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1 General Information

The MySQL (R) software delivers a very fast, multi-threaded, multi-user, and robust SQL (Structured Query Language) database server. MySQL Server is intended for mission-critical, heavy-load production systems as well as for embedding into mass-deployed software. MySQL is a registered trademark of MySQL AB.

The MySQL software is Dual Licensed. Users can choose to use the MySQL software as an Open Source product under the terms of the GNU General Public License (http://www.fsf.org/licenses/) or can purchase a standard commercial license from MySQL AB. See http://www.mysql.com/company/legal/licensing/ for more information on our licensing policies.

The following list describes some sections of particular interest in this manual:

Important:

Reports of errors (often called ``bugs''), as well as questions and comments, should be sent to http://bugs.mysql.com. See section 1.4.1.3 How to Report Bugs or Problems.

If you have found a sensitive security bug in MySQL Server, please let us know immediately by sending an email message to security@mysql.com.

1.1 About This Manual

This is the Reference Manual for the MySQL Database System. It documents MySQL up to Version 5.0.3-alpha, but is also applicable for older versions of the MySQL software (such as 3.23 or 4.0-production) because functional changes are indicated with reference to a version number.

Because this manual serves as a reference, it does not provide general instruction on SQL or relational database concepts. It also will not teach you how to use your operating system or command-line interpreter.

The MySQL Database Software is under constant development, and the Reference Manual is updated frequently as well. The most recent version of the manual is available online in searchable form at http://dev.mysