value of the main function xxx() is the function value, for long long and double functions. For string functions, the string is returned in the result and length arguments. result is a buffer at least 255 bytes long. Set these to the contents and length of the return value. For example:

memcpy(result, "result string", 13);
*length = 13;

The string function return value normally also points to the result.

To indicate a return value of NULL in the main function, set is_null to 1:

*is_null = 1;

To indicate an error return in the main function, set the error parameter to 1:

*error = 1;

If xxx() sets *error to 1 for any row, the function value is NULL for the current row and for any subsequent rows processed by the statement in which XXX() was invoked. (xxx() will not even be called for subsequent rows.) Note: in MySQL versions prior to 3.22.10, you should set both *error and *is_null:

*error = 1;
*is_null = 1;

14.1.4 Compiling and installing user-definable functions

Files implementing UDFs must be compiled and installed on the host where the server runs. This process is described below for the example UDF file `udf_example.cc' that is included in the MySQL source distribution. This file contains the following functions:

metaphon() returns a metaphon string of the string argument. This is something like a soundex string, but it's more tuned for English.
myfunc_double() returns the sum of the ASCII values of the characters in its arguments, divided by the sum of the length of its arguments.
myfunc_int() returns the sum of the length of its arguments.
lookup() returns the IP number for a hostname.
reverse_lookup() returns the hostname for an IP number. The function may be called with a string "xxx.xxx.xxx.xxx" or four numbers.

A dynamically-loadable file should be compiled as a sharable object file, using a command something like this:

shell> gcc -shared -o udf_example.so myfunc.cc

You can easily find out the correct compiler options for your system by running this command in the `sql' directory of your MySQL source tree:

shell> make udf_example.o

You should run a compile command similar to the one that make displays, except that you should remove the -c option near the end of the line and add -o udf_example.so to the end of the line. (On some systems, you may need to leave the -c on the command.)

Once you compile a shared object containing UDFs, you must install it and tell MySQL about it. Compiling a shared object from `udf_example.cc' produces a file named something like `udf_example.so' (the exact name may vary from platform to platform). Copy this file to some directory searched by ld, such as `/usr/lib'.

After the library is installed, notify mysqld about the new functions with these commands:

mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME "udf_example.so";
mysql> CREATE FUNCTION myfunc_double RETURNS REAL SONAME "udf_example.so";
mysql> CREATE FUNCTION myfunc_int RETURNS INTEGER SONAME "udf_example.so";
mysql> CREATE FUNCTION lookup RETURNS STRING SONAME "udf_example.so";
mysql> CREATE FUNCTION reverse_lookup RETURNS STRING SONAME "udf_example.so";

Functions can be deleted using DROP FUNCTION:

mysql> DROP FUNCTION metaphon;
mysql> DROP FUNCTION myfunc_double;
mysql> DROP FUNCTION myfunc_int;
mysql> DROP FUNCTION lookup;
mysql> DROP FUNCTION reverse_lookup;

The CREATE FUNCTION and DROP FUNCTION statements update the system table func in the mysql database. The function's name, type and shared library name are saved in the table. You must have the insert and delete privileges for the mysql database to create and drop functions.

You should not use CREATE FUNCTION to add a function that has already been created. If you need to reinstall a function, you should remove it with DROP FUNCTION and then reinstall it with CREATE FUNCTION. You would need to do this, for example, if you recompile a new version of your function, so that mysqld gets the new version. Otherwise the server will continue to use the old version.

Active functions are reloaded each time the server starts, unless you start mysqld with the --skip-grant-tables option. In this case, UDF initialization is skipped and UDFs are unavailable. (An active function is one that has been loaded with CREATE FUNCTION and not removed with DROP FUNCTION.)

14.2 Adding a new native function

The procedure for adding a new native function is described below. Note that you cannot add native functions to a binary distribution since the procedure involves modifying MySQL source code. You must compile MySQL yourself from a source distribution. Also note that if you migrate to another version of MySQL (e.g., when a new version is released), you will need to repeat the procedure with the new version.

To add a new native MySQL function, follow these steps:

Add one line to `sql_lex.cc' that defines the function name in the sql_functions[] array.
Add two lines to `sql_yacc.yy'. One indicates the preprocessor symbol that yacc should define (this should be added at the beginning of the file). Then define the function parameters and add an "item" with these parameters to the simple_expr parsing rule. For an example, check all occurrences of SOUNDEX in `sql_yacc.yy' to see how this is done.
In `item_func.h', declare a class inheriting from Item_num_func or Item_str_func, depending on whether your function returns a number or a string.
In `item_func.cc', add one of the following declarations, depending on whether you are defining a numeric or string function:
```
double   Item_func_newname::val()
longlong Item_func_newname::val_int()
String  *Item_func_newname::Str(String *str)
```
You should probably also define the following function:
```
void Item_func_newname::fix_length_and_dec()
```
This function should at least calculate max_length based on the given arguments. max_length is the maximum number of characters the function may return. This function should also set maybe_null = 0 if the main function can't return a NULL value.

All functions must be thread-safe.

For string functions, there are some additional considerations to be aware of:

The String *str argument provides a string buffer that may be used to hold the result.
The function should return the string that holds the result.
All current string functions try to avoid allocating any memory unless absolutely necessary!

15 MySQL ODBC Support

MySQL provides support for ODBC by means of the MyODBC program.

15.1 Operating systems supported by MyODBC

MyODBC is a 32-bit ODBC (2.50) level 0 driver for Windows95 and Windows NT. We hope somebody will port it to Windows 3.x.

15.2 How to report problems with MyODBC

ODBC has been tested with Access, Admndemo.exe, C++-Builder, ColdFusion (on Solaris), Delphi, Excel, FileMaker Pro, FoxPro, perl DBD-ODBC, Powerbuilder, VC++ and Visual Basic, among others.

If you encounter difficulties, we would like to have the log file from the ODBC manager (the log you get when requesting logs from ODBCADMIN) and a MyODBC log. This will help shed some light on any problems.

To get a MyODBC log, please add 4 to the "Options" argument on the MyODBC connect/configure screen. The log will be written to file `c:\tmp\myodbc.log'.

15.3 Programs known to work with MyODBC

Most programs should work with MyODBC, but for each of those listed below, we have tested it ourselves or gotten confirmation from some user that it works:

Program

Comment

Access

To make Access work:

You should have a primary key in the table.
You should have a timestamp.
Only use double float fields. Access fails when comparing with single floats.
Set 2 in the options field when connecting to MySQL.

Excel

Works. Some tips:

If you have problems with dates, try to select them as strings using the CONCAT() function. For example:
```
select CONCAT(rise_time), CONCAT(set_time)
    from sunrise_sunset;
```
Values retrieved as strings this way should be correctly recognized as time values by Excel97. The purpose of CONCAT() in this example is to fool ODBC into thinking the column is of "string type". Without the CONCAT(), ODBC knows the column is of time type, and Excel does not understand that. Note that this is a bug in Excel, because it automatically converts a string to a time. This would be great if the source was a text file, but is plain stupid when the source is an ODBC connection that reports exact types for each column.

odbcadmin

Test program for ODBC.

Delphi

You must use DBE 3.2 or newer.

C++Builder

Tested with BDE 3.0. The only known problem is that when the table schema changes, query fields are not updated. BDE however does not seem to recognize primary keys, only the index PRIMARY, though this has not been a problem.

15.4 How to fill in the various fields in the ODBC administrator program

There are three possibilities for specifying the server name on Windows95:

Use the IP address of the server.
Add a file `lmhosts' with the following info:
```
ip hostname
```
For example:
```
194.216.84.21 my
```
Configure the PC to use DNS.

Example of how to fill in the "ODBC setup":

Windows DNS name:   test
Description:        This is my test database
MySql Database:     test
Server:             194.216.84.21
User:               monty
Password:           my_password
Port:

The value for the Windows DNS name field is any name that is unique in your windows ODBC setup.

You don't have to specify values for the Server, User, Password or Port fields in the ODBC setup screen. However, if you do, the values will be used as the defaults later when you attempt to make a connection. You have the option of changing the values at that time.

If the port number is not given, the default port (3306) is used.

15.5 How to get the value of an `AUTO_INCREMENT` column in ODBC

A common problem is how to get the value of an automatically-generated ID from an INSERT. With ODBC, you can do something like this (assuming that auto is an AUTO_INCREMENT field):

INSERT INTO foo (auto,text) VALUES(NULL,'text');
SELECT LAST_INSERT_ID();

Or, if you are just going to insert the ID into another table, you can do this:

INSERT INTO foo (auto,text) VALUES(NULL,'text');
INSERT INTO foo2 (id,text) VALUES(LAST_INSERT_ID(),'text');

For the benefit of some ODBC applications (at least Delphi and Access), the following query can be used to find a newly-inserted row:

SELECT * FROM tbl_name WHERE auto IS NULL;

16 Problems and common errors

16.1 Some common errors when using MySQL

16.1.1 `MySQL server has gone away` error

The most common reason for the MySQL server has gone away error is that the server timed out and closed the connection. By default, the server closes the connection after 8 hours if nothing has happened.

If you have a script, you just have to issue the query again for the client to do an automatic reconnection.

You normally can get the following error codes in this case (which one you get is OS-dependent):

`CR_SERVER_GONE_ERROR`	The client couldn't send a question to the server.
`CR_SERVER_LOST`	The client didn't get an error when writing to the server, but it didn't get a full answer (or any answer) to the question.

You can also get these errors if you send a query to the server that is incorrect or too large. If mysqld gets a packet that is too large or out of order, it assumes that something has gone wrong with the client and closes the connection. If you need big queries (for example, if you are working with big BLOB columns), you can increase the query limit by starting mysqld with the -O max_allowed_packet=# option (default 1M). The extra memory is allocated on demand, so mysqld will use more memory only when you issue a big query or when mysqld must return a big result row!

16.1.2 `Can't connect to local MySQL server` error

A MySQL client can connect to the mysqld server in two different ways: Unix sockets, which connect through a file in the file system (default `/tmp/mysqld.sock'), or TCP/IP, which connects through a port number. Unix sockets are faster than TCP/IP but can only be used when connecting to a server on the same computer. Unix sockets are used if you don't specify a hostname or if you specify the special hostname localhost.

Here are some reasons the Can't connect to local MySQL server error might occur:

mysqld is not running. Check (using ps) that there is a process running named mysqld. If there is, you can check the server by trying these different connections (the port number and socket pathname might be different in your setup, of course):
```
shell> mysqladmin version
shell> mysqladmin -h `hostname` version
shell> mysqladmin -h `hostname` --port=3306 version
shell> mysqladmin --socket=/tmp/mysql.sock version
```
Note the use of backquotes rather than forward quotes with the hostname command; these cause the output of hostname (i.e., the current hostname) to be substituted into the mysqladmin command.
You are running on a system that uses MIT-pthreads. If you are running on a system that doesn't have native threads, mysqld uses the MIT-pthreads package. See section 4.2 Operating systems supported by MySQL. However, MIT-pthreads doesn't support Unix sockets, so on such a system you must always specify the hostname explicitly when connecting to the server. Try using this command to check the connection to the server:
```
shell> mysqladmin -h `hostname` version
```
Someone has removed the Unix socket that mysqld uses (default `/tmp/mysqld.sock'). You might have a cron job that removes the MySQL socket (e.g., a job that removes old files from the `/tmp' directory). You can always run mysqladmin version and check that the socket mysqladmin is trying to use really exists. The fix in this case is to change the cron job to not remove `mysqld.sock' or to place the socket somewhere else. You can specify a different socket location at MySQL configuration time with this command:
```
shell> ./configure --with-unix-socket-path=/path/to/socket
```
You can also start safe_mysqld with the --socket=/path/to/socket option and set the environment variable MYSQL_UNIX_PORT to the socket pathname before starting your MySQL clients. @item You have started the mysqld server with the --socket=/path/to/socket option. If you change the socket pathname for the server, you must also notify the MySQL clients about the new path. You can do this by setting the environment variable MYSQL_UNIX_PORT to the socket pathname or by providing the socket path as an argument to the clients. You can test the socket with this command:
```
shell> mysqladmin --socket=/path/to/socket version
```

16.1.3 `Host '...' is blocked` error

If you get a error like this:

Host 'hostname' is blocked because of many connection errors.  Unblock
with 'mysqladmin flush-hosts'

This means that mysqld has gotten a lot (max_connect_errors) of connect requests from the host 'hostname' that have been interrupted in the middle. After max_connect_errors failed requests, mysqld assumes that something is wrong (like a attack from a cracker), and blocks the site from further connections until someone executes the command mysqladmin flush-hosts.

By default, mysqld will block a host after 10 connection errors, but you can easily adjust this by starting the server like this:

shell> safe_mysqld -O max_connect_errors=10000 &

Note that if you get this error message for a given host, you should first check that there isn't anything wrong with TCP/IP connections from that host. If your TCP/IP connections aren't working, it won't do you any good to increase the value of the max_connect_errors variable!

16.1.4 `Out of memory` error

If you issue a query and get something like the following error:

mysql: Out of memory at line 42, 'malloc.c'
mysql: needed 8136 byte (8k), memory in use: 12481367 bytes (12189k)
ERROR 2008: MySQL client ran out of memory

Note that the error refers to the MySQL client mysql. The reason for this error is simply that the client does not have enough memory to store the whole result.

To remedy the problem, first check that your query is correct. Is it reasonable that it should return so many rows? If so, you can use mysql --quick, which uses mysql_use_result() to retrieve the result set. This places less of a load on the client (but more on the server).

16.1.5 `Packet too large` error

When a MySQL client or the mysqld server gets a packet bigger than max_allowed_packet bytes, it issues a Packet too large error and closes the connection.

If you are using the mysql client, you may specify a bigger buffer by starting the client with mysql --set-variable=max_allowed_packet=8M.

If you are using another client, such as DBI, you can set a bigger buffer size on the server side by starting mysqld with a command-line option to set max_allowed_packet to a larger size. For example, if you are expecting to store the full length of a BLOB into a table, you'll need to start the server with the --set-variable=max_allowed_packet=24M option.

16.1.6 `The table is full` error

This error occurs when an in-memory temporary table becomes larger than tmp_table_size. To avoid this problem, you can use the -O tmp_table_size=# option to mysqld to increase the temporary table size, or use the SQL option SQL_BIG_TABLES before you issue the problematic query. See section 7.24 SET OPTION syntax.

You can also start mysqld with the --big-tables option. This is exactly the same as using SQL_BIG_TABLES for all queries.

16.1.7 `Commands out of sync` error in client

If you get Commands out of sync; You can't run this command now in your client code, you are calling client functions in the wrong order!

This can happen, for example, if you are using mysql_use_result() and try to execute a new query before you have called mysql_free_result(). It can also happen if you try to execute two queries that return data without a mysql_use_result() or mysql_store_result() in between.

16.1.8 `Ignoring user` error

If you get the following error:

Found wrong password for user: 'some_user@some_host'; Ignoring user

This means that when mysqld was started or when it reloaded the permissions tables, it found an entry in the user table with an invalid password. As a result, the entry is simply ignored by the permission system.

Possible causes of and fixes for this problem:

You may be running a new version of mysqld with an old user table. You can check this by executing mysqlshow mysql user to see if the password field is shorter than 16 characters. If so, you can correct this condition by running the scripts/add_long_password script.
The user has an old password (8 chararacters long) and you didn't start mysqld with the --old-protocol option. Update the user in the user table with a new password or restart mysqld with --old-protocol.
You have specified a password in the user table without using the PASSWORD() function. Use mysql to update the user in the user table with a new password. Make sure to use the PASSWORD() function:
```
mysql> update user set password=PASSWORD('your password')
           where user='XXX';
```

16.2 How MySQL handles a full disk

When a disk full condition occurs, MySQL does the following:

It checks once every minute to see whether or not there is enough space to write the current row. If there is enough space, it continues as if nothing had happened.
Every 6 minutes it writes an entry to the log file warning about the disk full condition.

To alleviate the problem, you can take the following actions:

To continue, you only have to free enough disk space to insert all records.
To abort the thread, you must send a mysqladmin kill to the thread. The thread will be aborted when it checks the disk next time (in 1 minute).
Note that other threads may be waiting for the table that caused the "disk full" condition. If you have several "locked" threads, killing the one thread that is waiting on the disk full condition will allow the other threads to continue.

16.3 How to run SQL commands from a text file

The mysql client typically is used interactively, like this:

shell> mysql database

However, it's also possible to put your SQL commands in a file and tell mysql to read its input from that file. To do so, create a text file `text_file' that contains the commands you wish to execute. Then invoke mysql as shown below:

shell> mysql database < text_file

You can also start your text file with a USE db_name statement. In this case, it is unnecessary to specify the database name on the command line:

shell> mysql < text_file

See section 12.1 Overview of the different MySQL programs.

16.4 Where MySQL stores temporary files

MySQL uses the value of the TMPDIR environment variable as the pathname of the directory in which to store temporary files. If you don't have TMPDIR set, MySQL uses the system default, which is normally `/tmp' or `/usr/tmp'. If the file system containing your temporary file directory is too small, you should edit safe_mysqld to set TMPDIR to point to a directory in a file system where you have enough space! You can also set the temporary directory using the --tmpdir option to mysqld.

MySQL creates all temporary files as "hidden files". This ensures that the temporary files will be removed if mysqld is terminated. The disadvantage of using hidden files is that you will not see a big temporary file that fills up the file system in which the temporary file directory is located.

When sorting (ORDER BY or GROUP BY), MySQL normally uses one or two temporary files. The maximum disk-space needed is (length_of_what_is_sorted + sizeof(database_pointer)) * number_of_matched_rows * 2. sizeof(database_pointer) is usually 4, but may grow in the future for really big tables.

For some SELECT queries, MySQL also creates temporary SQL tables. These are not hidden and have names of the form `SQL_*'.

ALTER TABLE creates a temporary table in the same directory as the original table.

16.5 How to protect `/tmp/mysql.sock' from being deleted.

If you have problems with the fact that anyone can delete the MySQL communication socket `/tmp/mysql.sock' you can, on most versions of Unix, protect your `/tmp' file system by setting the sticky bit on it. Log in as root and do the following:

shell> chmod +s /tmp

This will protect your `/tmp' file system so that files can be deleted only by their owners or the superuser (root).

You can check if the sticky bit is set by executing ls -ld /tmp. If the last permission bit is t, the bit is set.

16.6 `Access denied` error

See section 6.4 How the privilege system works. And especially see section 6.10 Causes of Access denied errors.

16.7 How to run MySQL as a normal user

The MySQL server mysqld can be started and run by any user. In order to change mysqld to run as Unix user user_name, you must do the following:

Stop the server if it's running (use mysqladmin shutdown).
Change the database directories and files so that user_name has privileges to read and write files in them:
```
shell> chown -R user_name /your/path/to/mysql/var
```
Start the server as user user_name, or, if you are using MySQL 3.22 or later, start mysqld as the Unix root user and use the --user=user_name option. mysqld will switch to run as Unix user user_name before accepting any connections.
If you are using the mysql.server script to start mysqld when the system is rebooted, you should edit mysql.server to use su to run mysqld as user user_name, or to invoke mysqld with the --user option. (No changes to safe_mysqld are necessary.)

At this point, your mysqld process should be running fine and dandy as Unix user user_name. One thing hasn't changed, though: the contents of the permissions tables. By default (right after running the permissions table install script mysql_install_db), the MySQL user root is the only user with permission to access the mysql database or to create or drop databases. Unless you have changed those permissions, they still hold. This shouldn't stop you from accessing MySQL as the MySQL root user when you're logged in as a Unix user other than root; just specify the -u root option to the client program.

Note that accessing MySQL as root, by supplying -u root on the command line, has nothing to do with MySQL running as the Unix root user, or, indeed, as other Unix user. The access permissions and user names of MySQL are completely separate from Unix user names. The only connection with Unix user names is that if you don't provide a -u option when you invoke a client program, the client will try to connect using your Unix login name as your MySQL user name.

If your Unix box itself isn't secured, you should probably at least put a password on the MySQL root users in the access tables, since any user with an account on that machine can run mysql -u root db_name and do whatever he likes.

16.8 Problems with file permissions

If you have problems with file permissions, for example, if mysql issues the following error message when you create a table:

ERROR: Can't find file: 'path/with/filename.frm' (Errcode: 13)

Then the environment variable UMASK might be set incorrectly when mysqld starts up. The default umask value is 0660. You can change this behavior by starting safe_mysqld as follows:

shell> UMASK=384  # = 600 in octal
shell> export UMASK
shell> /path/to/safe_mysqld &

16.9 File not found

If you get ERROR '...' not found (errno: 23), Can't open file: ... (errno: 24) or any other error with errno 23 or errno 24 from MySQL, it means that you haven't allocated enough file descriptors for MySQL. You can use the perror utility to get a description of what the error number means:

shell> perror 23
File table overflow
shell> perror 24
Too many open files

The problem here is that mysqld is trying to keep open too many files simultaneously. You can either tell mysqld not to open so many files at once, or increase the number of file descriptors available to mysqld.

To tell mysqld to keep open fewer files at a time, you can make the table cache smaller by using the -O table_cache=32 option to safe_mysqld (the default is 64). Reducing the value of max_connections (default 90) will also reduce the number of open files.

To change the number of file descriptors available to mysqld, modify the safe_mysqld script. There is a commented-out line ulimit -n 256 in the script. You can remove the '#' character to uncomment this line, and change the number 256 to change the number of file descriptors available to mysqld.

ulimit can increase the number of file descriptors, but only up to the limit imposed by the operating system. If you need to increase the OS limit on the number of file descriptors available to each process, consult the documentation for your operating system.

16.10 Problems using `DATE` columns

The format of a DATE value is 'YYYY-MM-DD'. According to ANSI SQL, no other format is allowed. You should use this format in UPDATE expressions and in the WHERE clause of SELECT statements. For example:

mysql> SELECT * FROM tbl_name WHERE date >= '1997-05-05';

As a convenience, MySQL automatically converts a date to a number if the date is used in a numeric context (and vice versa). It is also smart enough to allow a 'relaxed' string form when updating and in a WHERE clause that compares a date to a TIMESTAMP, DATE or a DATETIME column. (Relaxed form means that any non-numeric character may be used as the separator between parts. For example, '1998-08-15' and '1998#08#15' are equivalent.) MySQL can also convert a string containing no separators (such as '19980815'), provided it makes sense as a date.

The special date '0000-00-00' can be stored and retrieved as '0000-00-00'. When using a '0000-00-00' date through MyODBC, it will automatically be converted to NULL in MyODBC 2.50.12 and above, because ODBC can't handle this kind of date.

Since MySQL performs the conversions described above, the following statements work:

mysql> INSERT INTO tbl_name (idate) VALUES (19970505);
mysql> INSERT INTO tbl_name (idate) VALUES ('19970505');
mysql> INSERT INTO tbl_name (idate) VALUES ('97-05-05');
mysql> INSERT INTO tbl_name (idate) VALUES ('1997.05.05');
mysql> INSERT INTO tbl_name (idate) VALUES ('1997 05 05');
mysql> INSERT INTO tbl_name (idate) VALUES ('0000-00-00');

mysql> SELECT idate FROM tbl_name WHERE idate >= '1997-05-05';
mysql> SELECT idate FROM tbl_name WHERE idate >= 19970505;
mysql> SELECT mod(idate,100) FROM tbl_name WHERE idate >= 19970505;
mysql> SELECT idate FROM tbl_name WHERE idate >= '19970505';

However, the following will not work:

mysql> SELECT idate FROM tbl_name WHERE STRCMP(idate,'19970505')=0;

STRCMP() is a string function, so it converts idate to a string and performs a string comparison. It does not convert '19970505' to a date and perform a date comparison.

Note that MySQL does no checking whether or not the date is correct. If you store an incorrect date, such as '1998-2-31', the wrong date will be stored. If the date cannot be converted to any reasonable value, a 0 is stored in the DATE field. This is mainly a speed issue and we think it is up to the application to check the dates, and not the server.

16.11 Case sensitivity in searches

By default, MySQL searches are case-insensitive (although there are some character sets that are never case insensitive, such as czech). That means that if you search with col_name LIKE 'a%', you will get all column values that start with A or a. If you want to make this search case-sensitive, use something like INDEX(col_name, "A")=0 to check a prefix. Or use STRCMP(col_name, "A") = 0 if the column value must be exactly "A".

Simple comparison operations (>=, >, = , < , <=, sorting and grouping) are based on each character's "sort value". Characters with the same sort value (like E, e and 'e) are treated as the same character!

LIKE comparisons are done on the uppercase value of each character (E == e but E <> 'e)

If you want a column always to be treated in case-sensitive fashion, declare it as BINARY. See section 7.6 CREATE TABLE syntax.

If you are using Chinese data in the so-called big5 encoding, you want to make all character columns BINARY. This works because the sorting order of big5 encoding characters is based on the order of ASCII codes.

16.12 Problems with `NULL` values

The concept of the NULL value is a common source of confusion for newcomers to SQL, who often think that NULL is the same thing as an empty string ". This is not the case! For example, the following statements are completely different:

mysql> INSERT INTO my_table (phone) VALUES (NULL);
mysql> INSERT INTO my_table (phone) VALUES ("");

Both statements insert a value into the phone column, but the first inserts a NULL value and the second inserts an empty string. The meaning of the first can be regarded as 'phone number is not known' and the meaning of the second can be regarded as 'she has no phone'.

In SQL, the NULL value is always false in comparison to any other value, even NULL. An expression that contains NULL always produces a NULL value unless otherwise indicated in the documentation for the operators and functions involved in the expression. All columns in the following example return NULL:

mysql> SELECT NULL,1+NULL,CONCAT('Invisible',NULL);

If you want to search for column values that are NULL, you cannot use the =NULL test. The following statement returns no rows, because expr = NULL is FALSE, for any expression:

mysql> SELECT * FROM my_table WHERE phone = NULL;

To look for NULL values, you must use the IS NULL test. The following shows how to find the NULL phone number and the empty phone number:

mysql> SELECT * FROM my_table WHERE phone IS NULL;
mysql> SELECT * FROM my_table WHERE phone = "";

In MySQL, as in many other SQL servers, you can't index columns that can have NULL values. You must declare such columns NOT NULL. Conversely, you cannot insert NULL into an indexed column.

When reading data with LOAD DATA INFILE, empty columns are updated with ". If you want a NULL value in a column, you should use \N in the text file. The literal word 'NULL' may also be used under some circumstances. See section 7.15 LOAD DATA INFILE syntax.

When using ORDER BY, NULL values are presented first. If you sort in descending order using DESC, NULL values are presented last. When using GROUP BY, all NULL values are regarded as equal.

To help with NULL handling, you can use the functions IS NULL, IS NOT NULL and IFNULL().

For some column types, NULL values are handled specially. If you insert NULL into the first TIMESTAMP column of a table, the current time is inserted. If you insert NULL into an AUTO_INCREMENT column, the next number in the sequence is inserted.

16.13 Deleting rows from related tables

As MySQL doesn't support sub selects or use of more than one table in the DELETE clause, you should use the following approach to delete rows from 2 related tables.

SELECT the rows based on some WHERE condition in the main table.
DELETE the rows in the main table based on the same condition.
DELETE FROM related_table WHERE related_column IN (selected_rows)

If the total number of characters in the query with related_columns is more than 1,000,0000 (the default value of max_packet_length), you should split it into smaller parts and execute multiple DELETE statements. You will probably get the fastest DELETE by only deleting 100-1000 related_column id's per time if the related_column is an index. If the related_column isn't an index, the speed is independent of the number of arguments in the IN part.

16.14 Solving problems with no matching rows

If you have a complicated query with many tables that doesn't return any rows, you should use the following procedure to find out what is wrong with your query:

Test the query with EXPLAIN and check if you can find something that is obviously wrong. See section 7.21 EXPLAIN syntax (Get information about a SELECT)
Select only those fields that are used in the WHERE clause.
Remove one table at a time from the query until it returns some rows. If the tables are big, its a good idea to use LIMIT 10 with the queries.
Do a SELECT against the last removed table with the column that should have matched a row in this table.
If you still can't find out what's wrong, create a minimal test that can be run with mysql test < query.sql that shows your problems. You can create a test file with mysqldump --quick database tables > query.sql. Take the file up in a editor, remove some insert lines (if there are too many of these) and add your select statement last in the file. Test that you still have your problem by doing:
```
shell> mysqladmin create test2
shell> mysql test2 < query.sql
```
Post the test file using mysqlbug to mysql@tcx.se.

17 Solving some common problems with MySQL

17.1 Database replication

The most general way to replicate a database is to use the update log. See section 9.2 The update log. This requires one database that acts as a master (to which data changes are made) and one or more other databases that act as slaves. To update a slave, just run mysql < update_log. Supply host, user and password options that are appropriate for the slave database, and use the update log from the master database as input.

If you never delete anything from a table, you can use a TIMESTAMP column to find out which rows are inserted or changed in the table since the last replication (by comparing to the time when you did the replication last time) and only copy the these rows to the mirror.

It is possible to make a two-way updating system using both the update log (for deletes) and timestamps (on both sides). But in that case you must be able to handle conflicts when the same data have been changed in both ends. You probably want to keep the old version to help with deciding what has been updated.

Because replication in this case is done with SQL statements, you should not use the following functions in statements that update the database, because they may not return the same value as in the original database:

DATABASE()
GET_LOCK() and RELEASE_LOCK()
RAND()
USER(), SYSTEM_USER() or SESSION_USER()
VERSION()

All time functions are safe to use, as the timestamp is sent to the mirror if needed. LAST_INSERT_ID() is also safe to use.

17.2 Database backups

Since MySQL tables are stored as files, it is easy to do a backup. To get a consistent backup, do a LOCK TABLES on the relevant tables. See section 7.23 LOCK TABLES/UNLOCK TABLES syntax. You only need a read lock; this allows other threads to continue to query the tables while you are making a copy of the files in the database directory. If you want to make a SQL level backup, you can use SELECT INTO OUTFILE.

Another way to backup a database is to use the mysqldump program:

Do a full backup of your databases:
```
shell> mysqldump --tab=/path/to/some/dir --lock-tables --quick
```
You can also simply copy all table files (`*.frm', `*.ISD' and `*.ISM'), as long as the server isn't updating anything.
Stop mysqld if it's running, then start it with the --log-update option. You will get log files with names of the form `hostname.n', where n is a number that is incremented each time you execute mysqladmin refresh or mysqladmin flush-logs, the FLUSH LOGS statement, or restart the server. These log files provide you with the information you need to replicate changes to the database that are made subsequent to the point at which you executed mysqldump.

If you have to restore something, try isamchk -r first. That should work in 99.9% of all cases. If isamchk fails, try the the following procedure:

Restore the original mysqldump backup.
Execute the following command to re-run the updates in the update logs:
```
shell> ls -1 -t -r hostname.[0-9]* | xargs cat | mysql
```

ls is used to get all the log files in the right order.

You can also do selective backups with SELECT * INTO OUTFILE 'file_name' FROM tbl_name and restore with LOAD DATA INFILE 'file_name' REPLACE ... To avoid duplicate records, you need a PRIMARY KEY or a UNIQUE key in the table. The REPLACE keyword causes old records to be replaced with new ones when a new record duplicates an old record on a unique key value.

17.3 Running multiple MySQL servers on the same machine

If you want to run multiple servers on the same machine (for example, an old and a new mysqld server), the easiest way is to compile the servers with a different TCP/IP port and a different socket file so they are not both listening to the same TCP/IP port or socket file.

Assume the existing server is configured for the default port number and socket file. Then configure the new server with a configure command something like this:

shell> ./configure  --with-tcp-port=port_number \
             --with-unix-socket=file_name \
             --prefix=/usr/local/mysql-3.22.9

Here port_number and file_name should be different than the default port number and socket file, and the --prefix value should specify an installation directory different than the one under which the existing MySQL installation is located.

You can check the socket and port used by any currently-executing MySQL server with this command:

shell> mysqladmin -h hostname --port port_number variables

If you have a MySQL server running on the port you used, you will get a list of some of the most important configurable variables in MySQL, including the socket name.

You should also edit the initialization script for your machine (probably `mysql.server') to start and kill multiple mysqld servers.

You don't have to recompile a new MySQL server just to start with a different port and socket. You can change the port and socket to be used by specifying them as options to safe_mysqld:

shell> ./bin/safe_mysqld --socket=file-name --port=file-name

If you run the new server on the same database directory as the old server with logging enabled, you should also specify a the name of the log files to safe_mysqld with --log and --log-update.

Warning: Normally you should never have two servers that update data in the same database! If your OS doesn't support fault-free system locking, this may lead to unpleasant surprises!

If you want to use another database directory for the second server you can use the --datadir=path option to safe_mysqld.

When you want to connect to a MySQL server that is running with a different port than the port that is compiled into your client, you can use one of the following methods:

Start the client with --host 'hostname' --port=port-numer or [--host localhost] --socket=file-name.
In your C or Perl programs, you can give the port and socket arguments when connecting to the MySQL server.
Set the MYSQL_UNIX_PORT and MYSQL_TCP_PORT environment variables to point to the Unix socket and TCP/IP port before you start your clients. If you normally use a specific socket or port you should place commands to set these environment variables in your `.login' file. See section 12.1 Overview of the different MySQL programs.
Specify the default socket and TCP/IP port in a `.my.cnf' file in your home directory. See section 4.14.4 Option files.

18 MySQL client tools and API's

18.1 MySQL C API

The C API code is distributed with MySQL. It is included in the mysqlclient library and allows C programs to access a database.

Many of the clients in the MySQL source distribution are written in C. If you are looking for examples showing how to use the C API, take a look at these clients.

Most of the other client APIs (all except Java) use this library to connect. So, for example, you can use the same environment variables as the ones used by other client programs. See section 12.1 Overview of the different MySQL programs.

The client has a maximum communication buffer size. The size of the buffer that is allocated initially (8192 bytes) is automatically increased up to the maximum size (the default for this is 24M). Since buffers are increased on demand (but not decreased until close), simply increasing the default limit doesn't cause more resources to be used. This size check is mostly a check for erroneous queries and communication packets.

The communication buffer must be big enough to contain a single SQL statement and one row of returned data (not at the same time, of course). Each thread's communication buffer is dynamically enlarged to handle any row or query up to the imposed limit. For example, if you have BLOB values that contain up to 16M of data, you must have at least 16M as your communication buffer limit (in both server and client). See section 10.1 Changing the size of MySQL buffers.

The MySQL server shrinks each communication buffer to net_buffer_length bytes after each query. The client doesn't shrink the buffer automatically. The client memory is reclaimed when the connection is closed.

18.2 C API datatypes

MYSQL: This structure represents a handle to one database connection. It is used for almost all MySQL functions.
MYSQL_RES: This structure represents the result of a query that returns rows (SELECT or SHOW). The information returned from a query is called the result set in the remainder of this section.
MYSQL_ROW: This is a type-safe representation of one row of data. It is currently implemented as an array of byte strings.
MYSQL_FIELD: This structure contains information about a field, such as the field's name, type and size. Its members are described in more detail below. You may obtain the MYSQL_FIELD structures for each field by calling mysql_fetch_field() repeatedly.
MYSQL_FIELD_OFFSET: This is a type-safe representation of an offset into a MySQL field list. (Used by mysql_field_seek().) Offsets are field numbers within a row, beginning at zero.
my_ulonglong: The type used for the number of rows and for mysql_insert_id(). This type provides a range of 0 - 1.84e19.

The MYSQL_FIELD structure contains the following members:

char * name

The name of the field.

char * table

The name of the table containing this field if it isn't a calculated field. For calculated fields, the table value is a NULL pointer.

char * def

The default value of this field (set only if you use mysql_list_fields()).

enum enum_field_types type

The type of the field. The type value may be one of the following:

Type name	Type meaning
`FIELD_TYPE_TINY`	`TINYINT` field
`FIELD_TYPE_ENUM`	`ENUM` field
`FIELD_TYPE_DECIMAL`	`DECIMAL` or `NUMERIC` field
`FIELD_TYPE_SHORT`	`SMALLINT` field
`FIELD_TYPE_LONG`	`INTEGER` field
`FIELD_TYPE_FLOAT`	`FLOAT` field
`FIELD_TYPE_DOUBLE`	`DOUBLE` or `REAL` field
`FIELD_TYPE_NULL`	`NULL`-type field
`FIELD_TYPE_TIMESTAMP`	`TIMESTAMP` field
`FIELD_TYPE_LONGLONG`	`BIGINT` field
`FIELD_TYPE_INT24`	`MEDIUMINT` field
`FIELD_TYPE_DATE`	`DATE` field
`FIELD_TYPE_TIME`	`TIME` field
`FIELD_TYPE_DATETIME`	`DATETIME` field
`FIELD_TYPE_YEAR`	`YEAR` field
`FIELD_TYPE_SET`	`SET` field
`FIELD_TYPE_BLOB`	`BLOB` or `TEXT` field (use `max_length` to determine the maximum length)
`FIELD_TYPE_STRING`	String (`CHAR` or `VARCHAR`) field
`FIELD_TYPE_CHAR`	Deprecated: use `FIELD_TYPE_TINY` instead

The IS_NUM() macro allows you to test if a field has a numeric type. Pass the type member to IS_NUM() and it will evaluate to TRUE if the field is numeric:

if (IS_NUM(field->type))
{
    printf("Field is numeric\n");
}

unsigned int length

The width of the field.

unsigned int max_length

The maximum width of the field for the selected set. If you used mysql_list_fields(), this contains the maximum length for the field.

unsigned int flags

Different bit-flags for the field These are the bits in flags that you may use:

Flag name	Flag meaning
`NOT_NULL_FLAG`	Field can't be `NULL`
`PRI_KEY_FLAG`	Field is part of a primary key
`UNIQUE_KEY_FLAG`	Field is part of a unique key
`MULTIPLE_KEY_FLAG`	Field is part of a key
`BLOB_FLAG`	Field is a `BLOB` or `TEXT`
`UNSIGNED_FLAG`	Field is `UNSIGNED`
`ZEROFILL_FLAG`	Field has the `ZEROFILL` attribute
`BINARY_FLAG`	Field has the `BINARY` attribute
`ENUM_FLAG`	Field is an `ENUM`
`AUTO_INCREMENT_FLAG`	Field has the `AUTO_INCREMENT` attribute
`TIMESTAMP_FLAG`	Field is a `TIMESTAMP`

The example below illustrates a typical use of the flag value:

if (field->flags & NOT_NULL_FLAG)
{
    printf("Field can't be null\n");
}

You may use the following convenience macros to determine the boolean status of the flags member:

`IS_PRI_KEY(flags)`	Is this field a primary key?
`IS_NOT_NULL(flags)`	Is this field defined as `NOT NULL`?
`IS_BLOB(flags)`	Is this field a `BLOB` or `TEXT`?

The use of BLOB_FLAG, ENUM_FLAG and TIMESTAMP_FLAG is deprecated, since they indicate the type of a field rather than an attribute of the type. It is preferable to test field->type against FIELD_TYPE_BLOB, FIELD_TYPE_ENUM or FIELD_TYPE_TIMESTAMP instead.

unsigned int decimals

The number of decimals for numeric fields.

18.3 C API function overview

The functions available in the C API are listed below. These functions are described in greater detail in the next section. See section 18.4 C API function descriptions.

mysql_affected_rows()	Returns the number of rows affected by the last `UPDATE`, `DELETE` or `INSERT` query.
mysql_close()	Closes a server connection.
mysql_connect()	Connects to a MySQL server. This function is deprecated; use `mysql_real_connect()` instead.
mysql_create_db()	Creates a database. This function is deprecated; use the SQL command `CREATE DATABASE` instead.
mysql_data_seek()	Seeks to an arbitrary row in a query result set.
mysql_debug()	Does a `DBUG_PUSH` with the given string.
mysql_drop_db()	Drops a database. This function is deprecated; use the SQL command `DROP DATABASE` instead.
mysql_dump_debug_info()	Makes the server dump debug information to the log.
mysql_eof()	Determines whether or not the last row of a result set has been read.
mysql_errno()	Returns the error number from the last MySQL function.
mysql_error()	Returns the error message from the last MySQL function.
mysql_escape_string()	Escapes a string for a SQL statement.
mysql_fetch_field()	Returns the type of the next table field.
mysql_fetch_field_direct()	Returns the type of a numbered table field.
mysql_fetch_fields()	Returns an array of all field structures.
mysql_fetch_lengths()	Returns the length for all columns in the current row.
mysql_fetch_row()	Fetches the next row from the result set.
mysql_field_seek()	Puts the column cursor on a specified column.
mysql_free_result()	Frees memory used by a result set.
mysql_get_client_info()	Returns client version information.
mysql_get_host_info()	Returns a string describing the connection.
mysql_get_proto_info()	Returns the protocol version used by the connection.
mysql_get_server_info()	Returns the server version number.
mysql_info()	Information about the most recently executed query.
mysql_init()	Get or initialize a MYSQL structure.
mysql_insert_id()	Returns the last ID generated for an `AUTO_INCREMENT` field.
mysql_list_dbs()	Returns database names matching a simple regular expression.
mysql_list_fields()	Returns field names matching a simple regular expression.
mysql_list_processes()	Returns a list of the current server threads.
mysql_list_tables()	Returns table names matching a simple regular expression.
mysql_num_fields()	Returns the number of columns in a result set.
mysql_num_rows()	Returns the number of rows in a result set.
mysql_ping()	Checks if the connection to the server is working.
mysql_query()	Executes a SQL query specified as a null-terminated string.
mysql_real_connect()	Connects to a MySQL server.
mysql_real_query()	Executes a SQL query specified as a counted string.
mysql_reload()	Tells the server to reload the access permissions tables.
mysql_row_tell()	Returns the row cursor.
mysql_select_db()	Connects to a database.
mysql_shutdown()	Shuts down the database server.
mysql_stat()	Returns the server status as a string.
mysql_store_result()	Reads a result set to the client.
mysql_thread_id()	Returns the current thread id.
mysql_use_result()	Initiates a dynamic result set for each row.

18.4 C API function descriptions

In the descriptions below, a parameter or return value of NULL means NULL in the sense of the C programming language, not a MySQL NULL value.

18.4.1 `mysql_affected_rows()`

my_ulonglong mysql_affected_rows(MYSQL *mysql)

18.4.1.1 Description

Returns the number of rows affected by the last UPDATE, DELETE or INSERT query. May be called immediately after mysql_query() for INSERT or UPDATE statements. For SELECT statements, this works like mysql_num_rows(). mysql_affected_rows() is currently implemented as a macro.

18.4.1.2 Return values

An integer > 0 indicates the number of rows affected or retrieved. Zero if no records matched the WHERE clause in the query or no query has yet been executed. -1 if the query returned an error or was called before mysql_store_result() was called for a SELECT query.

18.4.1.3 Errors

None.

18.4.1.4 Example

mysql_query(&mysql,"UPDATE products SET cost=cost*1.25 WHERE group=10");
printf("%d products updated",mysql_affected_rows(&mysql));

18.4.2 `mysql_close()`

void mysql_close(MYSQL *mysql)

18.4.2.1 Description

Closes a previously opened connection.

18.4.2.2 Return values

None.

18.4.2.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.3 `mysql_connect()`

MYSQL *mysql_connect(MYSQL *mysql, const char *host, const char *user, const char *passwd)

18.4.3.1 Description

This function is deprecated. It is preferable to use mysql_real_connect() instead.

mysql_connect() attempts to establish a connection to a MySQL database engine running on host. The value of host may be either a hostname or an IP address. The user parameter contains the user's MySQL login ID, and the passwd parameter contains the password for user. NOTE: Do not attempt to encrypt passwd before calling mysql_connect(). Password encryption is handled automatically by the client API.

If host is NULL or the string "localhost", a connection to the local host is assumed. If the OS supports sockets (Unix) or named pipes (Win32), they are used instead of TCP/IP to connect to the server.
If user is NULL, the current user is assumed. Under Windows ODBC, the current user must be specified explicitly. Under Unix, the current login name is assumed.
If passwd is NULL, only records in the user table for the user that have a blank password field will be checked for a match. This allows the database administrator to set up the MySQL privilege system in such a way that users get different privileges depending on whether or not they have specified a password.

mysql_connect() must complete successfully before you can execute any of the other API functions, with the exception of mysql_get_client_info().

You may optionally specify the first argument of mysql_connect() to be a NULL pointer. This will force the C API to allocate memory for the connection structure automatically and to free it when you call mysql_close(). The disadvantage of this approach is that you can't retrieve an error message from mysql_connect() if the connection fails.

If the first argument is not a NULL pointer, it should be the address of an existing MYSQL structure.

18.4.3.2 Return values

A MYSQL* connection handle if the connection was successful. A C NULL pointer if the connection was unsuccessful.

18.4.3.3 Errors

CR_CONN_HOST_ERROR: Failed to connect to the MySQL server.
CR_CONNECTION_ERROR: Failed to connect to the local MySQL server.
CR_IPSOCK_ERROR: Failed to create an IP socket.
CR_OUT_OF_MEMORY: Out of memory.
CR_SOCKET_CREATE_ERROR: Failed to create a Unix socket.
CR_UNKNOWN_HOST: Failed to find the IP address for the hostname.
CR_VERSION_ERROR: A protocol mismatch resulted from attempting to connect to a server with a client library that uses a different protocol version. This can happen if you use a very old client library to connect to a new server that wasn't started with the --old-protocol option.
CR_NAMEDPIPEOPEN_ERROR;: Failed to create a named pipe on Win32.
CR_NAMEDPIPEWAIT_ERROR;: Failed to wait for a named pipe on Win32.
CR_NAMEDPIPESETSTATE_ERROR;: Failed to get a pipe handler on Win32.

18.4.3.4 Example

MYSQL mysql;

if(!mysql_connect(&mysql, "host", "username", "password"))
  fprintf(stderr, "Failed to connect to database: Error: %s\n",
          mysql_error(&mysql));

18.4.4 `mysql_create_db()`

int mysql_create_db(MYSQL *mysql, const char *db)

18.4.4.1 Description

Creates the database named by the db argument.

This function is deprecated. It is preferable to use mysql_query() to issue a SQL CREATE DATABASE statement instead.

18.4.4.2 Return values

Zero if the database was successfully created. Non-zero if an error occurred.

18.4.4.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.4.4 Example

if(mysql_create_db(&mysql, "my_new_db"))
   fprintf(stderr, "Failed to create new database.  Error: %s\n",
           mysql_error(&mysql));

18.4.5 `mysql_data_seek()`

void mysql_data_seek(MYSQL_RES *res, unsigned int offset)

18.4.5.1 Description

Seeks to an arbitrary row in a query result set. This function may be used in conjunction only with mysql_store_result)(, not with mysql_use_result().

The offset can be any value: 0 <= offset <= mysql_num_rows() -1

18.4.5.2 Return values

None.

18.4.5.3 Errors

None.

18.4.6 `mysql_debug()`

void mysql_debug(char *debug)

18.4.6.1 Description

Does a DBUG_PUSH with the given string. mysql_debug() uses the Fred Fish debug library. To use this function, you must compile the client library to support debugging.

18.4.6.2 Return values

None.

18.4.6.3 Errors

None.

18.4.6.4 Example

The call shown below causes the client library to generate a trace file in `/tmp/client.trace' on the client machine:

mysql_debug("d:t:O,/tmp/client.trace");

18.4.7 `mysql_drop_db()`

int mysql_drop_db(MYSQL *mysql, const char *db)

18.4.7.1 Description

Drops the database named by the db argument.

This function is deprecated. It is preferable to use mysql_query() to issue a SQL DROP DATABASE statement instead.

18.4.7.2 Return values

Zero if the database was successfully dropped. Non-zero if an error occurred.

18.4.7.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.7.4 Example

if(mysql_drop_db(&mysql, "some_database"))
  fprintf(stderr, "Failed to drop the database: Error: %s\n",
          mysql_error(&mysql));

18.4.8 `mysql_dump_debug_info()`

int mysql_dump_debug_info(MYSQL *mysql)

18.4.8.1 Description

Instructs the server to dump some debug information to the log. The connected user must have process privileges for this to work.

18.4.8.2 Return values

Zero if the command was successful. Non-zero if the command failed.

18.4.8.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.9 `mysql_eof()`

my_bool mysql_eof(MYSQL_RES *result)

18.4.9.1 Description

When mysql_fetch_row() returns nothing, mysql_eof() returns a non-zero value if the end of the result set was reached and zero if an error occurred. If an error occurred, the preferred method of finding out what the was was is to call mysql_errno().

mysql_eof() may only be used with mysql_use_result(), not with mysql_store_result().

18.4.9.2 Return values

Zero if the end of the result set has not yet been reached. Non-zero if the end of the result set has been reached.

18.4.9.3 Errors

None.

18.4.9.4 Example

mysql_query(&mysql,"SELECT * FROM some_table");
result = mysql_use_result(&mysql);

while((row = mysql_fetch_row(result)))
{
        //do something with data
}

if(!mysql_eof(result))
{
        //mysql_fetch_row failed due to some error
}

18.4.10 `mysql_errno()`

unsigned int mysql_errno(MYSQL *mysql)

18.4.10.1 Description

Returns the error code for the last error that occurred on the connection specified by mysql. A return value of zero means that no error occurred. Client error message numbers are listed in `errmsg.h'. Server error message numbers are listed in `mysqld_error.h'

18.4.10.2 Return values:

An error code value. Zero if no error has occurred.

18.4.10.3 Errors

None.

18.4.11 `mysql_error()`

char *mysql_error(MYSQL *mysql)

18.4.11.1 Description

Returns the error message, if any, describing the last MySQL error that occurred on the connection specified by mysql. An empty string is returned if no error occurred. The language of the client error messages may be changed by recompiling the MySQL client library. You currently can choose between English or German client error messages.

18.4.11.2 Return values

A character string that describes the error.

18.4.11.3 Errors

None.

18.4.12 `mysql_escape_string()`

unsigned int mysql_escape_string(char *to, const char *from, unsigned int length)

18.4.12.1 Description

Encodes the string in from to an escaped SQL string that can be sent to the server in a SQL statement. The string pointed to by from must be length bytes long. You must allocate the to buffer to be at least length*2+1 bytes long. When mysql_escape_string() returns, the contents of to will be a NUL-terminated string. See section 7.1 Literals: how to write strings and numbers.

Characters encoded are `NUL' (ASCII 0), `\n', `\r', `\' and `''.

18.4.12.2 Example

char query[1000],*end;

 end=strmov(query,"INSERT INTO test_table values(");
 *end++='\"
 end+=mysql_escape_string(query,"What's this");
 *end++='\";
 *end++=','
 *end++='\"
 end+=mysql_escape_string(query,"binary data: \0\r\n");
 *end++='\"
 *end++=')';

  if (mysql_real_query(&mysql,query,(int) (end-query)))
  {
     fprintf(stderr, "Failed to insert row, Error: %s\n",
             mysql_error(&mysql));
  }

The strmov() function above is included in the mysqlclient library and works like strcpy() but returns a pointer to the terminating null of the first argument.

18.4.12.3 Return values

The length of the value placed into to, not including the terminating null character.

18.4.12.4 Errors

None.

18.4.13 `mysql_fetch_field()`

MYSQL_FIELD *mysql_fetch_field(MYSQL_RES *result)

18.4.13.1 Description

Returns the definition of one column as a MYSQL_FIELD structure. Call this function repeatedly to retrieve information about all columns in the result set.

mysql_fetch_field() is reset to return information about the first field each time you execute a new SELECT query. The field returned by mysql_fetch_field() is also affected by calls to mysql_field_seek().

When querying for the length of a BLOB without retrieving a result, MySQL returns the default blob length, which is 8192, when doing a SELECT on the table. After you retrieve a result, column_object->max_length contains the length of the biggest value for this column in the specific query.

The 8192 size is chosen because MySQL doesn't know the maximum length for the BLOB. This should be made configurable sometime.

18.4.13.2 Return values

The MYSQL_FIELD structure of the current column (NULL is returned if no columns are left).

18.4.13.3 Errors

None.

18.4.13.4 Example

MYSQL_FIELD *field;

while((field = mysql_fetch_field(result)))
{
    printf("field name %s\n", field->name);
}

18.4.14 `mysql_fetch_fields()`

MYSQL_FIELD *mysql_fetch_fields(MYSQL_RES * result)

18.4.14.1 Description

Returns an array of all MYSQL_FIELD structures for a result. Each structure provides the field definition for one column of the result set.

18.4.14.2 Return values

An array of MYSQL_FIELD structures for all columns of a result set.

18.4.14.3 Errors

None.

18.4.14.4 Example

unsigned int num_fields;
unsigned int i;
MYSQL_FIELD *fields;

num_fields = mysql_num_fields(result);
fields = mysql_fetch_fields(result);
for(i = 0; i < num_fields; i++)
{
   printf("Field %u is %s\n", i, fields[i].name);
}

18.4.15 `mysql_fetch_field_direct()`

MYSQL_FIELD * mysql_fetch_field_direct(MYSQL_RES * result, unsigned int fieldnr)

18.4.15.1 Description

Given a field number fieldnr, returns the fieldnr column's field definition of a result set as a MYSQL_FIELD structure. fieldnr begins at zero. You may use this function to retrieve the definition for any arbitrary column.

18.4.15.2 Return values

The MYSQL_FIELD structure of the specified column.

18.4.15.3 Errors

None.

18.4.15.4 Example

unsigned int num_fields;
unsigned int i;
MYSQL_FIELD *field;

num_fields = mysql_num_fields(result);
for(i = 0; i < num_fields; i++)
{
    field = mysql_fetch_field_direct(result, i);
    printf("Field %u is %s\n", i, field->name);
}

18.4.16 `mysql_fetch_lengths()`

unsigned long *mysql_fetch_lengths(MYSQL_RES *result)

18.4.16.1 Description

Returns the lengths of the columns of the current row. If you have binary data, you must use this function to determine the size of the data. If you copy the data, this length information is also useful for optimization, because you can avoid calling strlen().

18.4.16.2 Return values

An array of unsigned long integers representing the size of each column (does not include terminating NUL characters). A C NULL pointer if there is an error.

18.4.16.3 Errors

NULL is returned if you call this before calling mysql_fetch_row() or after retrieving all rows in the result.

18.4.16.4 Example

MYSQL_ROW row;
unsigned int * lengths;
unsigned int num_fields;
unsigned int i;

row = mysql_fetch_row(result);
if (row)
{
    num_fields = mysql_num_fields(result);
    lengths = mysql_fetch_lengths(result);
    for(i = 0; i < num_fields; i++)
    {
         printf("Column %u is %lu bytes in length.\n", i, lengths[i]);
    }
}

18.4.17 `mysql_fetch_row()`

MYSQL_ROW mysql_fetch_row(MYSQL_RES *result)

18.4.17.1 Description

Retrieves the next row of a result set. Returns NULL when there are no more rows to retrieve. When used with mysql_use_result(), data are dynamically retrieved from the server and thus errors may occur in this situation.

18.4.17.2 Return values

A MYSQL_ROW structure for the next row, or NULL if there is an error or there are no more rows to retrieve.

18.4.17.3 Errors

CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.17.4 Example

MYSQL_ROW row;
unsigned int num_fields;
unsigned int i;

num_fields = mysql_num_fields(result);
while ((row = mysql_fetch_row(result)))
{
   unsigned long *lengths;
   lengths = mysql_fetch_lengths(result);
   for(i = 0; i < num_fields; i++)
   {
       printf("[%.*s] ", (int) lengths[i],row[i]);
   }
   printf("\n");
}

18.4.18 `mysql_field_seek()`

MYSQL_FIELD_OFFSET mysql_field_seek(MYSQL_RES *result, MYSQL_FIELD_OFFSET offset)

18.4.18.1 Description

Sets the field cursor to the given offset. The next call to mysql_fetch_field() will retrieve the column associated with that offset.

To seek to the beginning of a row, pass an offset value of zero.

18.4.18.2 Return values

The previous value of the field cursor.

18.4.18.3 Errors

None.

18.4.19 `mysql_field_tell()`

MYSQL_FIELD_OFFSET mysql_field_tell(MYSQL_RES *result)

18.4.19.1 Description

Returns the position of the field cursor used for the last mysql_fetch_field(). This value can be used as an argument to mysql_field_seek().

18.4.19.2 Return values

The current offset of the field cursor.

18.4.19.3 Errors

None.

18.4.20 `mysql_free_result()`

void mysql_free_result(MYSQL_RES *result)

18.4.20.1 Description

Frees the memory allocated for a result set by mysql_store_result(), mysql_use_result(), mysql_list_dbs(), etc. When you are done with the result set, you must free the memory it uses by calling mysql_free_result().

18.4.20.2 Return values

None.

18.4.20.3 Errors

None.

18.4.21 `mysql_get_client_info()`

char *mysql_get_client_info(void)

18.4.21.1 Description

Returns a string that represents the client library version.

18.4.21.2 Return values

A character string that represents the MySQL client library version.

18.4.21.3 Errors

None.

18.4.22 `mysql_get_host_info()`

char *mysql_get_host_info(MYSQL *mysql)

18.4.22.1 Description

Returns a string describing the type of connection in use, including the server host name.

18.4.22.2 Return values

A character string representing the server host name and the connection type.

18.4.22.3 Errors

None.

18.4.23 `mysql_get_proto_info()`

unsigned int mysql_get_proto_info(MYSQL *mysql)

18.4.23.1 Description

Returns the protocol version used by current connection.

18.4.23.2 Return values

An unsigned integer representing the protocol version used by the current connection.

18.4.23.3 Errors

None.

18.4.24 `mysql_get_server_info()`

char *mysql_get_server_info(MYSQL *mysql)

18.4.24.1 Description

Returns a string that represents the server version number.

18.4.24.2 Return values

A character string that represents the server version number.

18.4.24.3 Errors

None.

18.4.25 `mysql_info()`

char * mysql_info(MYSQL *mysql)

18.4.25.1 Description

Retrieves a string providing information about the most recently executed query. The format of the string varies depending on the type of query, as described below (the numbers are illustrative only; the string will contain values appropriate for the query):

INSERT INTO ... SELECT ...: String format: Records: 100 Duplicates: 0 Warnings: 0
LOAD DATA INFILE ...: String format: Records: 1 Deleted: 0 Skipped: 0 Warnings: 0
ALTER TABLE: String format: Records: 3 Duplicates: 0 Warnings: 0
INSERT INTO TABLE ... VALUES (...),(...),(...)...: String format: Records: 3 Duplicates: 0 Warnings: 0

18.4.25.2 Return values

A character string representing additional information about the query that was most recently executed. A NULL pointer if no information is available for the query.

18.4.25.3 Errors

None.

18.4.26 `mysql_init()`

MYSQL * mysql_init(MYSQL *mysql)

18.4.26.1 Description

Allocates or initializes a MYSQL object suitable for mysql_real_connect(). If the argument is a NULL pointer, the function allocates, initializes and returns a new object, otherwise the object is initialized and the address to the object is returned. If a new object is allocated, mysql_close() will free this object.

18.4.26.2 Return values

An initialized MYSQL* handle or a NULL pointer if there wasn't enough memory to allocate a new object.

18.4.26.3 Errors

In case of low memory a NULL is returned.

18.4.27 `mysql_insert_id()`

my_ulonglong mysql_insert_id(MYSQL *mysql)

18.4.27.1 Description

Returns the ID generated for an AUTO_INCREMENT field. Use this function after you have performed an INSERT query into a table that contains an AUTO_INCREMENT field.

18.4.27.2 Return values

The value of the last AUTO_INCREMENT field updated.

18.4.27.3 Errors

None.

18.4.28 `mysql_kill()`

int mysql_kill(MYSQL *mysql, unsigned long pid)

18.4.28.1 Description

Asks the server to kill the thread specified by pid.

18.4.28.2 Return values

Zero on success. Non-zero on failure.

18.4.28.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.29 `mysql_list_dbs()`

MYSQL_RES *mysql_list_dbs(MYSQL *mysql, const char *wild)

18.4.29.1 Description

Returns a result set consisting of database names on the server that match the simple regular expression specified by the wild argument. wild may contain the wildcard characters `%' or `_', or may be a NULL pointer to match all databases. Calling mysql_list_dbs() is similar to executing the query SHOW databases [LIKE wild].

You must free the result set with mysql_free_result().

18.4.29.2 Return values

A MYSQL_RES result set for success. NULL if there is a failure.

18.4.29.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_OUT_OF_MEMORY: Out of memory.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.30 `mysql_list_fields()`

MYSQL_RES *mysql_list_fields(MYSQL *mysql, const char *table, const char *wild)

18.4.30.1 Description

Returns a result set consisting of field names in the given table that match the simple regular expression specified by the wild argument. wild may contain the wildcard characters `%' or `_', or may be a NULL pointer to match all fields. Calling mysql_list_fields() is similar to executing the query SHOW fields FROM table [LIKE wild].

You must free the result set with mysql_free_result().

18.4.30.2 Return values

A MYSQL_RES result set for success. NULL if there is a failure.

18.4.30.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.31 `mysql_list_processes()`

MYSQL_RES *mysql_list_processes(MYSQL *mysql)

18.4.31.1 Description

Returns a result set describing the current server threads. This is the same kind of information as that reported by mysqladmin processlist.

You must free the result set with mysql_free_result().

18.4.31.2 Return values

A MYSQL_RES result set for success. NULL if there is a failure.

18.4.31.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.32 `mysql_list_tables()`

MYSQL_RES *mysql_list_tables(MYSQL *mysql, const char *wild)

18.4.32.1 Description

Returns a result set consisting of table names in the current database that match the simple regular expression specified by the wild argument. wild may contain the wildcard characters `%' or `_', or may be a NULL pointer to match all tables. Calling mysql_list_tables() is similar to executing the query SHOW tables [LIKE wild].

You must free the result set with mysql_free_result().

18.4.32.2 Return values

A MYSQL_RES result set for success. NULL if there is a failure.

18.4.32.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.33 `mysql_num_fields()`

unsigned int mysql_num_fields(MYSQL_RES *result)

18.4.33.1 Description

Returns the number of columns in a result set.

18.4.33.2 Return values

An unsigned integer representing the number of fields in a result set.

18.4.33.3 Errors

None.

18.4.34 `mysql_num_rows()`

int mysql_num_rows(MYSQL_RES *result)

18.4.34.1 Description

Returns the number of rows in the result set.

If you use mysql_use_result(), mysql_num_rows() will not return the correct value until all the rows in the result set have been retrieved.

18.4.34.2 Return values

The number of rows in the result set.

18.4.34.3 Errors

None.

18.4.35 `mysql_ping()`

int mysql_ping(MYSQL *mysql)

18.4.35.1 Description

Checks if the connection to the server is working. If it has gone down, an automatic reconnection will be attempted.

This function can be used in clients that stay silent for a long while, to check (and reconnect) if the server has closed the connection.

18.4.35.2 Return values

Zero if the server is alive. Any other value indicates an error.

18.4.35.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.36 `mysql_query()`

int mysql_query(MYSQL *mysql, const char *query)

18.4.36.1 Description

Executes the SQL query pointed to by the null-terminated string query.

mysql_query() cannot be used for queries that contain binary data. (Binary data may contain the `\0' character, which would be interpreted as the end of the query string.) For such cases, use mysql_real_query() instead.

18.4.36.2 Return values

Zero if the query was successful. Non-zero if the query failed.

18.4.36.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.37 `mysql_real_connect()`

MYSQL *mysql_real_connect(MYSQL *mysql, const char *host, const char *user, const char *passwd, const char *db, uint port, const char *unix_socket, uint client_flag)

18.4.37.1 Description

Attempts to establish a connection to a MySQL database engine running on host. The value of host may be either a hostname or an IP address. The user parameter contains the user's MySQL login ID, and the passwd parameter contains the password for user. NOTE: Do not attempt to encrypt passwd before calling mysql_real_connect(). Password encryption is handled automatically by the client API.

Note that before calling mysql_real_connect() you have to call mysql_init() to get or initialize a MYSQL structure.

If host is NULL or the string "localhost", a connection to the local host is assumed. If the OS supports sockets (Unix) or named pipes (Win32), they are used instead of TCP/IP to connect to the server.
If user is NULL, the current user is assumed. Under Windows ODBC, the current user must be specified explicitly. Under Unix, the current login name is assumed.
If passwd is NULL, only records in the user table for the user that have a blank password field will be checked for a match. This allows the database administrator to set up the MySQL privilege system in such a way that users get different privileges depending on whether or not they have specified a password.
If db is not NULL, the connection will set the default database to this value.
If port is not 0, the value will be used as the port number for the TCP/IP connection. Note that it's the host parameter that decides the type of the connection.
If unix_socket is not NULL, the string specifies the socket or named pipe that should be used. Note that it's the host parameter that decides the type of the connection.

The value of client_flag is usually 0, but can be set to a combination of the following flags is very special circumstances:

Flag name	Flag meaning
`CLIENT_FOUND_ROWS`	Return the number of found rows, not the number of affected rows
`CLIENT_NO_SCHEMA`	Don't allow the `database.table.column`
`CLIENT_COMPRESS`	Use compression protocol
`CLIENT_ODBC`	The client is an ODBC client. This changes `mysqld` to be more ODBC-friendly.

mysql_real_connect() must complete successfully before you can execute any of the other API functions, with the exception of mysql_get_client_info().

You may optionally specify the first argument of mysql_real_connect() to be a NULL pointer. This will force the C API to allocate memory for the connection structure automatically and to free it when you call mysql_close(). The disadvantage of this approach is that you can't retrieve an error message from mysql_real_connect() if the connection fails.

If the first argument is not a NULL pointer, it should be the address of an existing MYSQL structure.

18.4.37.2 Return values

A MYSQL* connection handle if the connection was successful. A C NULL pointer if the connection was unsuccessful.

18.4.37.3 Errors

CR_CONN_HOST_ERROR: Failed to connect to the MySQL server.
CR_CONNECTION_ERROR: Failed to connect to the local MySQL server.
CR_IPSOCK_ERROR: Failed to create an IP socket.
CR_OUT_OF_MEMORY: Out of memory.
CR_SOCKET_CREATE_ERROR: Failed to create a Unix socket.
CR_UNKNOWN_HOST: Failed to find the IP address for the hostname.
CR_VERSION_ERROR: A protocol mismatch resulted from attempting to connect to a server with a client library that uses a different protocol version. This can happen if you use a very old client library to connect to a new server that wasn't started with the --old-protocol option.
CR_NAMEDPIPEOPEN_ERROR;: Failed to create a named pipe on Win32.
CR_NAMEDPIPEWAIT_ERROR;: Failed to wait for a named pipe on Win32.
CR_NAMEDPIPESETSTATE_ERROR;: Failed to get a pipe handler on Win32.

18.4.37.4 Example

NEED EXAMPLE HERE

18.4.38 `mysql_real_query()`

int mysql_real_query(MYSQL *mysql, const char *query, unsigned int length)

18.4.38.1 Description

Executes the SQL query pointed to by query, which should be a string length bytes long. You must use mysql_real_query() for queries that contain binary data, since binary data may contain the `\0' character. In addition, mysql_real_query() is faster than mysql_query() since it does not call strlen() on the query.

18.4.38.2 Return values

Zero if the query was successful. Non-zero if the query failed.

18.4.38.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.39 `mysql_reload()`

int mysql_reload(MYSQL *mysql)

18.4.39.1 Description

Asks the MySQL server to reload the access permissions tables. The connected user must have reload privileges.

18.4.39.2 Return values

Zero on success. Non-zero on failure.

18.4.39.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.40 `mysql_row_tell()`

unsigned int mysql_row_tell(MYSQL_RES *result)

18.4.40.1 Description

Returns the current position of the row cursor for the last mysql_fetch_row(). This value can be used as an argument to mysql_row_seek().

18.4.40.2 Return values

The current offset of the row cursor.

18.4.40.3 Errors

None.

18.4.41 `mysql_select_db()`

int mysql_select_db(MYSQL *mysql, const char *db)

18.4.41.1 Description

Instructs the current connection specified by mysql to use the database specified by db as the default (current) database. In subsequent queries, this database becomes the default for table references that do not indicate an explicit database specifier.

mysql_select_db() fails unless the connected user can be authenticated as having permission to use the database.

18.4.41.2 Return values

Zero on success. Non-zero on failure.

18.4.41.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.42 `mysql_shutdown()`

int mysql_shutdown(MYSQL *mysql)

18.4.42.1 Description

Asks the database server to shutdown. The connected user must have shutdown privileges.

18.4.42.2 Return values

Zero on success. Non-zero on failure.

18.4.42.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.43 `mysql_stat()`

char *mysql_stat(MYSQL *mysql)

18.4.43.1 Description

Returns information similar to that provided by mysqladmin status as a character string. This includes uptime in seconds and the number of running threads, questions, reloads and open tables.

18.4.43.2 Return values

A character string describing the server status. NULL if the command failed.

18.4.43.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.44 `mysql_store_result()`

MYSQL_RES *mysql_store_result(MYSQL *mysql)

18.4.44.1 Description

Reads the result of a query to the client, allocates a MYSQL_RES structure, and places the results into this structure. You must call mysql_store_result() or mysql_use_result() after every query which successfully retrieves data.

An empty result set is returned if there are no rows returned.

You must call mysql_free_result() once you are done with the result set.

18.4.44.2 Return values

A MYSQL_RES result structure with the results. NULL if there was an error.

18.4.44.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_OUT_OF_MEMORY: Out of memory.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.45 `mysql_thread_id()`

unsigned long mysql_thread_id(MYSQL * mysql)

18.4.45.1 Description

Returns the thread id of the current connection. This value can be used as an argument to mysql_kill() to kill the thread.

18.4.45.2 Return values

The thread id of the current connection.

18.4.45.3 Errors

None.

18.4.46 `mysql_use_result()`

MYSQL_RES *mysql_use_result(MYSQL *mysql)

18.4.46.1 Description

mysql_use_result() reads the result of a query directly from the server without storing it in a temporary table or local buffer. This is somewhat faster and uses much less memory than mysql_store_result(). In this case the client will only allocate memory for the current row and a communication buffer of size max_allowed_packet. On the other hand, you shouldn't use mysql_use_result() if you are doing a lot of processing for each row at the client side, or if the output is sent to a screen on which the user may type a ^S (stop scroll). This would tie up the server and then other threads couldn't update the used tables.

When using mysql_use_result(), you must execute mysql_fetch_row() until you get back a NULL value, otherwise the next query will get results from the previous query. The C API will give the error Commands out of sync; You can't run this command now if you forget to do this!

You may not use mysql_data_seek(), mysql_num_rows() or mysql_affected_rows() with a result returned from mysql_use_result(), nor may you issue other queries until the mysql_use_result() has finished.

You must call mysql_free_result() once you are done with the result set.

18.4.46.2 Return values

A MYSQL_RES result structure. NULL if there was an error.

18.4.46.3 Errors

CR_COMMANDS_OUT_OF_SYNC: Commands were executed in an improper order.
CR_OUT_OF_MEMORY: Out of memory.
CR_SERVER_GONE_ERROR: The MySQL server has gone away.
CR_SERVER_LOST: The connection to the server was lost during the query.
CR_UNKNOWN_ERROR: An unknown error occurred.

18.4.47 Why is it that after `mysql_query()` returns success, `mysql_store_result()` sometimes returns `NULL?`

It is possible for mysql_store_result() to return NULL following a successful call to mysql_query(). When this happens, it means one of the following conditions occurred:

There was a malloc() failure.
The data couldn't be read (error on connection).
The query returned no data (INSERT, UPDATE or DELETE).

You can always check whether or not the statement should have produced a non-empty result by calling mysql_num_fields(). If mysql_num_fields() returns zero, the result is empty and the last query was a statement that does not return values (for example, an INSERT or a DELETE). If mysql_num_fields() returns a non-zero value, the statement should have produced a non-empty result.

You can also test for an error by calling mysql_error() or mysql_errno().

18.4.48 What results can I get from a query?

In addition to the result set returned by a query, you can also get the following information:

mysql_affected_rows() returns the number of affected rows in the last query when doing an INSERT, UPDATE or DELETE. An exception is that if DELETE is used without a WHERE clause, the table is truncated, which is much faster! In this case, mysql_affected_rows() returns zero for the number of records affected.
mysql_insert_id() returns the ID generated by the last query that inserted a row into a table with an AUTO_INCREMENT index. See section 18.4.49 How can I get the unique ID for the last inserted row?.
Some queries (LOAD DATA INFILE ..., INSERT INTO ... SELECT ..., UPDATE) return additional info. The result is returned by mysql_info(). mysql_info() returns a NULL pointer if there is no additional information.

18.4.49 How can I get the unique ID for the last inserted row?

If you insert a record in a table containing a column that has the AUTO_INCREMENT attribute, you can get the given ID with the mysql_insert_id() function.

You can also retrieve the ID by using the LAST_INSERT_ID() function in a query string that you pass to mysql_query().

You can check if an AUTO_INCREMENT index is used by executing the following code. This also checks if the query was an INSERT with an AUTO_INCREMENT index:

if (mysql_error(MYSQL)[0] == 0 &&
    mysql_num_fields(MYSQL_RESULT) == 0 &&
    mysql_insert_id(MYSQL) != 0)
  used_id = mysql_insert_id(MYSQL);

18.4.50 Problems linking with the C API

When linking with the C API, you can get the following errors on some systems:

gcc -g -o client test.o -L/usr/local/lib/mysql -lmysqlclient -lsocket -lnsl

Undefined        first referenced
 symbol          in file
floor            /usr/local/lib/mysql/libmysqlclient.a(password.o)
ld: fatal: Symbol referencing errors. No output written to client

This means that on your system you must include the math library (-lm) at the end of the compile/link line.

18.4.51 How to make a thread-safe client

The client is `almost' thread-safe. The biggest problem is that `net.c' (the file containing the subroutines that read from sockets) is not interrupt-safe. This was done with the thought that you might want to have your own alarm that can break a long read to a server.

The standard client libraries are not compiled with the thread options.

To get a thread-safe client, use the -lmysys, -lstring and -ldbug libraries and net_serv.o that the server uses.

When using a threaded client, you can make great use of the the routines in the `thr_alarm.c' file. If you are using routines from the mysys library, the only thing you must remember is to call my_init() first!

All functions except mysql_connect() are currently thread-safe.

To make mysql_connect() thread-safe, you must recompile the client with this command:

shell> CPPFLAGS=-DTHREAD_SAFE_CLIENT ./configure ...

You may get some errors because of undefined symbols when linking the standard client, because the pthread libraries are not included by default.

The resulting `libmysqld.a' library is now thread-safe.

Two threads can't use the same connection handle (returned by mysql_connect()) at the same time, even if two threads can use different MYSQL_RES pointers that were created with mysql_store_result().

18.5 MySQL Perl API's

Since DBI/DBD now is the recommended Perl interface, mysqlperl is not documented here.

18.5.1 `DBI` with `DBD::mysql`

DBI is a generic interface for many databases. That means that you can write a script that works with many different database engines without change. You need a DataBase Driver (DBD) defined for each database type. For MySQL, this driver is called DBD::mysql.

For more information on the Perl5 DBI, please visit the DBI web page and read the documentation:

http://www.hermetica.com/technologia/DBI/

For more information on Object Oriented Programming (OOP) as defined in Perl5, see the Perl OOP page:

http://language.perl.com/info/documentation.html

18.5.1.1 The `DBI` interface

Portable DBI methods

`connect`	Establishes a connection to a database server
`prepare`	Prepares a SQL statement for execution
`do`	Prepares and executes a SQL statement
`disconnect`	Disconnects from the database server
`quote`	Quotes string or `BLOB` values to be inserted
`execute`	Executes prepared statements
`fetchrow_array`	Fetches the next row as an array of fields.
`fetchrow_arrayref`	Fetches next row as a reference array of fields
`fetchrow_hashref`	Fetches next row as a reference to a hashtable
`fetchall_arrayref`	Fetches all data as an array of arrays
`finish`	Finishes a statement and let the system free resources
`rows`	Returns the number of rows affected
`data_sources`	Returns an array of databases available on localhost
`ChopBlanks`	Controls whether `fetchrow_*` methods trim spaces
`NUM_OF_PARAMS`	The number of placeholders in the prepared statement
`NULLABLE`	Which columns can be `NULL`

MySQL-specific methods

`insertid`	The latest `AUTO_INCREMENT` value
`is_blob`	Which column are `BLOB` values
`is_key`	Which columns are keys
`is_num`	Which columns are numeric
`is_pri_key`	Which columns are primary keys
`is_not_null`	Which columns CANNOT be `NULL`. See `NULLABLE`.
`length`	Maximum possible column sizes
`max_length`	Maximum column sizes actually present in result
`NAME`	Column names
`NUM_OF_FIELDS`	Number of fields returned
`table`	Table names in returned set
`type`	All column types
`_CreateDB`	Create a database
`_DropDB`	Drop a database. THIS IS DANGEROUS.

The Perl methods are described in more detail below:

connect($data_source, $username, $password)

Use the connect method to make a database connection to the data source. The $data_source value should begin with DBI:driver_name:. Example uses of connect with the DBD::mysql driver:

$dbh = DBI->connect("DBI:mysql:$database", $user, $password);
$dbh = DBI->connect("DBI:mysql:$database:$hostname",
                    $user, $password);
$dbh = DBI->connect("DBI:mysql:$database:$hostname:$port",
                    $user, $password);

If the user name and/or password are undefined, DBI uses the values of the DBI_USER and DBI_PASS environment variables, respectively. If you don't specify a hostname, it defaults to 'localhost'. If you don't specify a port number, it defaults to the default MySQL port (3306).

prepare($statement)

Prepares a SQL statement for execution by the database engine and returns a statement handle ($sth) which you can use to invoke the execute method. Example:

$sth = $dbh->prepare($statement)
    or die "Can't prepare $statement: $dbh->errstr\n";

do($statement)

The do method prepares and executes a SQL statement and returns the number of rows affected. This method is generally used for "non-select" statements which cannot be prepared in advance (due to driver limitations) or which do not need to executed more than once (inserts, deletes, etc.). Example:

$rc = $dbh->do($statement)
        or die "Can't execute $statement: $dbh- >errstr\n";

disconnect

The disconnect method disconnects the database handle from the database. This is typically called right before you exit from the program. Example:

$rc = $dbh->disconnect;

quote($string)

The quote method is used to "escape" any special characters contained in the string and to add the required outer quotation marks. Example:

$sql = $dbh->quote($string)

execute

The execute method executes the prepared statement. For non-SELECT statements, it returns the number of rows affected. For SELECT statements, execute only starts the SQL query in the database. You need to use one of the fetch_* methods described below to retrieve the data. Example:

$rv = $sth->execute or die "can't execute the query: $sth->errstr;

fetchrow_array

This method fetches the next row of data and returns it as an array of field values. Example:

while(@row = $sth->fetchrow_array) {
        print qw($row[0]\t$row[1]\t$row[2]\n);
}

fetchrow_arrayref

This method fetches the next row of data and returns it as a reference to an array of field values. Example:

while($row_ref = $sth->fetchrow_arrayref) {
        print qw($row_ref->[0]\t$row_ref->[1]\t$row_ref->[2]\n);
}

fetchrow_hashref

This method fetches a row of data and returns a reference to a hash table containing field name/value pairs. This method is not nearly as efficient as using array references as demonstrated above. Example:

while($hash_ref = $sth->fetchrow_hashref) {
        print qw($hash_ref->{firstname}\t$hash_ref->{lastname}\t\
                $hash_ref- > title}\n);
}

fetchall_arrayref

This method is used to get all the data (rows) to be returned from the SQL statement. It returns a reference to an array of arrays of references to each row. You access/print the data by using a nested loop. Example:

my $table = $sth->fetchall_arrayref
                or die "$sth->errstr\n";
my($i, $j);
for $i ( 0 .. $#{$table} ) {
        for $j ( 0 .. $#{$table->[$i]} ) {
                print "$table->[$i][$j]\t";
        }
        print "\n";
}

finish

Indicates that no more data will be fetched from this statement handle. You call this method to free up the statement handle and any system resources it may be holding. Example:

$rc = $sth->finish;

rows

Returns the number of rows affected (updated, deleted, etc.) from the last command. This is usually used after a do or non-SELECT execute statement. Example:

$rv = $sth->rows;

NULLABLE

Returns a reference to an array of boolean values; for each element of the array, a value of TRUE indicates that this column may contain NULL values. Example:

$null_possible = $sth->{NULLABLE};

NUM_OF_FIELDS

This attribute indicates the number of fields returned by a SELECT or SHOW FIELDS statement. You may use this for checking whether a statement returned a result: A zero value indicates a non-SELECT statement like INSERT, DELETE or UPDATE. Example:

$nr_of_fields = $sth->{NUM_OF_FIELDS};

data_sources($driver_name)

This method returns an array containing names of databases available to the MySQL server on the host 'localhost'. Example:

@dbs = DBI->data_sources("mysql");

ChopBlanks

This attribute determines whether the fetchrow_* methods will chop leading and trailing blanks from the returned values. Example:

$sth->{'ChopBlanks'} =1;

MySQL-specific methods

insertid

If you use the AUTO_INCREMENT feature of MySQL, the new auto-incremented values will be stored here. Example:

$new_id = $sth->{insertid};

is_blob

Returns a reference to an array of boolean values; for each element of the array, a value of TRUE indicates that the respective column is a BLOB. Example:

$keys = $sth->{is_blob};

is_key

Returns a reference to an array of boolean values; for each element of the array, a value of TRUE indicates that the respective column is a key. Example:

$keys = $sth->{is_key};

is_num

Returns a reference to an array of boolean values; for each element of the array, a value of TRUE indicates that the respective column contains numeric values. Example:

$nums = $sth->{is_num};

is_pri_key

Returns a reference to an array of boolean values; for each element of the array, a value of TRUE indicates that the respective column is a primary key. Example:

$pri_keys = $sth->{is_pri_key};

is_not_null

Returns a reference to an array of boolean values; for each element of the array, a value of FALSE indicates that this column may contain NULL values. Example:

$not_nulls = $sth->{is_not_null};

It is preferable to use the NULLABLE attribute (described above), since that is a DBI standard.

length

max_length

Each of these methods returns a reference to an array of column sizes. The length array indicates the maximum possible sizes (that is, the sizes the columns are declared to be in the table description). The max_length array indicates the maximum sizes actually present in the result table. Example:

$lengths = $sth->{length};
$max_lengths = $sth->{max_length};

NAME

Returns a reference to an array of column names. Example:

$names = $sth->{NAME};

table

Returns a reference to an array of table names. Example:

$tables = $sth->{table};

type

Returns a reference to an array of column types. Example:

$types = $sth->{type};

_CreateDB

Creates a database. This method is deprecated. It is preferable to issue a CREATE DATABASE statement using the do method instead, since do is a DBI standard.

_DropDB

Drops a database. THIS IS DANGEROUS. This method is deprecated. It is preferable to issue a DROP DATABASE statement using the do method instead, since do is a DBI standard.

18.5.1.2 More `DBI`/`DBD` information

You can use the perldoc command to get more information about DBI.

perldoc DBI
perldoc DBI::FAQ
perldoc mysql

You can also use the pod2man, pod2html, etc., tools to translate to other formats.

And of course you can find the latest DBI information at the DBI web page:

http://www.hermetica.com/technologia/DBI/

18.6 MySQL Java connectivity (JDBC)

There are 2 supported JDBC drivers for MySQL (the twz and mm driver). You can find a copy of these at http://www.tcx.se/Contrib. For documentation consult any JDBC documentation and the drivers own documentation for MySQL specific features.

18.7 MySQL PHP API's

PHP documentation.

18.8 MySQL C++ API's

Insert pointers/descriptions for C++.

18.9 MySQL Python API's

Insert pointers/descriptions for Python.

18.10 MySQL TCL API's

Insert pointers/descriptions for TCL.

19 How MySQL compares to other databases

19.1 How MySQL compares to `mSQL`

This section has been written by the MySQL developers, so it should be read with that in mind. But there are NO factual errors that we know of.

For a list of all supported limits, functions and types, see the crash-me web page.

Performance

For a true comparison of speed, consult the growing MySQL benchmark suite. See section 11 The MySQL benchmark suite. Because there is no thread creation overhead, a small parser, few features and simple security, mSQL should be quicker at:

Tests that perform repeated connects and disconnects, running a very simple query during each connection.
INSERT operations into very simple tables with few columns and keys.
CREATE TABLE and DROP TABLE.
SELECT on something that isn't an index. (A table scan is very easy.)

Since these operations are so simple, it is hard to be better at them when you have a higher startup overhead. After the connection is established, MySQL should perform much better. On the other hand, MySQL is much faster than mSQL (and most other SQL implementions) on the following:

Complex SELECT operations.
Retrieving large results (MySQL has a better, faster and safer protocol).
Tables with variable-length strings, since MySQL has more efficent handling and can have indexes on VARCHAR columns.
Handling tables with many columns.
Handling tables with large record lengths.
SELECT with many expressions.
SELECT on large tables.
Handling many connections at the same time. MySQL is fully multi-threaded. Each connection has its own thread, which means that no thread has to wait for another (unless a thread is modifying a table another thread wants to access.) In mSQL, once one connection is established, all others must wait until the first has finished, regardless of whether the connection is running a query that is short or long. When the first connection terminates, the next can be served, while all the others wait again, etc.
Joins. mSQL can become pathologically slow if you change the order of tables in a SELECT. In the benchmark suite, a time more than 15000 times slower than MySQL was seen. This is due to mSQL's lack of a join optimizer to order tables in the optimal order. However, if you put the tables in exactly the right order in mSQL 2, the join will be relatively fast! See section 11 The MySQL benchmark suite.
ORDER BY and GROUP BY.
DISTINCT.
Using TEXT or BLOB columns.

SQL Features

GROUP BY and HAVING. mSQL does not support GROUP BY at all. MySQL supports a full GROUP BY with both HAVING and the following functions: COUNT(), AVG(), MIN(), MAX(), SUM() and STD(). COUNT(*) is optimized to return very quickly if the SELECT retrieves from one table, no other columns are retrieved and there is no WHERE clause. MIN() and MAX() may take string arguments.
INSERT and UPDATE with calculations. MySQL can do calculations in an INSERT or UPDATE. For example:
```
mysql> UPDATE SET x=x*10+y WHERE x<20;
```
Aliasing. MySQL has column aliasing.
Qualifying column names. In MySQL, if a column name is unique among the tables used in a query, you do not have to use the full qualifier.
SELECT with functions. MySQL has many functions (too many to list here; see section 7.3 Functions for use in SELECT and WHERE clauses).

Disk space efficiency

That is, how small can you make your tables? MySQL has very precise types, so you can create tables that take very little space. An example of a useful MySQL datatype is the MEDIUMINT that is 3 bytes long. If you have 100,000,000 records, saving even one byte per record is very important. mSQL2 has a more limited set of column types, so it is more difficult to get small tables.

Stability

This is harder to judge objectively. For a discussion of MySQL stability, see section 1.5 How stable is MySQL?. We have no experience with mSQL stability, so we cannot say anything about that.

Price

Another important issue is the license. MySQL has a more flexible license than mSQL, and is also less expensive than mSQL. Whichever product you choose to use, remember to at least consider paying for a license or email support. (You are required to get a license if you include MySQL with a product that you sell, of course.)

Perl interfaces

MySQL has basically the same interfaces to Perl as mSQL with some added features.

JDBC (Java)

MySQL currently has 4 JDBC drivers:

The gwe driver: A Java interface by GWE technologies (not supported anymore).
The jms driver: An improved gwe driver by Xiaokun Kelvin ZHU
The twz driver: A type 4 JDBC driver by Terrence W. Zellers and educational use.
The mm driver: A type 4 JDBC driver by Mark Matthews

The recommended drivers are the twz or mm driver. Both are reported to work excellently. We know that mSQL has a JDBC driver, but we have too little experience with it to compare.

Rate of development

MySQL has a very small team of developers, but we are quite used to coding C and C++ very rapidly. Since threads, functions, GROUP BY and so on are still not implemented in mSQL, it has a lot of catching up to do. To get some perspective on this, you can view the mSQL `HISTORY' file for the last year and compare it with the News section of the MySQL Reference Manual (see section D MySQL change history). It should be pretty obvious which one has developed most rapidly.

Utility programs

Both mSQL and MySQL have many interesting third-party tools. Since it is very easy to port upward (from mSQL to MySQL), almost all the interesting applications that are available for mSQL are also available for MySQL. MySQL comes with a simple msql2mysql program that fixes differences in spelling between mSQL and MySQL for the most-used C API functions. For example, it changes instances of msqlConnect() to mysql_connect(). Converting a client program from mSQL to MySQL usually takes a couple of minutes.

19.1.1 How to convert `mSQL` tools for MySQL

According to our experience, it would just take a few hours to convert tools such as msql-tcl and msqljava that use the mSQL C API so that they work with the MySQL C API.

The conversion procedure is:

Run the shell script msql2mysql on the source. This requires the replace program, which is distributed with MySQL.
Compile.
Fix all compiler errors.

Differences between the mSQL C API and the MySQL C API are:

MySQL uses a MYSQL structure as a connection type (mSQL uses an int).
mysql_connect() takes a pointer to a MYSQL structure as a parameter. It is easy to define one globally or to use malloc() to get one. mysql_connect() also takes 2 parameters for specifying the user and password. You may set these to NULL, NULL for default use.
mysql_error() takes the MYSQL structure as a parameter. Just add the parameter to your old msql_error() code if you are porting old code.
MySQL returns an error number and a text error message for all errors. mSQL returns only a text error message.
Some incompatibilities exist as a result of MySQL supporting multiple connections to the server from the same process.

19.1.2 How `mSQL` and MySQL client/server communications protocols differ

There are enough differences that it is impossible (or at least not easy) to support both.

The most significant ways in which the MySQL protocol differs from the mSQL protocol are listed below:

A message buffer may contain many result rows.
The message buffers are dynamically enlarged if the query or the result is bigger than the current buffer, up to a configurable server and client limit.
All packets are numbered to catch duplicated or missing packets.
All column values are sent in ASCII. The lengths of columns and rows are sent in packed binary coding (1, 2 or 3 bytes).
MySQL can read in the result unbuffered (without having to store the full set in the client).
If a single write/read takes more than 30 seconds, the server closes the connection.
If a connection is idle for 8 hours, the server closes the connection.

19.1.3 How `mSQL` 2.0 SQL syntax differs from MySQL

Column types

MySQL

Has the following additional types (among others; see section 7.6 CREATE TABLE syntax):

ENUM type for one of a set of strings.
SET type for many of a set of strings.
BIGINT type for 64-bit integers.

MySQL also supports the following additional type attributes:

UNSIGNED option for integer columns.
ZEROFILL option for integer columns.
AUTO_INCREMENT option for integer columns that are a PRIMARY KEY. See section 18.4.49 How can I get the unique ID for the last inserted row?.
DEFAULT value for all columns.

mSQL2

mSQL column types correspond to the MySQL types shown below:

`mSQL` type	Corresponding MySQL type
`CHAR(len)`	`CHAR(len)`
`TEXT(len)`	`TEXT(len)`. `len` is the maximal length. And `LIKE` works.
`INT`	`INT`. With many more options!
`REAL`	`REAL`. Or `FLOAT`. Both 4- and 8-byte versions are available.
`UINT`	`INT UNSIGNED`
`DATE`	`DATE`. Uses ANSI SQL format rather than `mSQL`'s own.
`TIME`	`TIME`
`MONEY`	`DECIMAL(12,2)`. A fixed-point value with two decimals.

Index creation

MySQL: Indexes may be specified at table creation time with the CREATE TABLE statement.
mSQL: Indexes must be created after the table has been created, with separate CREATE INDEX statements.

To insert a unique identifier into a table

MySQL: Use AUTO_INCREMENT as a column type specifier. See section 18.4.49 How can I get the unique ID for the last inserted row?.
mSQL: Create a SEQUENCE on a table and select the _seq column.

To obtain a unique identifier for a row

MySQL: Add a PRIMARY KEY or UNIQUE key to the table.
mSQL: Use the _rowid column. Observe that _rowid may change over time depending on many factors.

To get the time a column was last modified

MySQL: Add a TIMESTAMP column to the table. This column is automatically set to the current time for INSERT or UPDATE statements if you don't give the column a value or if you give it a NULL value.
mSQL: Use the _timestamp column.

NULL value comparisons

MySQL: MySQL follows ANSI SQL and a comparison with NULL is always NULL.
mSQL: In mSQL, NULL = NULL is TRUE. You must change =NULL to IS NULL and <>NULL to IS NOT NULL when porting old code from mSQL to MySQL.

String comparisons

MySQL: Normally, string comparisons are performed in case-independent fashion with the sort order determined by the current character set (ISO-8859-1 Latin1 by default). If you don't like this, declare your columns with the BINARY attribute, which causes comparisons to be done according to the ASCII order used on the MySQL server host.
mSQL: All string comparisons are performed in case-sensitive fashion with sorting in ASCII order.

Case-insensitive searching

MySQL: LIKE is a case-insensitive or case-sensitive operator, depending on the columns involved. If possible, MySQL uses indexes if the LIKE argument doesn't start with a wildcard character.
mSQL: Use CLIKE.

Handling of trailing spaces

MySQL: Strips all spaces at the end of CHAR and VARCHAR columns. Use a TEXT column if this behavior is not desired.
mSQL: Retains trailing space.

WHERE clauses

MySQL

MySQL correctly prioritizes everything (AND is evaluated before OR). To get mSQL behavior in MySQL, use parentheses (as shown below).

mSQL

Evaluates everything from left to right. This means that some logical calculations with more than three arguments cannot be expressed in any way. It also means you must change some queries when you upgrade to MySQL. You do this easily by adding parentheses. Suppose you have the following mSQL query:

mysql> SELECT * FROM table WHERE a=1 AND b=2 OR a=3 AND b=4;

To make MySQL evaluate this the way that mSQL would, you must add parentheses:

mysql> SELECT * FROM table WHERE (a=1 AND (b=2 OR (a=3 AND (b=4))));

Access control

MySQL: Has tables to store grant (permission) options per user, host and database. See section 6.4 How the privilege system works.
mSQL: Has a file `mSQL.acl' in which you can grant read/write privileges for users.

19.2 How MySQL compares to PostgreSQL

PostgreSQL has some more advanced features like user-defined types, triggers, rules and transactions. But it lacks many of the standard types and functions from ANSI SQL and ODBC. See the crash-me web page for a complete list of which limits, types and functions are supported or unsupported.

Normally, PostgreSQL is much slower than MySQL. See section 11 The MySQL benchmark suite. This is due largely to their transactions system. If you really need transactions and can afford to pay the speed penalty, you should take a look at PostgreSQL.

A Some MySQL users

19.2.1 Some Web search engines

19.2.2 Some Information search engines concentrated on some area

19.2.3 Some Domain/Internet/Web and related services

Registry of Web providers that support MySQL
Monolith Internet Services runs MySQL supporting nearly a million rows and over 100,000 users.
Online Database
BigBiz Internet Services
The Virt Gazette
Global InfoNet Inc
WebHosters - A Guide to WWW Providers
Internet information server
A pro-Linux/tech news and comment/discussion site
WorldNet Communications - An Internet Services Provider
Web consultant for net citizens
Search site for training courses in the UK

19.2.4 Web sites that use `PHP` and MySQL

Jgaa's Internet - Official Support Site running via apache
Ionline - online publication: MySQL, PHP, Java, Web programming, DB development
BaBoo(Browse and bookmark). Free web-based bookmark manager and Calendar
Course Schedule System at Pensacola Junior College

19.2.5 Uncategorized pages

19.2.6 Some MySQL consultants

19.2.7 Programming

The Perl CPAN Testers results page

Send any additions to this list to webmaster@tcx.se.

B Contributed programs

Many users of MySQL have contributed very useful support tools and addons.

A list of what is available at http://www.tcx.se/Contrib (or any mirror) is shown below. If you want to build MySQL support for the Perl DBI/DBD interface, you should fetch the Data-Dumper, DBI, and Msql-Mysql-modules files and install them. See section 4.10 Perl installation comments.

00-README: This listing.
Data-Dumper-2.09.tar.gz: Perl Data-Dumper module. Useful with DBI/DBD support.
DBI-1.02.tar.gz: Perl DBI module.
Msql-Mysql-modules-1.2005.tar.gz: Perl DBD module.
Db.py: Python module with caching. By gandalf@rosmail.com.
Old-Versions: Previous versions of things found here that you probably won't be interested in.
Vdb-dflts-2.1.tar.gz: This is a new version of a set of library utilities intended to provide a generic interface to SQL database engines such that your application becomes a 3-tiered application. The advantage is that you can easily switch between and move to other database engines by implementing one file for the new backend without needing to make any changes to your applications. By damian@cablenet.net.
access_to_mysql.txt: Paste this function into an Access module of a database which has the tables you want to export. See also exportsql. By Brian Andrews.
dbf2mysql-1.10d.tar.gz: Convert between `.dbf' files and MySQL tables. By Maarten Boekhold, boekhold@cindy.et.tudelft.nl, and Michael Widenius. This converter can't handle MEMO fields.
KAMXbase1.0.tar.gz: Convert between `.dbf' files and MySQL tables. Perl module written by Pratap Pereira pereira@ee.eng.ohio-state.edu, extened by Kevin A. McGrail kmcgrail@digital1.peregrinehw.com. This converter can handle MEMO fields.
delphi-interface.gz: Delphi interface to libmysql.dll, by Blestan Tabakov,
dump2h-1.20.gz: Convert from mysqldump output to a C header file. By Harry Brueckner,
emacs-sql-mode.tar.gz: Raw port of a SQL mode for XEmacs. Supports completion. Original by Peter D. Pezaris pez@atlantic2.sbi.com and partial MySQL port by David Axmark.
exportsql.txt: A script that is similar to access_to_mysql.txt, except that this one is fully configurable, has better type conversion (including detection of TIMESTAMP fields), provides warnings and suggestions while converting, quotes all special characters in text and binary data, and so on. It will also convert to mSQL v1 and v2, and is free of charge for anyone. See http://www.cynergi.net/prod/exportsql/ for latest version. By Pedro Freire, support@cynergi.net.
findres.pl: Find reserved words in tables. By Nem W Schlecht.
genquery.zip
http://www.odbsoft.com/cook/sources.htm: This package has various functions for generating html code from an SQL table structure and for generating SQL statements (Select, Insert, Update, Delete) from an html form. You can build a complete forms interface to an SQL database (query, add, update, delete) without any programming! By Marc Beneteau, marc@odbsoft.com.
handicap.tar.gz: Performance handicapping system for yachts. Uses PHP. By
hylalog-1.0.tar.gz: Store hylafax outgoing faxes in a MySQL database. By Sinisa Milivojevic, sinisa@coresinc.com.
importsql.txt: A script that does the exact reverse of exportsql.txt. That is, it imports data from MySQL into an Access database via ODBC. This is very handy when combined with exportSQL, since it lets you use Access for all DB design and administration, and synchronize with your actual MySQL server either way. Free of charge. See http://www.netdive.com/freebies/importsql/ for any updates. Created by Laurent Bossavit of NetDIVE.
jms130b.zip: JDBC driver for MySQL. Also contains a command line client and other examples. This driver is now rather old and one of the other drivers is recommended. By Xiaokun Kelvin ZHU and GWE Technologies.
twz1jdbcForMysql-1.0.3-GA.tar.gz: New JDBC driver for MySQL. This is a production release and is actively developed. By Terrence W. Zellers, You can always find the newest driver at http://www.voicenet.com/~zellert/tjFM.
mod_auth_mysql-2.20.tar.gz: Apache authentication module for MySQL. By Zeev Suraski, Please register this module at: http://bourbon.netvision.net.il/mysql/mod_auth_mysql/register.html. The registering information is only used for statistical purposes and will encourage further development of this module!
mod_log_mysql-1.05.tar.gz: MySQL logging module for Apache. By Zeev Suraski,
mrtg-mysql-1.0.tar.gz: MySQL status plotting with MRTG, by Luuk de Boer, luuk@wxs.nl.
mysqladmin-atif-1.0.tar.gz: WWW MySQL administrator for the user, db and host tables. By Tim Sailer, modified by Atif Ghaffar aghaffar@artemedia.ch.
mysqltcl-1.53.tar.gz: Tcl interface for MySQL. Based on `msqltcl-1.50.tar.gz'. Updated by Tobias Ritzau, tobri@ida.liu.se.
MySQLmodule-1.4.tar.gz: Python interface for MySQL. By Joseph Skinner,
mypasswd-2.0.tar.gz: Extra for mod_auth_mysql. This is a little tool that allows you to add/change user records storing group and/or password entries in MySQL tables. By Harry Brueckner,
mysql-c++-0.02.tar.gz: MySQL C++ wrapper library. By Roland Haenel,
C++ API: MySQL C++ API; More than just a wrapper library. By kevina@clark.net.
mysql-passwd.README
mysql-passwd-1.2.tar.gz: Extra for mod_auth_mysql. This is a two-part system for use with mod_auth_mysql.
mysql_watchdog.pl: Monitor the MySQL daemon for possible lockups. By Yermo Lamers,
mysql-webadmin-1.0a8-rz.tar.gz: A tool written in PHP-FI to administrate MySQL databases remotely over the web within a Web-Browser. By Peter Kuppelwieser, Not maintained anymore!
phpMyAdmin_1.1.0.tar.gz
phpMyAdmin home page: A PHP3 tool in the spirit of mysql-webadmin, by Tobias Ratschiller, tobias@dnet.it
mysqladm.tar.gz: MySQL Web Database Administration written in Perl. By Tim Sailer.
mysqladm-2.tar.gz: Updated version of `mysqladm.tar.gz', by High Tide.
nsapi_auth_mysql.tar: Netscape Web Server API (NSAPI) functions to authenticate (BASIC) users against MySQL tables. By Yuan John Jiang.
mysqlwinadmn.zip: Win32 GUI (binary only) to administrate a database, by David B. Mansel,
pam_mysql.tar.gz: This module authenticates users via pam, using MySQL.
pike-mysql-1.4.tar.gz: MySQL module for pike. For use with the Roxen web server.
radius-0.3.tar.gz: Patches for radiusd to make it support MySQL. By Wim Bonis,
jradius-diff.gz: MySQL support for Livingston's Radius 2.01. Authentication and Accounting. By Jose de Leon, jdl@thevision.net
sqlscreens-0.4.1.tar.gz: TCL/TK code to generate database screens. By Jean-Francois Dockes.
squile.tar.gz: Module for guile that allows guile to interact with SQL databases. By Hal Roberts.
mm.mysql.jdbc-0.9d.tar.gz: Another new java JDBC driver. By Mark Matthews,
useradm.tar.gz: MySQL administrator in PHP. By Ofni Thomas
wmtcl.doc
wmtcl.lex: With this you can write HTML files with inclusions of TCL code. By
wuftpd-2.4.2b12+mysql_support.tar.gz: Patches to add logging to MySQL for WU-ftpd. By Zeev Suraski,
wuftpd-2.4.2.18+mysql_support.1.tar.gz: Update to the above by takeshi@SoftAgency.co.jp
www-sql-0.5.3.lsm
www-sql-0.5.3.md5
www-sql-0.5.3.tar.gz: A CGI program that parses an HTML file containing special tags, parses them and inserts data from a MySQL database.
xmysql-1.9.tar.gz
xmysql home page: A front end to the MySQL database engine. It allows for simple queries and table maintenance, as well as batch queries. By Rick Mehalick, dblhack@wt.net. Requires xforms 0.88 to work.
xmysqladmin-0.4.4.tar.gz: A front end to the MySQL database engine. It allows reloads, status check, process control, isamcheck, grant/revoke privileges, creating databases, dropping databases, create, alter, browse and drop tables. By Gilbert Therrien, gilbert@ican.net.

C Contributors to MySQL

Contributors to the MySQL distribution are listed below, in somewhat random order:

Michael (Monty) Widenius

Has written the following parts of MySQL:

All the main code in mysqld.
New functions for the string library.
Most of the mysys library.
The NISAM library (A B-tree index file handler with index compression and different record formats).
The heap library. A memory table system with our superior full dynamic hashing. In use since 1981 and published around 1984.
The replace program (look into it, it's COOL!).
MyODBC, the ODBC driver for Windows95.
Fixing bugs in MIT-pthreads to get it to work for MySQL. And also Unireg, a curses-based application tool with many utilities.
Porting of mSQL tools like msqlperl, DBD/DBI and DB2mysql.

David Axmark

Coordinator and main writer for the Reference Manual, including enhancements to texi2html. Also automatic website updating from this manual.
Autoconf and Automake support.
The licensing stuff.
Parts of all the text files. Nowadays only the `README' is left. The rest ended up in the manual.
Our Mail master.
Lots of testing of new features.
Our in house 'free' software lawyer.
Mailing list maintainer (who never has the time to do it right...)
Our original portability code (more than 10 years old now). Nowadays only some parts of mysys are left.
Someone for Monty to call in the middle of the night when he just got that new feature to work. :-)

Paul DuBois

Help with making the Reference Manual correct and understandable.

Gianmassimo Vigazzola qwerg@mbox.vol.it or qwerg@tin.it

The initial port to Win32/NT.

Kim Aldale

Rewriting Monty's and David's attempts at English into English.

Allan Larsson (The BOSS at TcX)

For all the time he has allowed Monty to spend on this 'maybe useful' tool (MySQL). Dedicated user (and bug finder) of Unireg & MySQL.

Per Eric Olsson

For more or less constructive criticism and real testing of the dynamic record format.

Irena Pancirov irena@mail.yacc.it

Win32 port with Borland compiler.

David J. Hughes

For the effort to make a shareware SQL database. We at TcX started with mSQL, but found that it couldn't satisfy our purposes so instead we wrote a SQL interface to our application builder Unireg. mysqladmin and mysql are programs that were largely influenced by their mSQL counterparts. We have put a lot of effort into making the MySQL syntax a superset of mSQL. Many of the API's ideas are borrowed from mSQL to make it easy to port free mSQL programs to MySQL. MySQL doesn't contain any code from mSQL. Two files in the distribution (`client/insert_test.c' and `client/select_test.c') are based on the corresponding (non-copyrighted) files in the mSQL distribution, but are modified as examples showing the changes necessary to convert code from mSQL to MySQL. (mSQL is copyrighted David J. Hughes.)

Fred Fish

For his excellent C debugging and trace library. Monty has made a number of smaller improvements to the library (speed and additional options).

Richard A. O'Keefe

For his public domain string library.

Henry Spencer

For his regex library, used in WHERE column REGEXP regexp.

Free Software Foundation

From whom we got an excellent compiler (gcc), the libc library (from which we have borrowed `strto.c' to get some code working in Linux) and the readline library (for the mysql client).

Free Software Foundation & The XEmacs development team

For a really great editor/environment used by almost everybody at TcX/detron.

Igor Romanenko igor@frog.kiev.ua

mysqldump (previously msqldump, but ported and enhanced by Monty).

Tim Bunce, Alligator Descartes

For the DBD (Perl) interface.

Andreas Koenig a.koenig@mind.de

For the Perl interface to MySQL.

Eugene Chan eugene@acenet.com.sg

For porting PHP to MySQL.

Michael J. Miller Jr. mke@terrapin.turbolift.com

For the growing MySQL user manual. And a lot of spelling/language fixes for the FAQ.

Giovanni Maruzzelli maruzz@matrice.it

For porting iODBC (Unix ODBC).

Chris Provenzano

Portable user level pthreads. From the copyright: This product includes software developed by Chris Provenzano, the University of California, Berkeley, and contributors. We are currently using version 1_60_beta6 patched by Monty (see `mit-pthreads/Changes-mysql').

Xavier Leroy Xavier.Leroy@inria.fr

The author of LinuxThreads (used by MySQL on Linux).

Zarko Mocnik zarko.mocnik@dem.si

Sorting for Slovenian language and the `cset.tar.gz' module that makes it easier to add other character sets.

"TAMITO" tommy@valley.ne.jp

The _MB character set macros and the ujis and sjis character sets.

Yves Carlier Yves.Carlier@rug.ac.be

mysqlaccess, a program to show the access rights for a user.

Rhys Jones rhys@wales.com (And GWE Technologies Limited)

For the JDBC, a module to extract data from MySQL with a Java client.

Dr Xiaokun Kelvin ZHU X.Zhu@brad.ac.uk

Further development of the JDBC driver and other MySQL-related Java tools.

James Cooper pixel@organic.com

For setting up a searchable mailing list archive at his site.

Rick Mehalick Rick_Mehalick@i-o.com

For xmysql, a graphical X client for MySQL.

Doug Sisk sisk@wix.com

For providing RPM packages of MySQL for RedHat Linux.

Diemand Alexander V. axeld@vial.ethz.ch

For providing RPM packages of MySQL for RedHat Linux/Alpha.

Antoni Pamies Olive toni@readysoft.es

For providing RPM versions of a lot of MySQL clients for Intel and Sparc.

Jay Bloodworth jay@pathways.sde.state.sc.us

For providing RPM versions for MySQL 3.21 versions.

Jochen Wiedmann wiedmann@neckar-alb.de

For maintaining the Perl DBD::mysql module.

Therrien Gilbert gilbert@ican.net, Jean-Marc Pouyot jmp@scalaire.fr

French error messages.

Petr snajdr, snajdr@pvt.net

Czech error messages.

Jaroslaw Lewandowski jotel@itnet.com.pl

Polish error messages.

Miguel Angel Fernandez Roiz

Spanish error messages.

Roy-Magne Mo rmo@www.hivolda.no

Norwegian error messages and testing of 3.21.#.

Timur I. Bakeyev root@timur.tatarstan.ru

Russian error messages.

brenno@dewinter.com

Italian error messages.

Dirk Munzinger dirk@trinity.saar.de

German error messages.

David Sacerdote davids@secnet.com

Ideas for secure checking of DNS hostnames.

Wei-Jou Chen jou@nematic.ieo.nctu.edu.tw

Some support for Chinese(BIG5) characters.

Zeev Suraski bourbon@netvision.net.il

FROM_UNIXTIME() time formatting, ENCRYPT() functions, and bison adviser. Active mailing list member.

Luuk de Boer luuk@wxs.nl

Ported (and extended) the benchmark suite to DBI/DBD. Some new date functions.

Jay Flaherty fty@utk.edu

Big parts of the Perl DBI/DBD section in the manual.

Paul Southworth pauls@etext.org, Ray Loyzaga yar@cs.su.oz.au

Proof-reading of the Reference Manual.

Alexis Mikhailov root@medinf.chuvashia.su

User definable functions (UDFs); CREATE FUNCTION and DROP FUNCTION.

Ross Wakelin R.Wakelin@march.co.uk

Help to set up InstallShield for MySQL-Win32.

Jethro Wright III jetman@li.net

The `libmysql.dll' library.

James Pereria jpereira@iafrica.com

Mysqlmanager, a Win32 GUI tool for administrating MySQL.

Curt Sampson cjs@portal.ca

Porting of MIT-pthreads to NetBSD/Alpha and NetBSD 1.3/i386.

Sinisa Milivojevic sinisa@coresinc.com

Compression (with zlib) to the client/server protocol. Perfect hashing for the lexical analyzer phase.

Antony T. Curtis antony.curtis@olcs.net

Porting of MySQL to OS/2.

Other contributors, bugfinders and testers: James H. Thompson, Maurizio Menghini, Wojciech Tryc, Luca Berra, Zarko Mocnik, Wim Bonis, Elmar Haneke, jehamby@lightside, psmith@BayNetworks.COM, Mike Simons, Jaakko Hyv@"atti.

And lots of bug report/patches from the folks on the mailing list.

And a big tribute to those that help us answer questions on the mysql@tcx.se mailing list:

Daniel Koch dkoch@amcity.com: IRIX setup.
Luuk de Boer luuk@wxs.nl: Benchmark questions.
Tim Sailer tps@users.buoy.com: DBD-mysql questions.
Boyd Lynn Gerber gerberb@zenez.com: SCO related questions.
Richard Mehalick RM186061@shellus.com: xmysql-releated questions and basic installation questions.
Zeev Suraski bourbon@netvision.net.il: Apache module configuration questions (log & auth), PHP-related questions, SQL syntax related questions and other general questions.
Francesc Guasch frankie@citel.upc.es: General questions.
Jonathan J Smith jsmith@wtp.net: Questions pertaining to OS-specifics with Linux, SQL syntax, and other things that might be needing some work.
David Sklar sklar@student.net: Using MySQL from PHP and Perl.
Alistair MacDonald A.MacDonald@uel.ac.uk: Not yet specified, but is flexible and can handle Linux and maybe HP-UX. Will try to get user to use mysqlbug.
John Lyon jlyon@imag.net: Questions about installing MySQL on Linux systems, using either `.rpm' files, or compiling from source.
Lorvid Ltd. lorvid@WOLFENET.com: Simple billing/license/support/copyright issues.
Patrick Sherrill patrick@coconet.com: ODBC and VisualC++ interface questions.
Randy Harmon rjharmon@uptimecomputers.com: DBD, Linux, some SQL syntax questions.

D MySQL change history

19.3 Changes in release 3.22.x (Alpha version)

The 3.22 version has faster and safer connect code and a lot of new nice enhancements. The reason for not including these changes in the 3.21 version is mainly that we are trying to avoid big changes to 3.21 to keep it as stable as possible. As there aren't really any MAJOR changes, upgrading to 3.22 should be very easy and painless.

3.22 also includes the new DBD-mysql (1.2000) driver that can use the new connect protocol!

19.3.1 Changes in release 3.22.10

mysql_install_db will not anymore start the MySQL server! You should start mysqld with safe_mysqld after installing it! The MySQL RPM will however start the server as before.
Added --bootstrap to mysqld and recoded mysql_install_db to use this. This will make it easier to install MySQL with RPMs.
Changed +, - (sign and minus), *, /, %, ABS() and MOD() to be BIGINT aware (64 bit safe).
Fixed a bug in ALTER TABLE that caused mysqld to crash.
MySQL now always reports the conflicting key values when one gets a duplicate key entry. (Before this was only reported for INSERT).
New syntax: INSERT INTO table_name SET column=value,column=value...
Most errors in the `.err' log are now prefixed with a time stamp.
Added option MYSQL_INIT_COMMAND to mysql_options() to make a query on connect or reconnect.
Added option MYSQL_READ_DEFAULT_FILE and MYSQL_READ_DEFAULT_GROUP to mysql_options() to read the following parameters from the MySQL option files: port, socket, compress, password, pipe, timeout, user, init-command, host and database.
Added maybe_null to the UDF structure.
Added option IGNORE to INSERT statemants with many rows.
Fixed some problems with sorting of the koi8 character sets; Users of koi8 MUST run isamchk -rq on each table that has an index on a CHAR or VARCHAR column.
New script mysql_setpermission, by Luuk de Boer, allows one to easily create new users with permissions for specific databases.
Allow use of hexadecimal strings (0x...) when specifying a constant string (like in the column separators with LOAD DATA INFILE).
Ported to OS/2 (thanks to Antony T. Curtis antony.curtis@olcs.net).
Added more variables to SHOW STATUS and changed format of output to be like SHOW VARIABLES.
Added extended-status command to mysqladmin which will show the new status variables.

19.3.2 Changes in release 3.22.9

SET SQL_LOG_UPDATE=0 caused a lockup of the server.
New SQL command: FLUSH [ TABLES | HOSTS | LOGS | PRIVILEGES ] [, ...]
New SQL command: KILL thread_id
Added casts and changed include files to make MySQL easier to compile on AIX and DEC OSF1 4.x
Fixed conversion problem when using ALTER TABLE from a INT to a short CHAR() column.
Added SELECT HIGH_PRIORITY; This will get a lock for the SELECT even if there is a thread waiting for another SELECT to get a WRITE LOCK.
Moved wild_compare to string class to be able to use LIKE on BLOB/TEXT columns with \0.
Added ESCAPE option to LIKE
Added a lot more output to mysqladmin debug.
You can now start mysqld on Win32 with the --flush option. This will flush all tables to disk after each update. This makes things much safer on NT/Win98 but also MUCH slower.

19.3.3 Changes in release 3.22.8

Czech character sets should now work much better. You must also install ftp://www.tcx.se/pub/mysql/Downloads/Patches/czech-3.22.8-patch. This patch should also be installed if you are using a character set with uses my_strcoll()! The patch should always be safe to install (for any system), but as this patch changes ISAM internals it's not yet in the default distribution.
DATE_ADD() and DATE_SUB() didn't work with group functions.
mysql will now also try to reconnect on USE DATABASE commands.
Fix problem with ORDER BY and LEFT JOIN and const tables.
Fixed problem with ORDER BY if the first ORDER BY column was a key and the rest of the ORDER BY columns wasn't part of the key.
Fixed a big problem with OPTIMIZE TABLE.
MySQL clients on NT will now by default first try to connect with named pipes and after this with TCP/IP.
Fixed a problem with DROP TABLE and mysqladmin shutdown on Win32 (a fatal bug from 3.22.6).
Fixed problems with TIME columns and negative strings.
Added an extra thread signal loop on shutdown to avoid some error messages from the client.
MySQL now uses the next available number as extension for the update log file.
Added patches for UNIXWARE 7.

19.3.4 Changes in release 3.22.7

You can now use the /*! ... */ syntax to hide MySQL-specific keywords when you write portable code. MySQL will parse the code inside the comments as if the surrounding /*! and */ comment characters didn't exist.
OPTIMIZE TABLE table_name can now be used to reclaim disk space after many deletes. Currently, this uses ALTER TABLE to re-generate the table, but in the future it will use an integrated isamchk for more speed.
Upgraded libtool to get the configure more portable.
Fixed slow UPDATE and DELETE operations when using DATETIME or DATE keys.
Changed optimizer to make it better at deciding when to do a full join and when using keys.
You can now use mysqladmin proc to display information about your own threads. Only users with the Process_priv privilege can get information about all threads.
Added handling of formats YYMMDD, YYYYMMDD, YYMMDDHHMMSS for numbers when using DATETIME and TIMESTAMP types. (Formerly these formats only worked with strings.)
Added connect option CLIENT_IGNORE_SPACE to allow use of spaces after function names and before `(' (Powerbuilder requires this). This will make all function names reserved words.
Added the --log-long-format option to mysqld to enable timestamps and INSERT_ID's in the update log.
Added --where option to mysqldump (patch by Jim Faucette).
The lexical analyzer now uses "perfect hashing" for faster parsing of SQL statements.

19.3.5 Changes in release 3.22.6

Faster mysqldump.
For the LOAD DATA INFILE statement, you can now use the new LOCAL keyword to read the file from the client. mysqlimport will automatically use LOCAL when importing with the TCP/IP protocol.
Fixed small optimize problem when updating keys.
Changed makefiles to support shared libraries.
MySQL-NT can now use named pipes, which means that you can now use MySQL-NT without having to install TCP/IP.

19.3.6 Changes in release 3.22.5

All table lock handing is changed to avoid some very subtle deadlocks when using DROP TABLE, ALTER TABLE, DELETE FROM TABLE and mysqladmin flush-tables under heavy usage. Changed locking code to get better handling of locks of different types.
Updated DBI to 1.00 and DBD to 1.2.0.
Added a check that the error message file contains error messages suitable for the current version of mysqld. (To avoid errors if you accidentally try to use an old error message file.)
All count structures in the client (affected_rows, insert_id...) are now of type BIGINT to allow use of 64-bit values. This required a minor change in the MySQL protocol which should affect only old clients when using tables with AUTO_INCREMENT values > 24M.
The return type of mysql_fetch_lengths() has changed from uint * to ulong *. This may give a warning for old clients but should work on most machines.
Change mysys and dbug libraries to allocate all thread variables in one struct. This makes it easier to make a threaded `libmysql.dll' library.
Use the result from gethostname() (instead of uname()) when constructing pid file names.
New better compressed server/client protocol.
COUNT(), STD() and AVG() are extended to handle more than 4G rows.
You can now store values in the range -838:59:59 <= x <= 838:59:59 in a TIME column.
WARNING: INCOMPATIBLE CHANGE!! If you set a TIME column to too short a value, MySQL now assumes the value is given as: [[[D ]HH:]MM:]SS instead of HH[:MM[:SS]].
TIME_TO_SEC() and SEC_TO_TIME() can now handle negative times and hours up to 32767.
Added new option SET OPTION SQL_LOG_UPDATE={0|1} to allow users with process_priv privilege to bypass the update log. (Modified patch from Sergey A Mukhin violet@rosnet.net.)
Fixed fatal bug in LPAD().
Initialize line buffer in `mysql.cc' to make BLOB reading from pipes safer.
Added -O max_connect_errors=# option to mysqld. Connect errors are now reset for each correct connection.
Increased the default value of max_allowed_packet to 1M in mysqld.
Added --low-priority-updates option to mysqld, to give UPDATE operations lower priority than retrievals. You can now use {INSERT | REPLACE | UPDATE | DELETE} LOW_PRIORITY ... You can also use SET OPTION LOW_PRIORITY_UPDATES={0|1} to change the priority for one thread. One side effect is that LOW_PRIORITY is now a reserved word. :(
Add support for INSERT INTO table ... VALUES(...),(...),(...), to allow inserting multiple rows with a single statement.
INSERT INTO tbl_name is now also cached when used with LOCK TABLES. (Previously only INSERT ... SELECT and LOAD DATA INFILE were cached.)

Allow GROUP BY functions with HAVING:

mysql> SELECT col FROM table GROUP BY col HAVING COUNT(*)>0;

mysqld will now ignore trailing ';' characters in queries. This is to make it easier to migrate from some other SQL servers that require the trailing ';'.
Fix for corrupted fixed-format output generated by SELECT INTO OUTFILE.
WARNING: INCOMPATIBLE CHANGE!! Added Oracle GREATEST() and LEAST() functions. You must now use these instead of the MAX() and MIN() functions to get the largest/smallest value from a list of values. These can now handle REAL, BIGINT and string (CHAR or VARCHAR) values.
WARNING: INCOMPATIBLE CHANGE!! DAYOFWEEK() had offset 0 for Sunday. Changed the offset to 1.
Give an error for queries that mix GROUP BY columns and fields when there is no GROUP BY specification.
Added --vertical option to mysql, for printing results in vertical mode.
Index-only optimization; some queries are now resolved using only indexes. Until MySQL 4.0, this works only for numeric columns. See section 10.4 How MySQL uses indexes.
Lots of new benchmarks.
A new C API chapter and lots of other improvements in the manual.

19.3.7 Changes in release 3.22.4

Added --tmpdir option to mysqld, for specifying the location of the temporary file directory.
MySQL now automatically changes a query from an ODBC client:
```
SELECT ... FROM table WHERE auto_increment_column IS NULL
```
to:
```
SELECT ... FROM table WHERE auto_increment_column == LAST_INSERT_ID()
```
This allows some ODBC programs (Delphi, Access) to retrieve the newly inserted row to fetch the AUTO_INCREMENT id.
DROP TABLE now waits for all users to free a table before deleting it.
Fixed small memory leak in the new connect protocol.
New functions BIN(), HEX() and CONV() for converting between different number bases.
Added function SUBSTRING() with 2 arguments.
If you created a table with a record length smaller than 5, you couldn't delete rows from the table.
Added optimization to remove const reference tables from ORDER BY and GROUP BY.
mysqld now automatically disables system locking on Linux and Win32, and for systems that use MIT-pthreads. You can force the use of locking with the --enable-locking option.
Added --console option to mysqld, to force a console window (for error messages) when using Win32.
Fixed table locks for Win32.
Allow '$' in identifiers.
Changed name of user-specific configuration file from `my.cnf' to `.my.cnf' (Unix only).
Added DATE_ADD() and DATE_SUB() functions.

19.3.8 Changes in release 3.22.3

Fixed a lock problem (bug in MySQL 3.22.1) when closing temporary tables.
Added missing mysql_ping() to the client library.
Added --compress option to all MySQL clients.
Changed byte to char in `mysql.h' and `mysql_com.h'.

19.3.9 Changes in release 3.22.2

Searching on multiple constant keys that matched > 30% of the rows didn't always use the best possible key.
New functions <<, >>, RPAD() and LPAD().
You can now save default options (like passwords) in a configuration file (`my.cnf').
Lots of small changes to get ORDER BY to work when no records are found when using fields that are not in GROUP BY (MySQL extension).
Added --chroot option to mysqld, to start mysqld in a chroot environment (by Nikki Chumakov nikkic@cityline.ru).
Trailing space is now ignored when comparing case-sensitive strings; this should fix some problems with ODBC and flag 512!
Fixed a core-dump bug in the range optimizer.
Added --one-thread option to mysqld, for debugging with LinuxThreads (or glibc). (This replaces the -T32 flag)
Added DROP TABLE IF EXISTS to prevent an error from occurring if the table doesn't exist.
IF and EXISTS are now reserved words (they would have to be sooner or later).
Added lots of new options to mysqldump.
Server error messages are now in `mysqld_error.h'.
The server/client protocol now supports compression.
All bug fixes from MySQL 3.21.32.

19.3.10 Changes in release 3.22.1

Added new C API function mysql_ping().
Added new API functions mysql_init() and mysql_options(). You now MUST call mysql_init() before you call mysql_real_connect(). You don't have to call mysql_init() if you only use mysql_connect().
Added mysql_options(...,MYSQL_OPT_CONNECT_TIMEOUT,...) so you can set a timeout for connecting to a server.
Added --timeout option to mysqladmin, as a test of mysql_options().
Added AFTER column and FIRST options to ALTER TABLE ... ADD columns. This makes it possible to add a new column at some specific location within a row in an existing table.
WEEK() now takes an optional argument to allow handling of weeks when the week starts on Monday (some European countries). By default, WEEK() assumes the week starts on Sunday.
TIME columns weren't stored properly (bug in MySQL 3.22.0).
UPDATE now returns information about how many rows were matched and updated, and how many 'warnings' occurred when doing the update.
Fixed incorrect result from FORMAT(-100,2).
ENUM and SET columns were compared in binary (case-sensitive) fashion; changed to be case insensitive.

19.3.11 Changes in release 3.22.0

New (backward compatible) connect protocol that allows you to specify the database to use when connecting, to get much faster connections to a specific database. The mysql_real_connect() call is changed to:
```
mysql_real_connect(MYSQL *mysql, const char *host, const char *user,
                   const char *passwd, const char *db, uint port,
                   const char *unix_socket, uint client_flag)
```
Each connection is handled by its own thread, rather than by the master accept() thread. This fixes permanently the telnet bug that was a topic on the mail list some time ago.
All TCP/IP connections are now checked with backward resolution of the hostname to get better security. mysqld now has a local hostname resolver cache so connections should actually be faster than before, even with this feature.
A site automatically will be blocked from future connections if someone repeatedly connects with an 'improper header' (like when one uses telnet).
You can now refer to tables in different databases with references of the form tbl_name@db_name or db_name.tbl_name. This makes it possible to give a user read access to some tables and write access to others simply by keeping them in different databases!
Added --user option to mysqld, to allow it to run as another Unix user (if it is started as the Unix root user).
Added caching of users and access rights (for faster access rights checking)
Normal users (not anonymous ones) can change their password with mysqladmin password 'new_password'. This uses encrypted passwords that are not logged in the normal MySQL log!
All important string functions are now coded in assembler for x86 Linux machines. This gives a speedup of 10% in many cases.
For tables that have many columns, the column names are now hashed for much faster column name lookup (this will speed up some benchmark tests a lot!)
Some benchmarks are changed to get better individual timing. (Some loops were so short that a specific test took < 2 seconds. The loops have been changed to take about 20 seconds to make it easier to compare different databases. A test that took 1-2 seconds before now takes 11-24 seconds, which is much better)
Re-arranged SELECT code to handle some very specific queries involving group functions (like COUNT(*)) without a GROUP BY but with HAVING. The following now works:
```
mysql> SELECT count(*) as C FROM table HAVING C > 1;
```
Changed the protocol for field functions to be faster and avoid some calls to malloc().
Added -T32 option to mysqld, for running all queries under the main thread. This makes it possible to debug mysqld under Linux with gdb!
Added optimization of not_null_column IS NULL (needed for some Access queries).
Allow STRAIGHT_JOIN to be used between two tables to force the optimizer to join them in a specific order.
String functions now return VARCHAR rather than CHAR and the column type is now VARCHAR for fields saved as VARCHAR. This should make the MyODBC driver better, but may break some old MySQL clients that don't handle FIELD_TYPE_VARCHAR the same way as FIELD_TYPE_CHAR.
CREATE INDEX and DROP INDEX are now implemented through ALTER TABLE. CREATE TABLE is still the recommended (fast) way to create indexes.
Added --set-variable option wait_timeout to mysqld.
Added time column to mysqladmin processlist to show how long a query has taken or how long a thread has slept.
Added lots of new variables to show variables and some new to show status.
Added new type YEAR. YEAR is stored in 1 byte with allowable values of 0, and 1901 to 2155.
Added new DATE type that is stored in 3 bytes rather than 4 bytes. All new tables are created with the new date type if you don't use the --old-protocol option to mysqld.
Fixed bug in record caches; for some queries, you could get Error from table handler: # on some operating systems.
Added --enable-assembler option to configure, for x86 machines (tested on Linux + gcc). This will enable assembler functions for the most important string functions for more speed!

19.4 Changes in release 3.21.x

19.4.1 Changes in release 3.21.33

Fixed problem when sending SIGHUP to mysqld; mysqld core dumped when starting from boot on some systems.
Fixed problem with losing a little memory for some connections.
DELETE FROM tbl_name without a WHERE condition is now done the long way when you use LOCK TABLES or if the table is in use, to avoid race conditions.
INSERT INTO TABLE (timestamp_column) VALUES (NULL); didn't set timestamp.

19.4.2 Changes in release 3.21.32

Fixed some possible race conditions when doing many reopen/close on the same tables under heavy load! This can happen if you execute mysqladmin refresh often. This could in some very rare cases corrupt the header of the index file and cause error 126 or 138.
Fixed fatal bug in refresh() when running with the --skip-locking option. There was a 'very small' time gap after a mysqladmin refresh when a table could be corrupted if one thread updated a table while another thread did mysqladmin refresh and another thread started a new update ont the same table before the first thread had finished. A refresh (or --flush-tables) will now not return until all used tables are closed!
SELECT DISTINCT with a WHERE clause that didn't match any rows returned a row in some contexts (bug only in 3.21.31).
GROUP BY + ORDER BY returned one empty row when no rows where found.
Fixed a bug in the range optimizer that wrote Use_count: Wrong count for ... in the error log file.

19.4.3 Changes in release 3.21.31

Fixed a sign extension problem for the TINYINT type on IRIX.
Fixed problem with LEFT("constant_string",function).
Fixed problem with FIND_IN_SET().
LEFT JOIN core dumped if the second table is used with a constant WHERE/ON expression that uniquely identifies one record.
Fixed problems with DATE_FORMAT() and incorrect dates. DATE_FORMAT() now ignores '%' to make it possible to extend it more easily in the future.

19.4.4 Changes in release 3.21.30

mysql now returns an exit code > 0 if the query returned an error.
Saving of command line history to file in mysql client. By Tommy Larsen tommy@mix.hive.no.
Fixed problem with empty lines that were ignored in `mysql.cc'.
Save the pid of the signal handler thread in the pid file instead of the pid of the main thread.
Added patch by tommy@valley.ne.jp to support Japanese characters SJIS and UJIS.
Changed safe_mysqld to redirect startup messages to 'hostname'.err instead of 'hostname'.log to reclaim file space on mysqladmin refresh.
ENUM always had the first entry as default value.
ALTER TABLE wrote two entries to the update log.
sql_acc() now closes the mysql privilege tables after a reload to save table space and memory.
Changed LOAD DATA to use less memory with tables and BLOB columns.
Sorting on a function which made a division / 0 produced a wrong set in some cases.
Fixed SELECT problem with LEFT() when using the czech character set.
Fixed problem in isamchk; it couldn't repair a packed table in a very unusual case.
SELECT statements with & or | (bit functions) failed on columns with NULL values.
When comparing a field = field, where one of the fields was a part key, only the length of the part key was compared.

19.4.5 Changes in release 3.21.29

LOCK TABLES + DELETE from tbl_name never removed locks properly.
Fixed problem when grouping on an OR function.
Fixed permission problem with umask() and creating new databases.
Fixed permission problem on result file with SELECT ... INTO OUTFILE ...
Fixed problem in range optimizer (core dump) for a very complex query.
Fixed problem when using MIN(integer) or MAX(integer) in GROUP BY.
Fixed bug on Alpha when using integer keys. (Other keys worked on Alpha).
Fixed bug in WEEK("XXXX-xx-01").

19.4.6 Changes in release 3.21.28

Fixed socket permission (clients couldn't connect to Unix socket on Linux).
Fixed bug in record caches; for some queries, you could get Error from table handler: # on some operating systems.

19.4.7 Changes in release 3.21.27

Added user level lock functions GET_LOCK(string,timeout), RELEASE_LOCK(string).
Added opened_tables to show status.
Changed connect timeout to 3 seconds to make it somewhat harder for crackers to kill mysqld through telnet + TCP/IP.
Fixed bug in range optimizer when using WHERE key_part_1 >= something AND key_part_2 <= something_else.
Changed configure for detection of FreeBSD 3.0 9803xx and above
WHERE with string_column_key = constant_string didn't always find all rows if the column had many values differing only with characters of the same sort value (like e and 'e).
Strings keys looked up with 'ref' were not compared in case-sensitive fashion.
Added umask() to make log files non-readable for normal users.
Ignore users with old (8-byte) password on startup if not using --old-protocol option to mysqld.
SELECT which matched all key fields returned the values in the case of the matched values, not of the found values. (Minor problem.)

19.4.8 Changes in release 3.21.26

FROM_DAYS(0) now returns "0000-00-00".
In DATE_FORMAT(), PM and AM were swapped for hours 00 and 12.
Extended the default maximum key size to 256.
Fixed bug when using BLOB/TEXT in GROUP BY with many tables.
An ENUM field that is not declared NOT NULL has NULL as the default value. (Previously, the default value was the first enumeration value.)
Fixed bug in the join optimizer code when using many part keys on the same key: INDEX (Organization,Surname(35),Initials(35)).
Added some tests to the table order optimizer to get some cases with SELECT ... FROM many_tables much faster.
Added a retry loop around accept() to possibly fix some problems on some Linux machines.

19.4.9 Changes in release 3.21.25

Changed typedef 'string' to typedef 'my_string' for better portability.
You can now kill threads that are waiting on a disk full condition.
Fixed some problems with UDF functions.
Added long options to isamchk. Try isamchk --help.
Fixed a bug when using 8 bytes long (alpha); filesort() didn't work. Affects DISTINCT, ORDER BY and GROUP BY on 64-bit processors.

19.4.10 Changes in release 3.21.24

Dynamic loadable functions. Based on source from Alexis Mikhailov.
You couldn't delete from a table if no one had done a SELECT on the table.
Fixed problem with range optimizer with many OR operators on key parts inside each other.
Recoded MIN() and MAX() to work properly with strings and HAVING.
Changed default umask value for new files from 0664 to 0660.
Fixed problem with LEFT JOIN and constant expressions in the ON part.
Added Italian error messages from brenno@3cord.philips.nl.
configure now works better on OSF1 (tested on 4.0D).
Added hooks to allow LIKE optimization with international character support.
Upgraded DBI to 0.93.

19.4.11 Changes in release 3.21.23

The following symbols are now reserved words: TIME, DATE, TIMESTAMP, TEXT, BIT, ENUM, NO, ACTION, CHECK, YEAR, MONTH, DAY, HOUR, MINUTE, SECOND, STATUS, VARIABLES.
Setting a TIMESTAMP to NULL in LOAD DATA INFILE ... didn't set the current time for the TIMESTAMP.
Fix BETWEEN to recognize binary strings. Now BETWEEN is case sensitive.
Added --skip-thread-priority option to mysqld, for systems where mysqld's thread scheduling doesn't work properly (BSDI 3.1).
Added ODBC functions DAYNAME() and MONTHNAME().
Added function TIME_FORMAT(). This works like DATE_FORMAT(), but takes a time string ('HH:MM:DD') as argument.
Fixed unlikely(?) key optimizer bug when using ORs of key parts inside ANDs.
Added command 'variables' to mysqladmin.
A lot of small changes to the binary releases.
Fixed a bug in the new protocol from MySQL 3.21.20.
Changed ALTER TABLE to work with Win32 (Win32 can't rename open files). Also fixed a couple of small bugs in the Win32 version.
All standard MySQL clients are now ported to MySQL-Win32.
MySQL can now be started as a service on NT.

19.4.12 Changes in release 3.21.22

Starting with this version, all MySQL distributions will be configured, compiled and tested with crash-me and the benchmarks on the following platforms: SunOS 5.6 sun4u, SunOS 5.5.1 sun4u, SunOS 4.14 sun4c, SunOS 5.6 i86pc, IRIX 6.3 mips5k, HP-UX 10.20 hppa, AIX 4.2.1 ppc, OSF1 V4.0 alpha, FreeBSD 2.2.2 i86pc and BSDI 3.1 i386.
Fix COUNT(*) problems when the WHERE clause didn't match any records. (Bug from 3.21.17.)
Removed that NULL = NULL is true. Now you must use IS NULL or IS NOT NULL to test whether or not a value is NULL. (This is according to ANSI SQL but may break old applications that are ported from mSQL.) You can get the old behavior by compiling with -DmSQL_COMPLIANT.
Fixed bug that core dumped when using many LEFT OUTER JOIN clauses.
Fixed bug in ORDER BY on string formula with possible NULL values.
Fixed problem in range optimizer when using <= on sub index.
Added functions DAYOFYEAR(), DAYOFMONTH(), MONTH(), YEAR(), WEEK(), QUARTER(), HOUR(), MINUTE(), SECOND() and FIND_IN_SET().
Added command SHOW VARIABLES.

Added support of 'long constant strings' from ANSI SQL:

mysql> SELECT 'first ' 'second';       -> 'first second'

Upgraded mSQL-Mysql-modules to 1.1825.
Upgraded mysqlaccess to 2.02.
Fixed problem with Russian character set and LIKE.
Ported to OpenBSD 2.1.
New Dutch error messages.

19.4.13 Changes in release 3.21.21a

Configure changes for some operating systems.

19.4.14 Changes in release 3.21.21

Fixed optimizer bug when using WHERE data_field = date_field2 AND date_field2 = constant.
Added command SHOW STATUS.
Removed `manual.ps' from the source distribution to make it smaller.

19.4.15 Changes in release 3.21.20

Changed the maximum table name and column name lengths from 32 to 64.
Aliases can now be of 'any' length.
Fixed mysqladmin stat to return the right number of queries.
Changed protocol (downward compatible) to mark if a column has the AUTO_INCREMENT attribute or is a TIMESTAMP. This is needed for the new Java driver.
Added Hebrew sorting order by Zeev Suraski.
Solaris 2.6: Fixed configure bugs and increased maximum table size from 2G to 4G.

19.4.16 Changes in release 3.21.19

Upgraded DBD to 1823. This version implements mysql_use_result in DBD-Mysql.
Benchmarks updated for empress (by Luuk).
Fixed a case of slow range searching.
Configure fixes (`Docs' directory).
Added function REVERSE() (by Zeev Suraski).

19.4.17 Changes in release 3.21.18

Issue error message if client C functions are called in wrong order.
Added automatic reconnect to the `libmysql.c' library. If a write command fails, an automatic reconnect is done.
Small sort sets no longer use temporary files.
Upgraded DBI to 0.91.
Fixed a couple of problems with LEFT OUTER JOIN.
Added CROSS JOIN syntax. CROSS is now a reserved word.
Recoded yacc/bison stack allocation to be even safer and to allow MySQL to handle even bigger expressions.
Fixed a couple of problems with the update log.
ORDER BY was slow when used with key ranges.

19.4.18 Changes in release 3.21.17

Changed documentation string of --with-unix-socket-path to avoid confusion.
Added ODBC and ANSI SQL style LEFT OUTER JOIN.
The following are new reserved words: LEFT, NATURAL, USING.
The client library now uses the value of the environment variable MYSQL_HOST as the default host if it's defined.
SELECT column, SUM(expr) now returns NULL for column when there are matching rows.
Fixed problem with comparing binary strings and BLOBs with ASCII characters over 127.
Fixed lock problem: when freeing a read lock on a table with multiple read locks, a thread waiting for a write lock would have been given the lock. This shouldn't affect data integrity, but could possibly make mysqld restart if one thread was reading data that another thread modified.
LIMIT offset,count didn't work in INSERT ... SELECT.
Optimized key block caching. This will be quicker than the old algorithm when using bigger key caches.

19.4.19 Changes in release 3.21.16

Added ODBC 2.0 & 3.0 functions POWER(), SPACE(), COT(), DEGREES(), RADIANS(), ROUND(2 arg) and TRUNCATE().
WARNING: INCOMPATIBLE CHANGE!! LOCATE() parameters were swapped according to ODBC standard. Fixed.
Added function TIME_TO_SEC().
In some cases, default values were not used for NOT NULL fields.
Timestamp wasn't always updated properly in UPDATE SET ... statements.
Allow empty strings as default values for BLOB and TEXT, to be compatible with mysqldump.

19.4.20 Changes in release 3.21.15

WARNING: INCOMPATIBLE CHANGE!! mysqlperl is now from Msql-Mysql-modules. This means that connect() now takes host, database, user, password arguments! The old version took host, database, password, user.
Allow DATE '1997-01-01', TIME '12:10:10' and TIMESTAMP '1997-01-01 12:10:10' formats required by ANSI SQL. WARNING: INCOMPATIBLE CHANGE!! This has the unfortunate side-effect that you no longer can have columns named DATE, TIME or TIMESTAMP. :( Old columns can still be accessed through tablename.columnname!)
Changed Makefiles to hopefully work better with BSD systems. Also, `manual.dvi' is now included in the distribution to avoid having stupid make programs trying to rebuild it.
readline library upgraded to version 2.1.
A new sortorder german-1. That is a normal ISO-Latin1 with a german sort order.
Perl DBI/DBD is now included in the distribution. DBI is now the recommended way to connect to MySQL from Perl.
New portable benchmark suite with DBD, with test results from mSQL 2.0.3, MySQL, PostgreSQL 6.2.1 and Solid server 2.2.
crash-me is now included with the benchmarks; This is a Perl program designed to find as many limits as possible in a SQL server. Tested with mSQL, PostgreSQL, Solid and MySQL.
Fixed bug in range-optimizer that crashed MySQL on some queries.
Table and column name completion for mysql command line tool, by Zeev Suraski and Andi Gutmans.
Added new command REPLACE that works like INSERT but replaces conflicting records with the new record. REPLACE INTO TABLE ... SELECT ... works also.
Added new commands CREATE DATABASE db_name and DROP DATABASE db_name.
Added RENAME option to ALTER TABLE: ALTER TABLE name RENAME AS new_name.
make_binary_distribution now includes `libgcc.a' in `libmysqlclient.a'. This should make linking work for people who don't have gcc.
Changed net_write() to my_net_write() because of a name conflict with Sybase.
New function DAYOFWEEK() compatible with ODBC.
Stack checking and bison memory overrun checking to make MySQL safer with weird queries.

19.4.21 Changes in release 3.21.14b

Fixed a couple of small configure problems on some platforms.

19.4.22 Changes in release 3.21.14a

Ported to SCO Openserver 5.0.4 with FSU-threads.
HP-UX 10.20 should work.
Added new function DATE_FORMAT().
Added NOT IN.
Added automatic removal of 'ODBC function conversions': {fn now() }
Handle ODBC 2.50.3 option flags.
Fixed comparison of DATE and TIME values with NULL.
Changed language name from germany to german to be consistent with the other language names.
Fixed sorting problem on functions returning a FLOAT. Previously, the values were converted to INTs before sorting.
Fixed slow sorting when sorting on key field when using key_column=constant.
Sorting on calculated DOUBLE values sorted on integer results instead.
mysql no longer needs a database argument.
Changed the place where HAVING should be. According to ANSI, it should be after GROUP BY but before ORDER BY. MySQL 3.20 incorrectly had it last.
Added Sybase command USE DATABASE to start using another database.
Added automatic adjusting of number of connections and table cache size if the maximum number of files that can be opened is less than needed. This should fix that mysqld doesn't crash even if you haven't done a ulimit -n 256 before starting mysqld.
Added lots of limit checks to make it safer when running with too little memory or when doing weird queries.

19.4.23 Changes in release 3.21.13

Added retry of interrupted reads and clearing of errno. This makes Linux systems much safer!
Fixed locking bug when using many aliases on the same table in the same SELECT.
Fixed bug with LIKE on number key.
New error message so you can check whether the connection was lost while the command was running or whether the connection was down from the start.
Added --table option to mysql to print in table format. Moved time and row information after query result. Added automatic reconnect of lost connections.
Added != as a synonym for <>.
Added function VERSION() to make easier logs.
New multi-user test `tests/fork_test.pl' to put some strain on the thread library.

19.4.24 Changes in release 3.21.12

Fixed ftruncate() call in MIT-pthreads. This made isamchk destroy the `.ISM' files on (Free)BSD 2.# systems.
Fixed broken __P_ patch in MIT-pthreads.
Many memory overrun checks. All string functions now return NULL if the returned string should be longer than max_allowed_packet bytes.
Changed the name of the INTERVAL type to ENUM, because INTERVAL is used in ANSI SQL.
In some cases, doing a JOIN + GROUP + INTO OUTFILE, the result wasn't grouped.
LIKE with '_' as last character didn't work. Fixed.
Added extended ANSI SQL TRIM() function.
Added CURTIME().
Added ENCRYPT() function by Zeev Suraski.
Fixed better FOREIGN KEY syntax skipping. New reserved words: MATCH, FULL, PARTIAL.
mysqld now allows IP number and hostname to the --bind-address option.
Added SET OPTION CHARACTER SET cp1251_koi8 to enable conversions of data to/from cp1251_koi8.
Lots of changes for Win95 port. In theory, this version should now be easily portable to Win95.
Changed the CREATE COLUMN syntax of NOT NULL columns to be after the DEFAULT value, as specified in the ANSI SQL standard. This will make mysqldump with NOT NULL and default values incompatible with MySQL 3.20.
Added many function name aliases so the functions can be used with ODBC or ANSI SQL92 syntax.
Fixed syntax of ALTER TABLE tbl_name ALTER COLUMN col_name SET DEFAULT NULL.
Added CHAR and BIT as synonyms for CHAR(1).
Fixed core dump when updating as a user who has only select privilege.
INSERT ... SELECT ... GROUP BY didn't work in some cases. An Invalid use of group function error occurred.
When using LIMIT, SELECT now always uses keys instead of record scan. This will give better performance on SELECT and a WHERE that matches many rows.
Added Russian error messages.

19.4.25 Changes in release 3.21.11

Configure changes.
MySQL now works with the new thread library on BSD/OS 3.0.
Added new group functions BIT_OR() and BIT_AND().
Added compatibility functions CHECK and REFERENCES. CHECK is now a reserved word.
Added ALL option to GRANT for better compatibility. (GRANT is still a dummy function.)
Added partly-translated dutch messages.
Fixed bug in ORDER BY and GROUP BY with NULL columns.
Added function last_insert_id() to retrieve last AUTO_INCREMENT value. This is intended for clients to ODBC that can't use the mysql_insert_id() API function, but can be used by any client.
Added --flush-logs option to mysqladmin.
Added command STATUS to mysql.
Fixed problem with ORDER BY/GROUP BY because of bug in gcc.
Fixed problem with INSERT ... SELECT ... GROUP BY.

19.4.26 Changes in release 3.21.10

New mysqlaccess.
CREATE now supports all ODBC types and the mSQL TEXT type. All ODBC 2.5 functions are also supported (added REPEAT). This provides better portability.
Added text types TINYTEXT, TEXT, MEDIUMTEXT and LONGTEXT. These are actually BLOBtypes, but all searching is done in case-insensitive fashion.
All old BLOB fields are now TEXT fields. This only changes that all searching on strings is done in case-sensitive fashion. You must do an ALTER TABLE and change the field type to BLOB if you want to have tests done in case-sensitive fashion.
Fixed some configure issues.
Made the locking code a bit safer. Fixed very unlikely deadlock situation.
Fixed a couple of bugs in the range optimizer. Now the new range benchmark test-select works.

19.4.27 Changes in release 3.21.9

Added --enable-unix-socket=pathname option to configure.
Fixed a couple of portability problems with include files.
Fixed bug in range calculation that could return empty set when searching on multiple key with only one entry (very rare).
Most things ported to FSU threads, which should allow MySQL to run on SCO. See section 4.11.10 SCO notes.

19.4.28 Changes in release 3.21.8

Works now in Solaris 2.6.
Added handling of calculation of SUM() functions. For example, you can now use SUM(column)/COUNT(column).
Added handling of trigometric functions: PI(), ACOS(), ASIN(), ATAN(), COS(), SIN() and TAN().
New languages: norwegian, norwegian-ny and portuguese.
Fixed parameter bug in net_print() in `procedure.cc'.
Fixed a couple of memory leaks.
Now allow also the old SELECT ... INTO OUTFILE syntax.
Fixed bug with GROUP BY and SELECT on key with many values.
mysql_fetch_lengths() sometimes returned incorrect lengths when you used mysql_use_result(). This affected at least some cases of mysqldump --quick.
Fixed bug in optimization of WHERE const op field.
Fixed problem when sorting on NULL fields.
Fixed a couple of 64-bit (Alpha) problems.
Added --pid-file=# option to mysqld.
Added date formatting to FROM_UNIXTIME(), originally by Zeev Suraski.
Fixed bug in BETWEEN in range optimizer (Did only test = of the first argument).
Added machine-dependent files for MIT-pthreads i386-SCO. There is probably more to do to get this to work on SCO 3.5.

19.4.29 Changes in release 3.21.7

Changed `Makefile.am' to take advantage of Automake 1.2.
Added the beginnings of a benchmark suite.
Added more secure password handling.
Added new client function mysql_errno(), to get the error number of the error message. This makes error checking in the client much easier. This makes the new server incompatible with the 3.20.# server when running without --old-protocol. The client code is backward compatible. More information can be found in the `README' file!
Fixed some problems when using very long, illegal names.

19.4.30 Changes in release 3.21.6

Fixed more portability issues (incorrect sigwait and sigset defines).
configure should now be able to detect the last argument to accept().

19.4.31 Changes in release 3.21.5

Should now work with FreeBSD 3.0 if used with `FreeBSD-3.0-libc_r-1.0.diff', which can be found at http://www.tcx.se/Download/Patches.
Added new option -O tmp_table_size=# to mysqld.
New function FROM_UNIXTIME(timestamp) which returns a date string in 'YYYY-MM-DD HH:MM:DD' format.
New function SEC_TO_TIME(seconds) which returns a string in 'HH:MM:SS' format.
New function SUBSTRING_INDEX(), originally by Zeev Suraski.

19.4.32 Changes in release 3.21.4

Should now configure and compile on OSF1 4.0 with the DEC compiler.
Configuration and compilation on BSD/OS 3.0 works, but due to some bugs in BSD/OS 3.0, mysqld doesn't work on it yet.
Configuration and compilation on FreeBSD 3.0 works, but I couldn't get pthread_create to work.

19.4.33 Changes in release 3.21.3

Added reverse check lookup of hostnames to get better security.
Fixed some possible buffer overflows if filenames that are too long are used.
mysqld doesn't accept hostnames that start with digits followed by a '.', because the hostname may look like an IP number.
Added --skip-networking option to mysqld, to only allow socket connections. (This will not work with MIT-pthreads!)
Added check of too long table names for alias.
Added check if database name is okay.
Added check if too long table names.
Removed incorrect free() that killed the server on CREATE DATABASE or DROP DATABASE.
Changed some mysqld -O options to better names.
Added -O join_cache_size=# option to mysqld.
Added -O max_join_size=# option to mysqld, to be able to set a limit how big queries (in this case big = slow) one should be able to handle without specifying SET OPTION SQL_BIG_SELECTS=1. A # = is about 10 examined records. The default is 'unlimited'.
When comparing a TIME, DATE, DATETIME or TIMESTAMP column to a constant, the constant is converted to a time value before performing the comparison. This will make it easier to get ODBC (particularly Access97) to work with the above types. It should also make dates easier to use and the comparisons should be quicker than before.
Applied patch from Jochen Wiedmann that allows query() in mysqlperl to take a query with \0 in it.
Storing a timestamp with a 2-digit year (YYMMDD) didn't work.
Fix that timestamp wasn't automatically updated if set in an UPDATE clause.
Now the automatic timestamp field is the FIRST timestamp field.
SELECT * INTO OUTFILE, which didn't correctly if the outfile already existed.
mysql now shows the thread ID when starting or doing a reconnect.
Changed the default sort buffer size from 2M to 1M.

19.4.34 Changes in release 3.21.2

The range optimizer is coded, but only 85% tested. It can be enabled with --new, but it crashes core a lot yet...
More portable. Should compile on AIX and alpha-digital. At least the isam library should be relatively 64-bit clean.
New isamchk which can detect and fix more problems.
New options for isamlog.
Using new version of Automake.
Many small portability changes (from the AIX and alpha-digital port) Better checking of pthread(s) library.
czech error messages by snajdr@pvt.net.
Decreased size of some buffers to get fewer problems on systems with little memory. Also added more checks to handle 'out of memory' problems.
mysqladmin: you can now do mysqladmin kill 5,6,7,8 to kill multiple threads.
When the maximum connection limit is reached, one extra connection by a user with the PROCESS_ACL privilege is granted.
Added -O backlog=# option to mysqld.
Increased maximum packet size from 512K to 1024K for client.
Almost all of the function code is now tested in the internal test suite.
ALTER TABLE now returns warnings from field conversions.
Port changed to 3306 (got it reserved from ISI).
Added a fix for Visual FoxBase so that any schema name from a table specification is automatically removed.
New function ASCII().
Removed function BETWEEN(a,b,c). Use the standard ANSI synax instead: expr BETWEEN expr AND expr.
MySQL no longer has to use an extra temporary table when sorting on functions or SUM() functions.
Fixed bug that you couldn't use tbl_name.field_name in UPDATE.
Fixed SELECT DISTINCT when using 'hidden group'. For example:
```
mysql> SELECT DISTINCT MOD(some_field,10)
           FROM test GROUP BY some_field;
```
Note: some_field is normally in the SELECT part. ANSI SQL should require it.

19.4.35 Changes in release 3.21.0

New keywords used: INTERVAL, EXPLAIN, READ, WRITE, BINARY.
Added ODBC function CHAR(num,...).
New operator IN. This uses a binary search to find a match.
New command LOCK TABLES tbl_name [AS alias] {READ|WRITE} ...
Added --log-update option to mysqld, to get a log suitable for incremental updates.
New command EXPLAIN SELECT ... to get information about how the optimizer will do the join.
For easier client code, the client should no longer use FIELD_TYPE_TINY_BLOB, FIELD_TYPE_MEDIUM_BLOB, FIELD_TYPE_LONG_BLOB or FIELD_TYPE_VAR_STRING (as previously returned by mysql_list_fields). You should instead only use FIELD_TYPE_BLOB or FIELD_TYPE_STRING. If you want exact types, you should use the command SHOW FIELDS.
Added varbinary syntax: 0x###### which can be used as a string (default) or a number.
FIELD_TYPE_CHAR is renamed to FIELD_TYPE_TINY.
Changed all fields to C++ classes.
Removed FORM struct.
Fields with DEFAULT values no longer need to be NOT NULL.
New field types:

ENUM
A string which can take only a couple of defined values. The value is stored as a 1-3 byte number that is mapped automatically to a string. This is sorted according to string positions!
SET
A string which may have one or many string values separated with ','. The string is stored as a 1-, 2-, 3-, 4- or 8-byte number where each bit stands for a specific set member. This is sorted according to the unsigned value of the stored packed number.
Now all function calculation is done with double or long long. This will provide the full 64-bit range with bit functions and fix some conversions that previously could result in precision losses. One should avoid using unsigned long long columns with full 64-bit range (numbers bigger than 9223372036854775807) because calculations are done with signed long long.
ORDER BY will now put NULL field values first. GROUP BY will also work with NULL values.
Full WHERE with expressions.

New range optimizer that can resolve ranges when some keypart prefix is constant. Example:

mysql> SELECT * FROM tbl_name
           WHERE key_part_1="customer"
           AND key_part_2>=10 AND key_part_2<=10;

19.5 Changes in release 3.20.x

Changes from 3.20.18 to 3.20.32b are not documented here since the 3.21 release branched here. And the relevant changes are also documented as changes to the 3.21 version.

19.5.1 Changes in release 3.20.18

Added -p# (remove # directories from path) to isamlog. All files are written with a relative path from the database directory Now mysqld shouldn't crash on shutdown when using the --log-isam option.
New mysqlperl version. It is now compatible with msqlperl-0.63.
New DBD module available at http://www.tcx.se/Contrib site.
Added group function STD() (standard deviation).
The mysqld server is now compiled by default without debugging information. This will make the daemon smaller and faster.
Now one usually only has to specify the --basedir option to mysqld. All other paths are relative in a normal installation.
BLOB columns sometimes contained garbage when used with a SELECT on more than one table and ORDER BY.
Fixed that calculations that are not in GROUP BY work as expected (ANSI SQL extension). Example:
```
mysql> SELECT id,id+1 FROM table GROUP BY id;
```
The test of using MYSQL_PWD was reversed. Now MYSQL_PWD is enabled as default in the default release.
Fixed conversion bug which caused mysqld to core dump with Arithmetic error on Sparc-386.
Added --unbuffered option to mysql, for new mysqlaccess.
When using overlapping (unnecessary) keys and join over many tables, the optimizer could get confused and return 0 records.

19.5.2 Changes in release 3.20.17

You can now use BLOB columns and the functions IS NULL and IS NOT NULL in the WHERE clause.
All communication packages and row buffers are now allocated dynamically on demand. The default value of max_allowed_packet is now 65K for the server and 512K for the client. This is mainly used to catch incorrect packets that could trash all memory. The server limit may be changed when it is started.
Changed stack usage to use less memory.
Changed safe_mysqld to check for running daemon.
The ELT() function is renamed to FIELD(). The new ELT() function returns a value based on an index: FIELD() is the inverse of ELT() Example: ELT(2,"A","B","C") returns "B". FIELD("B","A","B","C") returns 2.
COUNT(field), where field could have a NULL value, now works.
A couple of bugs fixed in SELECT ... GROUP BY.
Fixed memory overrun bug in WHERE with many unoptimizable brace levels.
Fixed some small bugs in the grant code.
If hostname isn't found by get_hostname, only the IP is checked. Previously, you got Access denied.
Inserts of timestamps with values didn't always work.
INSERT INTO ... SELECT ... WHERE could give the error Duplicated field.
Added some tests to safe_mysqld to make it 'safer'.
LIKE was case sensitive in some places and case insensitive in others. Now LIKE is always case insensitive.
`mysql.cc': Allow '#' anywhere on the line.
New command SET OPTION SQL_SELECT_LIMIT=#. See the FAQ for more details.
New version of the mysqlaccess script.
Change FROM_DAYS() and WEEKDAY() to also take a full TIMESTAMP or DATETIME as argument. Before they only took a number of type YYYYMMDD or YYMMDD.
Added new function UNIX_TIMESTAMP(timestamp_column).

19.5.3 Changes in release 3.20.16

More changes in MIT-pthreads to get them safer. Fixed also some link bugs at least in SunOS.
Changed mysqld to work around a bug in MIT-pthreads. This makes multiple small SELECT operations 20 times faster. Now lock_test.pl should work.
Added mysql_FetchHash(handle) to mysqlperl.
The mysqlbug script is now distributed built to allow for reporting bugs that appear during the build with it.
Changed `libmysql.c' to prefer getpwuid() instead of cuserid().
Fixed bug in SELECT optimizer when using many tables with the same column used as key to different tables.
Added new latin2 and Russian KOI8 character tables.
Added support for a dummy GRANT command to satisfy Powerbuilder.

19.5.4 Changes in release 3.20.15

Fixed fatal bug packets out of order when using MIT-pthreads.
Removed possible loop when a thread waits for command from client and fcntl() fails. Thanks to Mike Bretz for finding this bug.
Changed alarm loop in `mysqld.cc' because shutdown didn't always succeed in Linux.
Removed use of termbits from `mysql.cc'. This conflicted with glibc 2.0.
Fixed some syntax errors for at least BSD and Linux.
Fixed bug when doing a SELECT as superuser without a database.
Fixed bug when doing SELECT with group calculation to outfile.

19.5.5 Changes in release 3.20.14

If one gives -p or --password option to mysql without an argument, the user is solicited for the password from the tty.
Added default password from MYSQL_PWD (by Elmar Haneke).
Added command 'kill' to mysqladmin to kill a specific MySQL thread.
Sometimes when doing a reconnect on a down connection this succeeded first on second try.
Fixed adding an AUTO_INCREMENT key with ALTER_TABLE.
AVG() gave too small value on some SELECTs with GROUP BY and ORDER BY.
Added new DATETIME type (by Giovanni Maruzzelli
Fixed that define DONT_USE_DEFAULT_FIELDS works.
Changed to use a thread to handle alarms instead of signals on Solaris to avoid race conditions.
Fixed default length of signed numbers. (George Harvey
Allow anything for CREATE INDEX.
Add prezeros when packing numbers to DATE, TIME and TIMESTAMP.
Fixed a bug in OR of multiple tables (gave empty set).
Added many patches to MIT-pthreads. This fixes at least one lookup bug.

19.5.6 Changes in release 3.20.13

Added ANSI SQL94 DATE and TIME types.
Fixed bug in SELECT with AND-OR levels.
Added support for Slovenian characters. The `Contrib' directory contains source and instructions for adding other character sets.
Fixed bug with LIMIT and ORDER BY.
Allow ORDER BY and GROUP BY on items that aren't in the SELECT list. (Thanks to Wim Bonis bonis@kiss.de, for pointing this out.)
Allow setting of timestamp values in INSERT.
Fixed bug with SELECT ... WHERE ... = NULL.
Added changes for glibc 2.0. To get glibc to work, you should add the `gibc-2.0-sigwait-patch' before compiling glibc.
Fixed bug in ALTER TABLE when changing a NOT NULL field to allow NULL values.
Added some ANSI92 synonyms as field types to CREATE TABLE. CREATE TABLE now allows FLOAT(4) and FLOAT(8) to mean FLOAT and DOUBLE.
New utility program mysqlaccess by Yves.Carlier@rug.ac.be. This program shows the access rights for a specific user and the grant rows that determine this grant.
Added WHERE const op field (by bonis@kiss.de).

19.5.7 Changes in release 3.20.11

When using SELECT ... INTO OUTFILE, all temporary tables are ISAM instead of HEAP to allow big dumps.
Changed date functions to be 'string functions'. This fixed some 'funny' side effects when sorting on dates.
Extended ALTER TABLE according to SQL92.
Some minor compability changes.
Added --port and --socket options to all utility programs and mysqld.
Fixed MIT-pthreads readdir_r(). Now mysqladmin create database and mysqladmin drop database should work.
Changed MIT-pthreads to use our tempnam(). This should fix the 'sort aborted' bug.
Added sync of records count in sql_update. This fixed slow updates on first connection. (Thanks to Vaclav Bittner for the test.)

19.5.8 Changes in release 3.20.10

New insert type: INSERT INTO ... SELECT ...
MEDIUMBLOB fixed.
Fixed bug in ALTER TABLE and BLOBs.
SELECT ... INTO OUTFILE now creates the file in the current database directory.
DROP TABLE now can take a list of tables.
Oracle synonym DESCRIBE (DESC).
Changes to make_binary_distribution.
Added some comments to installation instructions about configure's C++ link test.
Added --without-perl option to configure.
Lots of small portability changes.

19.5.9 Changes in release 3.20.9

ALTER TABLE didn't copy null bit. As a result, fields that were allowed to have NULL values were always NULL.
CREATE didn't take numbers as DEFAULT.
Some compatibility changes for SunOS.
Removed `config.cache' from old distribution.

19.5.10 Changes in release 3.20.8

Fixed bug with ALTER TABLE and multi-part keys.

19.5.11 Changes in release 3.20.7

New commands: ALTER TABLE, SELECT ... INTO OUTFILE and LOAD DATA INFILE.
New function: NOW().
Added new field file_priv to mysql/user table.
New script add_file_priv which adds the new field file_priv to the user table. This script must be executed if you want to use the new SELECT ... INTO and LOAD DATA INFILE ... commands with a version of MySQL earlier than 3.20.7.
Fixed bug in locking code, which made lock_test.pl test fail.
New files `NEW' and `BUGS'.
Changed `select_test.c' and `insert_test.c' to include `config.h'.
Added command 'status' to mysqladmin for short logging.
Increased maximum number of keys to 16 and maximum number of key parts to 15.
Use of sub keys. A key may now be a prefix of a string field.
Added -k option to mysqlshow, to get key information for a table.
Added long options to mysqldump.

19.5.12 Changes in release 3.20.6

Portable to more systems because of MIT-pthreads, which will be used automatically if configure cannot find a -lpthreads library.
Added GNU-style long options to almost all programs. Test with program --help.
Some shared library support for Linux.
The FAQ is now in `.texi' format and is available in `.html', `.txt' and `.ps' formats.
Added new SQL function RAND([init]).
Changed sql_lex to handle \0 unquoted, but the client can't send the query through the C API, because it takes a str pointer. You must use mysql_real_query() to send the query.
Added API function mysql_get_client_info().
mysqld now uses the N_MAX_KEY_LENGTH from `nisam.h' as the maximum allowed key length.
The following now works:
```
mysql> SELECT filter_nr,filter_nr FROM filter ORDER BY filter_nr;
```
Previously, this resulted in the error: Column: 'filter_nr' in order clause is ambiguous.
mysql now outputs '\0', '\t', '\n' and '\\' when encountering ASCII 0, tab, newline or '\' while writing tab-separated output. This is to allow printing of binary data in a portable format. To get the old behavior, use -r (or --raw).
Added german error messages (60 of 80 error messages translated).
Added new API function mysql_fetch_lengths(MYSQL_RES *), which returns an array of of column lengths (of type uint).
Fixed bug with IS NULL in WHERE clause.
Changed the optimizer a little to get better results when searching on a key part.
Added SELECT option STRAIGHT_JOIN to tell the optimizer that it should join tables in the given order.
Added support for comments starting with '--' in `mysql.cc' (Postgres syntax).
You can have SELECT expressions and table columns in a SELECT which are not used in the group part. This makes it efficient to implement lookups. The column that is used should be a constant for each group because the value is calculated only once for the first row that is found for a group.
```
mysql> SELECT id,lookup.text,sum(*) FROM test,lookup
         WHERE test.id=lookup.id GROUP BY id;
```
Fixed bug in SUM(function) (could cause a core dump).
Changed AUTO_INCREMENT placement in the SQL query:
```
INSERT into table (auto_field) values (0);
```
inserted 0, but it should insert an AUTO_INCREMENT value.
`mysqlshow.c': Added number of records in table. Had to change the client code a little to fix this.
mysql now allows doubled " or "" within strings for embedded ' or ".
New math functions: EXP(), LOG(), SQRT(), ROUND(), CEILING().

19.5.13 Changes in release 3.20.3

The configure source now compiles a thread-free client library -lmysqlclient. This is the only library that needs to be linked with client applications. When using the binary releases, you must link with -lmysql -lmysys -ldbug -lstrings as before.
New readline library from bash-2.0.
LOTS of small changes to configure and makefiles (and related source).
It should now be possible to compile in another directory using VPATH. Tested with GNU Make 3.75.
safe_mysqld and mysql.server changed to be more compatible between the source and the binary releases.
LIMIT now takes one or two numeric arguments. If one argument is given, it indicates the maximum number of rows in a result. If two arguments are given, the first argument indicates the offset of the first row to return, the second is the maximum number of rows. With this it's easy to do a poor man's next page/previous page WWW application.
Changed name of SQL function FIELDS() to ELT(). Changed SQL function INTERVALL() to INTERVAL().
Made SHOW COLUMNS a synonym for SHOW FIELDS. Added compatibility syntax FRIEND KEY to CREATE TABLE. In MySQL, this creates a non-unique key on the given columns.
Added CREATE INDEX and DROP INDEX as compatibility functions. In MySQL, CREATE INDEX only checks if the index exists and issues an error if it doesn't exist. DROP INDEX always succeeds.
`mysqladmin.c': added client version to version information.
Fixed core dump bug in sql_acl (core on new connection).
Removed host, user and db tables from database test in the distribution.
FIELD_TYPE_CHAR can now be signed (-128 - 127) or unsigned (0 - 255) Previously, it was always unsigned.
Bug fixes in CONCAT() and WEEKDAY().
Changed a lot of source to get mysqld to be compiled with SunPro compiler.
SQL functions must now have a '(' immediately after the function name (no intervening space). For example, 'user(' is regarded as beginning a function call, and 'user (' is regarded as an identifier user followed by a '(', not as a function call.

19.5.14 Changes in release 3.20.0

The source distribution is done with configure and Automake. It will make porting much easier. The readline library is included in the distribution.
Separate client compilation: the client code should be very easy to compile on systems which don't have threads.
The old Perl interface code is automatically compiled and installed. Automatic compiling of DBD will follow when the new DBD code is ported.
Dynamic language support: mysqld can now be started with Swedish or English (default) error messages.
New functions: INSERT(), RTRIM(), LTRIM() and FORMAT().
mysqldump now works correctly for all field types (even AUTO_INCREMENT). The format for SHOW FIELDS FROM tbl_name is changed so the Type column contains information suitable for CREATE TABLE. In previous releases, some CREATE TABLE information had to be patched when recreating tables.
Some parser bugs from 3.19.5 (BLOB and TIMESTAMP) are corrected. TIMESTAMP now returns different date information depending on its create length.
Changed parser to allow a database, table or field name to start with a number or '_'.
All old C code from Unireg changed to C++ and cleaned up. This makes the daemon a little smaller and easier to understand.
A lot of small bug fixes done.
New INSTALL files (not final version) and some info regarding porting.

19.6 Changes in release 3.19.x

19.6.1 Changes in release 3.19.5

Some new functions, some more optimization on joins.
Should now compile clean on Linux (2.0.x).
Added functions DATABASE(), USER(), POW(), LOG10() (needed for ODBC).
In a WHERE with an ORDER BY on fields from only one table, the table is now preferred as first table in a multi-join.
HAVING and IS NULL or IS NOT NULL now works.
A group on one column and a sort on a group function (SUM(), AVG()...) didn't work together. Fixed.
mysqldump: Didn't send password to server.

19.6.2 Changes in release 3.19.4

Fixed horrible locking bug when inserting in one thread and reading in another thread.
Fixed one-off decimal bug. 1.00 was output as 1.0.
Added attribute 'Locked' to process list as info if a query is locked by another query.
Fixed full magic timestamp. Timestamp length may now be 14, 12, 10, 8, 6, 4 or 2 bytes.
Sort on some numeric functions could sort incorrectly on last number.
IF(arg,syntax_error,syntax_error) crashed.
Added functions CEILING(), ROUND(), EXP(), LOG() and SQRT().
Enhanced BETWEEN to handle strings.

19.6.3 Changes in release 3.19.3

Fixed SELECT with grouping on BLOB columns not to return incorrect BLOB info. Grouping, sorting and distinct on BLOB columns will not yet work as expected (probably it will group/sort by the first 7 characters in the BLOB). Grouping on formulas with a fixed string size (use MID() on a BLOB) should work.
When doing a full join (no direct keys) on multiple tables with BLOB fields, the BLOB was garbage on output.
Fixed DISTINCT with calculated columns.

E Known errors and design deficiencies in MySQL

You cannot build in another directory when using MIT-pthreads. Since this requires changes to MIT-pthreads, we are not likely to fix this.
BLOBs can't 'reliably' be used in GROUP BY or ORDER BY or DISTINCT. Only the first max_sort_length characters (default 1024) are used when comparing BLOBbs in these cases. This can be changed with the -O max_sort_length option to mysql. A workaround for most cases is to use a substring: SELECT DISTINCT LEFT(blob,2048) FROM tbl_name.
Calculation is done with BIGINT or DOUBLE (both are normally 64 bits long). It depends on the function which precision one gets. The general rule is that bit functions are done with BIGINT precision, IF, and ELT() with BIGINT or DOUBLE precision and the rest with DOUBLE precision. One should try to avoid using bigger unsigned long long values than 63 bits (9223372036854775807) for anything else than bit fields!
All numeric types are treated as fixed-point fields. That means you must specify how many decimals a floating-point field shall have. All results will be returned with the correct number of decimals.
All string columns, except BLOB and TEXT columns, automatically have all trailing spaces removed when retrieved. For CHAR types this is okay, and may be regarded as a feature according to ANSI SQL92. The bug is that VARCHAR columns are treated the same way.
You can only have up to 255 ENUM and SET columns in one table.
An UPDATE that updates a key with a WHERE on the same key may fail because the key is used to search for records and will be found multiple times:
```
UPDATE SET KEY=KEY+1 WHERE KEY > 100;
```
This will be fixed in newer MySQL versions by not using keys that contain fields that are going to be updated. Until this fixed you can use the current workaround:
```
UPDATE SET KEY=KEY+1 WHERE KEY+0 > 100;
```
This will work because MySQL will not use index on expressions in the WHERE clause.
safe_mysqld re-directs all messages from mysqld to the mysqld log. One problem with this is that if you execute mysqladmin refresh to close and reopen the log, stdout and stderr are still redirected to the old log. If you use --log extensively, you should edit safe_mysqld to log to `'hostname'.err' instead of `'hostname'.log' so you can easily reclaim the space for the old log by deleting the old one and executing mysqladmin refresh.

For platform-specific bugs, see the sections about compiling and porting.

F List of things we want to add to MySQL in the future (The TODO)

Everything in this list is in the order it will be done. If you want to affect the priority order, please register a license or support us and tell us what you want to have done more quickly. See section 3 Licensing or When do I have/want to pay for MySQL?.

19.7 Things that must done in the real near future

Binary compatible ISAM tables.
Subqueries. 'select id from t where grp in (select grp from g where u > 100)
Delayed inserts for log tables.
FreeBSD and MIT-pthreads; Do sleeping threads take CPU?
Allow join on key parts (optimizing issue).
Fix that temporary HEAP tables are automatically converted to NISAM if they get too big. At the moment one gets 'error 135' or 'Table xxx is full' if one does a query which has to use a big temporary table.
Binary portable data tables (a new version of ISAM).
Add DISTINCT qualifier to COUNT(), SUM()...
Change conv_blob to handle BLOB as a TEXT field.
Entry for DECRYPT().
Remember FOREIGN key definitions in the `.frm' file.
Server side cursors.
Don't add automatic DEFAULT values to columns. Give an error when using an INSERT that doesn't contain a column that doesn't have a DEFAULT.
Caching of queries and results. This should be done as a separated module that examines each query and if this is query is in the cache the cached result should be returned. When one updates a table one should remove as few queries as possible from the cache. This should give a big speed bost on machines with much ram where queries are often repeated (like www applications). One idea would be to only cache queries of type: SELECT CACHED ....
Fix `libmysql.c' to allow two mysql_query() commands in a row without reading results or give a nice error message when one does this.
Optimize BIT type to take 1 bit (now BIT takes 1 char).
Check why MIT-pthreads ctime() doesn't work on some FreeBSD systems.
Check if locked threads take any CPU.
Add ORDER BY to update. This would be handy with functions like: generate_id(start,step).
Add an IMAGE option to LOAD DATA INFILE to not update TIMESTAMP and AUTO_INCREMENT fields.

Make LOAD DATA INFILE understand a syntax like:

LOAD DATA INFILE 'file_name.txt' INTO TABLE table_name
TEXT_FIELDS (text_field1, text_field2, text_field3)
SET table_field1=concatenate(text_field1, text_field2), table_field3=23
IGNORE text_field3

Allow strings with MIN(), MAX() (not group functions). These should be synonyms for LEAST(), GREATEST().
Demo procedure: analyze
Automatic output from mysql to netscape.
LOCK DATABASES. (with various options)
NATURAL JOIN.
Change sort to allocate memory in 'hunks' to get better memory utilization.
DECIMAL and NUMERIC types can't read exponential numbers; Field_decimal::store(const char *from,uint len) must be recoded to fix this.
Add ANSI SQL EXTRACT function.
Fix mysql.cc to do fewer malloc() calls when hashing field names.

Add functions:

MAKE_SET(set_bits,'aaa,bbb,ccc,ddd,eee") -> Returns a set string.
MAKE_SET(1 | 8, 'aaa,bbb,ccc,ddd,eee")   -> 'aaa,ddd'

EXPORT_SET(set_column,'Y','N',[separator],[number_of_set_values])

where separator is ',' by default and number_of_set_values is taken from the set_column (or is 64 if set_column is an expression). For example:

EXPORT_SET(9,'Y','N',',',5) -> Y,N,N,Y,N

Add use of t1 JOIN t2 ON ... and t1 JOIN t2 USING ... Currently, you can only use this syntax with LEFT JOIN.
Add full support for unsigned long long type.
A LOCK DATABASE function (for backups).
Function CASE.
Much more variables for show status. Counts for: INSERT/DELETE/UPDATE statements. Records reads and updated. Selects on 1 table and selects with joins. Mean number of tables in select. Key buffer read/write hits (logical and real). ORDER BY, GROUP BY, temporary tables created.
If you abort mysql in the middle of a query, you should open another connection and kill the old running query. Alternatively, an attempt should be made to detect this in the server.
Add a handler interface for table information so you can use it as a system table. This would be a bit slow if you requested information about all tables, but very flexible. SHOW INFO FROM tbl_name for basic table information should be implemented.
Allow mysqld to support many character sets at the same time.
Add optimization to make LEFT JOIN .. WHERE not_null_field IS NULL much faster. (This is often used to find rows that doesn't match)

19.8 Things that have to be done sometime

Implement a table optimizer by an analyze procedure call that returns a table like show fields with min and max value and the best MySQL type for that expression.
Implement function: get_changed_tables(timeout,table1,table2,...)
Implement function: LAST_UPDATED(tbl_name)
Atomic updates; This includes a language that one can even use for a set of stored procedures.
update items,month set items.price=month.price where items.id=month.id;
Change reading through tables to use memmap when possible. Now only compressed tables use memmap.
Make a SQL standard GRANT command with MySQL extensions.
Add a new privilege 'Show_priv' for SHOW commands.
Make the automatic timestamp code nicer. Add timestamps to the update log with SET TIMESTAMP=#;
Optimize the autoincrement code.
Use read/write mutex in some places to get more speed.
Full foreign key support. One probably wants to implement a procedural language first.
Simple views (first on one table, later on any expression).
Automatically close some tables if a table, temporary table or temporary files gets error 23 (not enough open files).
When one finds a field=#, change all occurrences of field to #. Now this is only done for some simple cases.
Change all const expressions with calculated expressions if possible.
Optimize key = expression. At the moment only key = field or key = constant are optimized.
Join some of the copy functions for nicer code.
Change `sql_yacc.yy' to an inline parser to reduce its size and get better error messages (5 days).
Change the parser to use only one rule per different number of arguments in function.
Use of full calculation names in the order part. (For ACCESS97)
UNION, MINUS, INTERSECT and FULL OUTER JOIN. (Currently only LEFT OUTER JOIN is supported)
Allow UNIQUE on fields that can be NULL.
SQL_OPTION MAX_SELECT_TIME=# to put a time limit on a query.
Make the update log to a database.
Negative LIMIT to retrieve data from the end.
Alarm around client connect/read/write functions.
Make a mysqld version which isn't multithreaded (3-5 days).
Please note the changes to safe_mysqld: according to FSSTND (which Debian tries to follow) PID files should go into `/var/run/<progname>.pid' and log files into `/var/log'. It would be nice if you could put the "DATADIR" in the first declaration of "pidfile" and "log", so the placement of these files can be changed with a single statement.
Better dynamic record layout to avoid fragmentation.
UPDATE SET blob=read_blob_from_file('my_gif') where id=1;

Allow sorting on RAND():

SELECT email,RAND() AS ran FROM info ORDER BY ran;

Allow a client to request logging.
Add use of zlib() for gzip-ed files to LOAD DATA INFILE.
Fix sorting and grouping of BLOB columns (partly solved now).
Stored procedures. This is currently not regarded to be very important as stored procedures are not very standardized yet. Another problem is that true stored procedures make it much harder for the optimizer and in many cases the result is slower than before We will, on the other hand, add a simple (atomic) update language that can be used to write loops and such in the MySQL server.
Change to use semaphores when counting threads. One should first implement a semaphore library to MIT-pthreads.
Don't assign a new AUTO_INCREMENT value when one sets a column to 0. Use NULL instead.

Time is given according to amount of work, not real time. TcX's main business is the use of MySQL not the development of it. But since TcX is a very flexible company, we have put a lot of resources into the development of MySQL.

19.9 Some things we don't have any plans to do

Transactions with rollback (we mainly do SELECTs, and because we don't do transactions, we can be much quicker on everything else). We will support some kind of atomic operations on multiple tables, though. Currently atomic operations can be done with LOCK TABLES/UNLOCK TABLES but we will make this more automatic in the future.

G Comments on porting to other systems

A working Posix thread library is needed for the server. On Solaris 2.5 we use SUN PThreads (the native thread support in 2.4 and earlier versions are not good enough) and on Linux we use LinuxThreads by Xavier Leroy, Xavier.Leroy@inria.fr.

The hard part of porting to a new Unix variant without good native thread support is probably to port MIT-pthreads. See `mit-pthreads/README' and Programming POSIX Threads.

The MySQL distribution includes a patched version of Provenzano's Pthreads from MIT (see MIT Pthreads web page). This can be used for some operating systems that do not have POSIX threads.

It is also possible to use another user level thread package named FSU Pthreads (see FSU pthread home page). This implementation is being used for the SCO port.

See the `thr_lock.c' and `thr_alarm.c' programs in the `mysys' directory for some tests/examples of these problems.

Both the server and the client need a working C++ compiler (we use gcc and have tried SparcWorks). Another compiler that is known to work is the IRIX cc.

To compile only the client use ./configure --without-server.

There is currently no support for only compiling the server. Nor is it likly to be added unless someone has a good reason for it.

If you want/need to change any `Makefile' or the configure script you must get Automake and Autoconf. We have used the automake-1.2 and autoconf-2.12 distributions.

All steps needed to remake everything from the most basic files.

/bin/rm */.deps/*.P
/bin/rm -f config.cache
aclocal
autoheader
aclocal
automake
autoconf
./configure --with-debug --prefix='your installation directory'

# The makefiles generated above need GNU make 3.75 or newer.
# (called gmake below)
gmake clean all install init-db

If you run into problems with a new port, you may have to do some debugging of MySQL! See section 19.10 Debugging MySQL.

NOTE: Before you start debugging mysqld, first get the test programs mysys/thr_alarm and mysys/thr_lock to work. This will ensure that your thread installation has even a remote chance to work!

19.10 Debugging MySQL

If you have some very specific problem, you can always try to debug MySQL. To do this you must configure MySQL with the option --with-debug. You can check whether or not MySQL was compiled with debugging by doing: mysqld --help. If the --debug flag is listed with the options then you have debugging enabled. mysqladmin ver also lists the mysqld version as mysql ... -debug in this case.

Start the mysqld server with a trace log in `/tmp/mysql.trace'. The log file will get very BIG.

mysqld --debug --log

or you can start it with

mysqld --debug=d,info,error,query,general,where:O,/tmp/mysql.trace

which only prints information with the most interesting tags.

When you configure MySQL for debugging you automatically enable a lot of extra safety check functions that monitor the health of mysqld. If they find something 'unexpected' an entry will be written to stderr, which safe_mysqld directs to the error log! This also means that if you are having some unexpected problems with MySQL, the first thing you, if you are using a source distribution, is to configure MySQL for debugging! (The second thing is of course to mail to mysql@tcx.se and ask for help. Please use the mysqlbug script for all bug reports or questions regarding a MySQL version you are using!

On most system you can also start mysqld in gdb to get more information if mysqld crashes.

shell> gdb libexec/mysqld
gdb>   run
...
back   # Do this when mysqld crashes
quit

On Linux you must use run --one-thread if you want to be able to debug mysqld threads. In this case you can only have one thread active at a time.

If mysqld hangs you can try to use some system tools like strace or /usr/proc/bin/pstack to examine where mysqld has hanged.

If mysqld starts to eat up CPU or memory you can use mysqladmin processlist status to find out if someone is executing some query that takes a long time. It may be a good idea to run mysqladmin -i10 processlist status in some window if you are experiencing performance problems.

If mysqld dies or hangs, you should start mysqld with --log.

If you are using a log file, mysqld --log, you should check the 'hostname' log files, that you can find in the database directory, for any queries that could cause a problem. Try the command EXPLAIN on all SELECT statements that takes a long time to ensure that mysqld are using indexes properly. See section 7.21 EXPLAIN syntax (Get information about a SELECT). You should also test complicated queries that didn't complete within the mysql command line tool.

If you find the text mysqld restarted in the error log file (normally named `hostname.err') you have probably found a query that causes mysqld to fail. If this happens you should check all your tables with isamchk (see section 13 Using isamchk for table maintenance and crash recovery), and test the queries in the MySQL log files if someone doesn't work. If you find such a query, try first upgrading to the newest MySQL version. If this doesn't help and you can't find anything in the mysql mail archive, you should report the bug to mysql@tcx.se. Links to mail archives are available at the online MySQL documentation page.

The command mysqladmin debug will dump some information about locks in use, used memory and query usage to the mysql log file. This may help solve some problems.

If the problem is that some tables are getting slower and slower you should try to repair the tables with isamchk to optimize the table layout. You should also check the slow queries with EXPLAIN. See section 13 Using isamchk for table maintenance and crash recovery.

You should also read the OS-specific section in this manual for problems that may be unique to your environment. See section 4.11 System-specific notes

19.11 Comments about RTS threads

I have tried to use the RTS thread packages with MySQL but stumbled on the following problems:

They use old version of a lot of POSIX calls and it is very tedious to make wrappers for all functions. I am inclined to think that it would be easier to change the thread libraries to the newest POSIX specification.

Some wrappers are already written. See `mysys/my_pthread.c' for more info.

At least the following should be changed:

pthread_get_specific should use one argument. sigwait should take two arguments. A lot of functions (at least pthread_cond_wait, pthread_cond_timedwait) should return the error code on error. Now they return -1 and set errno.

Another problem is that user-level threads use the ALRM signal and this aborts a lot of functions (read, write, open...). MySQL should do a retry on interrupt on all of these but it is not that easy to verify it.

The biggest unsolved problem is the following:

To get thread-level alarms I changed `mysys/thr_alarm.c' to wait between alarms with pthread_cond_timedwait(), but this aborts with error EINTR. I tried to debug the thread library as to why this happens, but couldn't find any easy solution.

If someone wants to try MySQL with RTS threads I suggest the following:

Change functions MySQL uses from the thread library to POSIX. This shouldn't take that long.
Compile all libraries with the -DHAVE_rts_threads.
Compile thr_alarm.
If there are some small differences in the implementation, they may be fixed by changing `my_pthread.h' and `my_pthread.c'.

Run thr_alarm. If it runs without any 'warning', 'error' or aborted messages, you are on the right track. Here follows a successful run on Solaris:

Main thread: 1
Tread 0 (5) started
Thread: 5  Waiting
process_alarm
Tread 1 (6) started
Thread: 6  Waiting
process_alarm
process_alarm
thread_alarm
Thread: 6  Slept for 1 (1) sec
Thread: 6  Waiting
process_alarm
process_alarm
thread_alarm
Thread: 6  Slept for 2 (2) sec
Thread: 6  Simulation of no alarm needed
Thread: 6  Slept for 0 (3) sec
Thread: 6  Waiting
process_alarm
process_alarm
thread_alarm
Thread: 6  Slept for 4 (4) sec
Thread: 6  Waiting
process_alarm
thread_alarm
Thread: 5  Slept for 10 (10) sec
Thread: 5  Waiting
process_alarm
process_alarm
thread_alarm
Thread: 6  Slept for 5 (5) sec
Thread: 6  Waiting
process_alarm
process_alarm

...
thread_alarm
Thread: 5  Slept for 0 (1) sec
end

19.12 Differences between different thread packages

MySQL is very dependent on the thread package used. So when choosing a good platform for MySQL, the thread package is very important.

There are at least three types of thread packages.

User threads in a single process. Thread switching is managed with alarms and the threads library manages all not thread-safe functions with locks. Read, write and select are usually managed with a thread-specific select that switches to another thread if the running threads have to wait for data. If the user thread packages are integrated in the standard libs (FreeBSD and BSDI threads) the thread package requires less overhead than thread packages that have to map all unsafe calls (MIT-pthreads, FSU-threads and RTS threads). In some environments (for example, SCO), all system calls are thread-safe so the mapping can be done very easily (FSU-threads on SCO). Downside: All mapped calls take a little time and it's quite tricky to be able to handle all situations. There are usually also some system calls that are not handled by the thread package (like MIT-pthreads and sockets). Thread scheduling isn't always optimal.
User threads in separate processes. Thread switching is done by the kernel and all data are shared between threads. The thread package manages the standard thread calls to allow sharing data between threads. LinuxThreads is using this method. Downside: Lots of processes. Thread creating is slow. If one thread dies the rest are usually left hanging and you must kill them all before restarting. Thread switching is somewhat expensive.
Kernel threads. Thread switching is handled by the thread library or the kernel and is very fast. Everything is done in one process but 'ps' may on some systems show the different threads. If one thread aborts the whole process aborts. Most system calls are thread-safe and should require very little overhead. Solaris, HP-UX, AIX and OSF1 has kernel threads.

In some systems kernel threads are managed by integrating user level threads in the system libraries. In such cases, the thread switching can only be done by the thread library and the kernel isn't really 'thread aware'.

H Description of MySQL regular expression syntax

A regular expression (regex) is a powerful way of specifying a complex search.

MySQL uses regular Henry Spencer's inplementation of regular expressions. And that is aimed to conform to POSIX 1003.2. MySQL uses the extended version.

This is a simplistic reference that skips the details. To get more exact information, see Henry Spencer's regex(7) manual page that is included in the source distribution. See section C Contributors to MySQL.

A regular expression describes a set of strings. The simplest regexp is one that has no special characters in it. For example, the regexp hello matches hello and nothing else.

Nontrivial regular expressions use certain special constructs so that they can match more than one string. For example, the regexp hello|word matches either the string hello or the string word.

As a more complex example, the regexp B[an]*s matches any of the strings Bananas, Baaaaas, Bs and any other string starting with a B, ending with an s, and containing any number of a or n characters in between.

A regular expression may use any of the following special characters/constructs:

^

Match the beginning of a string.

mysql> select "fo\nfo" REGEXP "^fo$";           -> 0
mysql> select "fofo" REGEXP "^fo";              -> 1

$

Match the end of a string.

mysql> select "fo\no" REGEXP "^fo\no$";         -> 1
mysql> select "fo\no" REGEXP "^fo$";            -> 0

.

Match any character (including newline).

mysql> select "fofo" REGEXP "^f.*";             -> 1
mysql> select "fo\nfo" REGEXP "^f.*";           -> 1

a*

Match any sequence of zero or more a characters.

mysql> select "Ban" REGEXP "^Ba*n";             -> 1
mysql> select "Baaan" REGEXP "^Ba*n";           -> 1
mysql> select "Bn" REGEXP "^Ba*n";              -> 1

a+

Match any sequence of one or more a characters.

mysql> select "Ban" REGEXP "^Ba+n";             -> 1
mysql> select "Bn" REGEXP "^Ba+n";              -> 0

a?

Match either zero or one a character.

mysql> select "Bn" REGEXP "^Ba?n";              -> 1
mysql> select "Ban" REGEXP "^Ba?n";             -> 1
mysql> select "Baan" REGEXP "^Ba?n";            -> 0

de|abc

Match either of the sequences de or abc.

mysql> select "pi" REGEXP "pi|apa";             -> 1
mysql> select "axe" REGEXP "pi|apa";            -> 0
mysql> select "apa" REGEXP "pi|apa";            -> 1
mysql> select "apa" REGEXP "^(pi|apa)$";        -> 1
mysql> select "pi" REGEXP "^(pi|apa)$";         -> 1
mysql> select "pix" REGEXP "^(pi|apa)$";        -> 0

(abc)*

Match zero or more instances of the sequence abc.

mysql> select "pi" REGEXP "^(pi)+$";            -> 1
mysql> select "pip" REGEXP "^(pi)+$";           -> 0
mysql> select "pipi" REGEXP "^(pi)+$";          -> 1

{1}

{2,3}

The is a more general way of writing regexps that match many occurrences of the previous atam.

a*: Can be written as a{0,}.
a+: Can be written as a{1,}.
a?: Can be written as a{0,1}.

To be more precise, an atom followed by a bound containing one integer i and no comma matches a sequence of exactly i matches of the atom. An atom followed by a bound containing one integer i and a comma matches a sequence of i or more matches of the atom. An atom followed by a bound containing two integers i and j matches a sequence of i through j (inclusive) matches of the atom. Both arguments must 0 >= value <= RE_DUP_MAX (default 255). If there are two arguments, the second must be greater than or equal to the first.

[a-dX]

[^a-dX]

Matches any character which is (or is not, if ^ is used) either a, b, c, d or X. To include a literal ] character, it must immediately follow the opening bracket [. To include a literal - character, it must be written first or last. So [0-9] matches any decimal digit. Any character that does not have a defined meaning inside a [] pair has no special meaning and matches only itself.

mysql> select "aXbc" REGEXP "[a-dXYZ]";         -> 1
mysql> select "aXbc" REGEXP "^[a-dXYZ]$";       -> 0
mysql> select "aXbc" REGEXP "^[a-dXYZ]+$";      -> 1
mysql> select "aXbc" REGEXP "^[^a-dXYZ]+$";     -> 0
mysql> select "gheis" REGEXP "^[^a-dXYZ]+$";    -> 1
mysql> select "gheisa" REGEXP "^[^a-dXYZ]+$";   -> 0

[[.characters.]]

The sequence of characters of that collating element. The sequence is a single element of the bracket expression's list. A bracket expression containing a multi-character collating element can thus match more than one character, e.g., if the collating sequence includes a ch collating element, then the regular expression [[.ch.]]*c matches the first five characters of chchcc.

[=character-class=]

An equivalence class, standing for the sequences of characters of all collating elements equivalent to that one, including itself. For example, if o and (+) are the members of an equivalence class, then [[=o=]], [[=(+)=]], and [o(+)] are all synonymous. An equivalence class may not be an endpoint of a range.

[:character_class:]

Within a bracket expression, the name of a character class enclosed in [: and :] stands for the list of all characters belonging to that class. Standard character class names are:

alnum	digit	punct
alpha	graph	space
blank	lower	upper
cntrl	print	xdigit

These stand for the character classes defined in the ctype(3) manual page. A locale may provide others. A character class may not be used as an endpoint of a range.

mysql> select "justalnums" REGEXP "[[:alnum:]]+";       -> 1
mysql> select "!!" REGEXP "[[:alnum:]]+";               -> 0

[[:<:]]

[[:>:]]

These match the null string at the beginning and end of a word respectively. A word is defined as a sequence of word characters which is neither preceded nor followed by word characters. A word character is an alnum character (as defined by ctype(3)) or an underscore (_).

mysql> select "a word a" REGEXP "[[:<:]]word[[:>:]]";      -> 1
mysql> select "a xword a" REGEXP "[[:<:]]word[[:>:]]";     -> 0

mysql> select "weeknights" REGEXP "^(wee|week)(knights|nights)$"; -> 1

I What is Unireg?

Unireg is our tty interface builder, but it uses a low level connection to our NISAM (which is used by MySQL) and because of this it is very quick. It has existed since 1979 (on Unix in C since ~1986).

Unireg has the following components:

One table viewer with updates/browsing.
Multi table viewer (with one scrolling region).
Table creator. (With lots of column tags you can't create with MySQL) This is WYSIWYG (for a tty). You design a screen and Unireg prompts for the column specification.
Report generator.
A lot of utilities (quick export/import of tables to/from text files, analysis of table contents...).
Powerful multi-table updates (which we use a lot) with a BASIC like language with LOTS of functions.
Dynamic languages (at present in Swedish and Finnish). If somebody wants an English version there are a few files that would have to be translated.
The ability to run updates interactively or in a batch.
Emacs-like key definitions with keyboard macros.
All this in a binary of 800k.
The convform utility. Changes `.frm' and text files between different character sets.
The pack_isam utility. Packs a NISAM table (makes it 50-80% smaller). The table can be read by MySQL like an ordinary table. Only one record has to be decompressed per access. Cannot handle BLOB columns or updates (yet).

We update most of our production databases with the Unireg interface and serve web pages through MySQL (and in some extreme cases the Unireg report generator).

Unireg takes about 3M of disk space and works on at least the following platforms: SUN OS 4.x, Solaris, Linux, HP-UX, ICL Unix, DNIX, SCO and MSDOS.

Unireg is currently only available in Swedish and Finnish.

The price tag for Unireg is 10,000 Swedish kr (about $1500 US), but this includes support. Unireg is distributed as a binary. (But all the ISAM sources can be found in MySQL). Usually we compile the binary for the customer at their site.

All new development is concentrated to MySQL.

J The MySQL server license for non Microsoft operating systems

MySQL FREE PUBLIC LICENSE (Version 4, March 5, 1995)

NOTE: This license is not the same as any of the GNU Licenses published by the Free Software Foundation. Its terms are substantially different from those of the GNU Licenses. If you are familiar with the GNU Licenses, please read this license with extra care.

This License applies to the computer program known as "MySQL". The "Program", below, refers to such program, and a "work based on the Program" means either the Program or any derivative work of the Program, as defined in the United States Copyright Act of 1976, such as a translation or a modification. The Program is a copyrighted work whose copyright is held by TcX Datakonsult AB and Monty Program KB and Detron HB.

This License does not apply when running "MySQL" on any Microsoft operating system. Microsoft operating systems include all versions of Microsoft Windows NT and Microsoft Windows.

BY MODIFYING OR DISTRIBUTING THE PROGRAM (OR ANY WORK BASED ON THE PROGRAM), YOU INDICATE YOUR ACCEPTANCE OF THIS LICENSE TO DO SO, AND ALL ITS TERMS AND CONDITIONS FOR COPYING, DISTRIBUTING OR MODIFYING THE PROGRAM OR WORKS BASED ON IT. NOTHING OTHER THAN THIS LICENSE GRANTS YOU PERMISSION TO MODIFY OR DISTRIBUTE THE PROGRAM OR ITS DERIVATIVE WORKS. THESE ACTIONS ARE PROHIBITED BY LAW. IF YOU DO NOT ACCEPT THESE TERMS AND CONDITIONS, DO NOT MODIFY OR DISTRIBUTE THE PROGRAM.

Licenses. Licensor hereby grants you the following rights, provided that you comply with all of the restrictions set forth in this License and provided, further, that you distribute an unmodified copy of this License with the Program:
1. You may copy and distribute literal (i.e., verbatim) copies of the Program's source code as you receive it throughout the world, in any medium.
2. You may modify the Program, create works based on the Program and distribute copies of such throughout the world, in any medium.
Restrictions. This license is subject to the following restrictions:
1. Distribution of the Program or any work based on the Program by a commercial organization to any third party is prohibited if any payment is made in connection with such distribution, whether directly (as in payment for a copy of the Program) or indirectly (as in payment for some service related to the Program, or payment for some product or service that includes a copy of the Program "without charge"; these are only examples, and not an exhaustive enumeration of prohibited activities). However, the following methods of distribution involving payment shall not in and of themselves be a violation of this restriction:
  1. Posting the Program on a public access information storage and retrieval service for which a fee is received for retrieving information (such as an on-line service), provided that the fee is not content-dependent (i.e., the fee would be the same for retrieving the same volume of information consisting of random data).
  2. Distributing the Program on a CD-ROM, provided that the files containing the Program are reproduced entirely and verbatim on such CD-ROM, and provided further that all information on such CD-ROM be redistributable for non-commercial purposes without charge.
2. Activities other than copying, distribution and modification of the Program are not subject to this License and they are outside its scope. Functional use (running) of the Program is not restricted, and any output produced through the use of the Program is subject to this license only if its contents constitute a work based on the Program (independent of having been made by running the Program).
3. You must meet all of the following conditions with respect to the distribution of any work based on the Program:
  1. If you have modified the Program, you must cause your work to carry prominent notices stating that you have modified the Program's files and the date of any change;
  2. You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole and at no charge to all third parties under the terms of this License;
  3. If the modified program normally reads commands interactively when run, you must cause it, at each time the modified program commences operation, to print or display an announcement including an appropriate copyright notice and a notice that there is no warranty (or else, saying that you provide a warranty). Such notice must also state that users may redistribute the Program only under the conditions of this License and tell the user how to view the copy of this License included with the Program. (Exception: if the Program itself is interactive but does not normally print such an announcement, your work based on the Program is not required to print an announcement.);
  4. You must accompany any such work based on the Program with the complete corresponding machine-readable source code, delivered on a medium customarily used for software interchange. The source code for a work means the preferred form of the work for making modifications to it. For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable code. However, the source code distributed need not include anything that is normally distributed (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs, unless that component itself accompanies the executable code;
  5. If you distribute any written or printed material at all with the Program or any work based on the Program, such material must include either a written copy of this License, or a prominent written indication that the Program or the work based on the Program is covered by this License and written instructions for printing and/or displaying the copy of the License on the distribution medium;
  6. You may not impose any further restrictions on the recipient's exercise of the rights granted herein. If distribution of executable or object code is made by offering the equivalent ability to copy from a designated place, then offering equivalent ability to copy the source code from the same place counts as distribution of the source code, even though third parties are not compelled to copy the source code along with the object code.
Reservation of Rights. No rights are granted to the Program except as expressly set forth herein. You may not copy, modify, sublicense, or distribute the Program except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense or distribute the Program is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.
Other Restrictions. If the distribution and/or use of the Program is restricted in certain countries for any reason, Licensor may add an explicit geographical distribution limitation excluding those countries, so that distribution is permitted only in or among countries not thus excluded. In such case, this License incorporates the limitation as if written in the body of this License.
Limitations. THE PROGRAM IS PROVIDED TO YOU "AS IS," WITHOUT WARRANTY. THERE IS NO WARRANTY FOR THE PROGRAM, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT OF THIRD PARTY RIGHTS. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL LICENSOR, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

K The MySQL license for Microsoft operating systems

MySQL shareware license for Microsoft operating systems (Version 1, September 4, 1998)

This License applies to the computer program known as "MySQL".

This License applies when running MySQL on any Microsoft operating system. Microsoft operating systems include all versions of Microsoft Windows NT and Microsoft Windows.

YOU SHOULD CAREFULLY READ THE FOLLOWING TERMS AND CONDITIONS BEFORE USING, COPYING OR DISTRIBUTING MySQL. BY USING, COPYING AND DISTRIBUTING MySQL, YOU INDICATE YOUR ACCEPTANCE OF THIS LICENSE TO DO SO, AND ALL ITS TERMS AND CONDITIONS FOR USING, COPYING AND DISTRIBUTING MySQL OR WORKS BASED ON IT. NOTHING OTHER THAN THIS LICENSE GRANTS YOU PERMISSION TO USE, COPY OR DISTRIBUTE MySQL OR ITS DERIVATIVE WORKS. THESE ACTIONS ARE PROHIBITED BY LAW. IF YOU DO NOT ACCEPT THESE TERMS AND CONDITIONS, DO NOT USE, COPY OR DISTRIBUTE MySQL.

Evaluation and License Registration. This is an evaluation version of MySQL for Win32. Subject to the terms below, you are hereby licensed to use MySQL for evaluation purposes without charge for a period of 30 days. If you use MySQL after the 30 day evaluation period the registration and purchase of a MySQL license is required. The price for a MySQL license is currently $200 and email support starts from $200/year. Quantity discounts are available. If you pay by credit card, the currency is FIM (Finish Marks) so the prices will differ slightly. The easiest way to register or find options about how to pay for MySQL is to use the license form at TcX's secure server at https://www.tcx.se/license.htmy. This can be used also when paying with credit card over the Internet. Other applicables for paying are SWIFT payments, cheques and credit cards. Payment should be made to:
```
Postgirot Bank AB
105 06 STOCKHOLM, SWEDEN

T.C.X DataKonsult AB
BOX 6434
11382 STOCKHOLM, SWEDEN

SWIFT address: PGSI SESS
Account number: 96 77 06 - 3
```
Specify: license(/support) and your name and email address. In Europe and Japan, EuroGiro (that should be cheaper) can be used to the same account. If you want to pay by cheque make it payable to "Monty Program KB" and mail it to the address below.
```
T.C.X DataKonsult AB
BOX 6434
11382 STOCKHOLM, SWEDEN
```
For more information about commercial licensing, please contact:
```
David Axmark
Kungsgatan 65 B
753 21 UPPSALA
SWEDEN
Voice Phone +46-18-10 22 80     (Swedish and English spoken)
Fax +46-8-729 69 05             (Email *much* preferred)
E-Mail: mysql-licensing@tcx.se
```
For more about the license prices and commercial supports, like e-mail supports, please refer to the MySQL manual. See section 3.1 How much MySQL costs. See section 3.2 How to get commercial support. The use of MySQL or any work based on MySQL after the 30-day evaluation period is in violation of international copyright laws.
Registered version of MySQL. After you have purcased a MySQL license we will send you a receipt by snail mail. You are allowed to use MySQL or any work based on MySQL after the 30-days evaluation period. The use of MySQL is, however, restricted to one physical computer, but there are no restrictions on concurrent uses of MySQL or the number of MySQL servers run on the computer. We will also e-mail you an address and password for a password-protected WWW page that always has the newest MySQL - Win32 version. Our current policy is that a user with the MySQL license can get free upgrades. The best way to ensure that you get the best possible support is to purcase commercial support!
Registration for use in education and university or government-sponsored research. You may obtain a MySQL license for the use in education and university or government-sponsored research for free. In that case, send a detailed application for licensing MySQL for such use to the email address The following information is required in the application:
- the name of the school or institute.
- a short description of the school or institute and of the type of education, resarch or other functions it provides.
- a detailed report of the use of MySQL in the institution.
In this case you will be provided with a license that entitles you to use MySQL in a specified manner.
Distribution. Provided that you verify that you are distributing an evaluation or educational/research version of MySQL you are hereby licensed to make as many literal (i.e., verbatim) copies of the evaluation version of MySQL and documentation as you wish.
Restrictions. The client code of MySQL is in the Public Domain or under the GPL (for example the code for readline) license. You are not allowed to modify, recompile, translate or create derivative works based upon any part of the server code of MySQL.
Reservation of Rights. No rights are granted to MySQL except as expressly set forth herein. You may not copy or distribute MySQL except as expressly provided under this License. Any attempt otherwise to copy or distribute MySQL is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.
Other Restrictions. If the distribution and/or use of MySQL is restricted in certain countries for any reason, the Licensor may add an explicit geographical distribution limitation excluding those countries, so that distribution is permitted only in or among countries not thus excluded. In such case, this License incorporates the limitation as if written in the body of this License.
Limitations. MySQL IS PROVIDED TO YOU "AS IS," WITHOUT WARRANTY. THERE IS NO WARRANTY FOR MySQL, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT OF THIRD PARTY RIGHTS. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF MySQL IS WITH YOU. SHOULD MySQL PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL THE LICENSOR, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE MySQL AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE MySQL (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF MySQL TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

SQL command, type and function index

! (logical NOT)

!= (not equal)

% (modulo)

% (wildcard character)

.my.cnf file, .my.cnf file, .my.cnf file, .my.cnf file, .my.cnf file, .my.cnf file, .my.cnf file, .my.cnf file

<= (less than or equal)

<> (not equal)

= (equal)

> (greater than)

>= (greater than or equal)

_ (wildcard character)

A

AUTO_INCREMENT, and NULL values

AVG()

B

BLOB, BLOB

C

carriage return (\r)

CC environment variable, CC environment variable

CEILING()

CFLAGS environment variable

CHAR, CHAR

Control flow functions

CXX environment variable, CXX environment variable, CXX environment variable, CXX environment variable

CXXFLAGS environment variable, CXXFLAGS environment variable

D

data_sources()

DATABASE()

DATE, DATE

Date and time functions

DBI->fetchall_arrayref

DBI->fetchrow_array

DBI->fetchrow_arrayref

DBI->fetchrow_hashref

E

ELT()

ENCRYPT()

ENUM, ENUM

Environment variable, CC, Environment variable, CC

Environment variable, CFLAGS

Environment variable, CXX, Environment variable, CXX, Environment variable, CXX

Environment variable, CXXFLAGS, Environment variable, CXXFLAGS

Environment variable, HOME

Environment variable, LOGIN

Environment variable, LOGNAME

Environment variable, MYSQL_DEBUG, Environment variable, MYSQL_DEBUG

Environment variable, MYSQL_HISTFILE

Environment variable, MYSQL_HOST

Environment variable, MYSQL_PWD, Environment variable, MYSQL_PWD

Environment variable, MYSQL_TCP_PORT, Environment variable, MYSQL_TCP_PORT, Environment variable, MYSQL_TCP_PORT, Environment variable, MYSQL_TCP_PORT

Environment variable, MYSQL_UNIX_PORT, Environment variable, MYSQL_UNIX_PORT, Environment variable, MYSQL_UNIX_PORT, Environment variable, MYSQL_UNIX_PORT

Environment variable, PATH

Environment variable, TMPDIR

Environment variable, USER

Environment variables, CXX

F

FROM_UNIXTIME(), FROM_UNIXTIME()

G

GET_LOCK()

GRANT

greater than (>)

greater than or equal (>=)

GREATEST()

GROUP BY functions

H

HEX()

HOME environment variable

host.frm, problems finding

HOUR()

I

INT

J

JOIN

L

LAST_INSERT_ID(), LAST_INSERT_ID()

less than or equal (<=)

LIKE, and indexes

LIKE, and wildcards

LOAD DATA INFILE, LOAD DATA INFILE

LOGNAME environment variable

M

Mathematical functions

Miscellaneous functions

mysql_affected_rows()

MYSQL_DEBUG environment variable, MYSQL_DEBUG environment variable

mysql_debug()

mysql_drop_db()

mysql_dump_debug_info()

mysql_eof()

mysql_errno()

mysql_error()

mysql_escape_string()

mysql_escape_string()

mysql_fetch_field()

mysql_fetch_field_direct()

mysql_fetch_fields()

mysql_fetch_lengths()

mysql_fetch_row()

mysql_field_seek()

mysql_field_tell()

mysql_free_result()

mysql_get_client_info()

mysql_get_host_info()

mysql_get_proto_info()

mysql_get_server_info()

MYSQL_HISTFILE environment variable

MYSQL_HOST environment variable

mysql_info(), mysql_info(), mysql_info(), mysql_info()

mysql_list_fields()

mysql_list_processes()

mysql_list_tables()

mysql_num_fields()

mysql_num_rows()

mysql_ping()

MYSQL_PWD environment variable, MYSQL_PWD environment variable

mysql_query()

mysql_real_connect()

mysql_store_result()

MYSQL_TCP_PORT environment variable, MYSQL_TCP_PORT environment variable, MYSQL_TCP_PORT environment variable, MYSQL_TCP_PORT environment variable

mysql_thread_id()

MYSQL_UNIX_PORT environment variable, MYSQL_UNIX_PORT environment variable, MYSQL_UNIX_PORT environment variable, MYSQL_UNIX_PORT environment variable

mysql_use_result()

N

NAME

NATURAL LEFT JOIN

NATURAL LEFT OUTER JOIN

NUL

NULL, NULL

NULL value

NULL values, and AUTO_INCREMENT columns

NULL values, and TIMESTAMP columns

NULLABLE

NUM_OF_FIELDS

NUMERIC

O

P

parentheses ( and )

PASSWORD(), PASSWORD(), PASSWORD()

PATH environment variable

Q

QUARTER()

quote()

R

S

SET, SET

String comparison functions

String functions

SUBDATE()

SUBSTRING(), SUBSTRING(), SUBSTRING()

T

tab (\t)

table

table_cache, table_cache

TAN()

TEXT, TEXT

TIME, TIME

TIME_FORMAT()

TIME_TO_SEC()

TIMESTAMP, TIMESTAMP

TIMESTAMP, and NULL values

TINYBLOB

TINYINT

TINYTEXT

TMPDIR environment variable

U

USE

USER environment variable

USER()

V

VARCHAR, VARCHAR

VERSION()

W

WEEK()

WEEKDAY()

wildcard character (%)

wildcard character (_)

Y

YEAR, YEAR

YEAR()

| (bitwise OR)

|| (logical OR)

Concept Index

A

Adding native functions

Adding user-definable functions

anonymous user, anonymous user, anonymous user

Arithmetic expressions

AUTO_INCREMENT, using with DBI

B

Backup

Big5 Chinese character encoding

Bug reports

C

C++ compiler cannot create executables

Case sensitivity

Casts

cc1plus problems

Checking tables for errors

Chinese

Choosing types

Choosing version

Client programs, building

Command line history

Commands out of sync

Compatibility, Oracle, Compatibility, Oracle, Compatibility, Oracle

Compatibility, Sybase

Configuration files

configure, running after prior invocation

Constant table, Constant table

Cost

D

Database mirroring

Database replication, Database replication

E

Environment variables, Environment variables

Environment variables.

F

fatal signall 11

FreeBSD troubleshooting

Full disk

Functions for SELECT and WHERE clauses

Functions, native, adding

Functions, user-definable, adding

G

Getting MySQL

grant tables, sorting, grant tables, sorting

Grouping of expressions

H

History file

host table, sorting

How to pronounce MySQL

I

Index

Index, leftmost prefix

Indexes

Indexes, and LIKE

Internal compiler errors

isamchk, isamchk

K

Keys

Keywords

L

Languages support

Leftmost prefix of index

Linking

M

make_binary_release

Manual information

Memory use

Mirroring, database

msql2mysql

Multi-byte characters

multi-part-index

MyODBC

mysql

MySQL

MySQL binary

MySQL mailing lists

MySQL mailing lists, subscribing to

MySQL mailing lists, unsubscribing from

MySQL source

MySQL version, MySQL version

mysql_install_db

mysqlaccess

mysqladmin

mysqladmin, mysqladmin, mysqladmin, mysqladmin, mysqladmin

mysqlbug

mysqld

mysqldump

mysqldump

mysqlimport

mysqlimport, mysqlimport

mysqlshow

N

Native functions, adding

Net etiquette, Net etiquette

NULL values vs. empty values

O

ODBC

optimizations

Oracle compatibility, Oracle compatibility, Oracle compatibility

P

pack_isam

Q

Quoting

Quoting binary data

R

Release numbers

replace

Replication

Replication, database, Replication, database

Reporting bugs

Reporting errors

Reserved words

Reserved words, exceptions

row-level locking

Running configure after prior invocation

S

safe_mysqld

Server functions

SHOW INDEX

Size of tables

Solaris troubleshooting

sorting, grant tables, sorting, grant tables

sql_yacc.cc problems

Stability

Startup parameters

Storage requirements

Strings, How to escape things

Support

Support, types

Sybase compatibility, Sybase compatibility

Symbolic links

System table

T

Table cache, Table cache, Table cache

Table size

Table, constant, Table, constant

Table, system

The table is full

Timezone problems, Timezone problems

TODO

Troubleshooting, FreeBSD

Troubleshooting, Solaris

U

Update log

user table, sorting

User-definable functions, adding

V

Version, Choosing

Version, Latest

Virtual memory problems while compiling

W

Which languages MySQL supports

Wildcards, and LIKE

Windows

Y

Year 2000 compliance

This document was generated on 4 November 1998 using the texi2html translator version 1.52 (extended by davida@detron.se).

Filename	Purpose
`/etc/my.cnf`	Global options
`DATADIR/my.cnf`	Server-specific options
`~/.my.cnf`	User-specific options

Column type	`Zero' value
`DATETIME`	`'0000-00-00 00:00:00'`
`DATE`	`'0000-00-00'`
`TIMESTAMP`	`00000000000000` (length depends on display size)
`TIME`	`'00:00:00'`
`YEAR`	`0000`

Table	Column	Column type
`tt`	`ActualPC`	`CHAR(10)`
`tt`	`AssignedPC`	`CHAR(10)`
`tt`	`ClientID`	`CHAR(10)`
`et`	`EMPLOYID`	`CHAR(15)`
`do`	`CUSTNMBR`	`CHAR(15)`

Table	Index
`tt`	`ActualPC`
`tt`	`AssignedPC`
`tt`	`ClientID`
`et`	`EMPLOYID` (primary key)
`do`	`CUSTNMBR` (primary key)

Table	Rows
`person_data`	71074
`lentus`	5291
`twin_project`	5286
`twin_data`	2012
`informant_data`	663
`harmony`	381
`postal_groups`	100

Name	Description
`MYSQL_UNIX_PORT`	The default socket; used for connections to `localhost`
`MYSQL_TCP_PORT`	The default TCP/IP port
`MYSQL_PWD`	The default password
`MYSQL_DEBUG`	Debug-trace options when debugging
`TMPDIR`	The directory where temporary tables/files are created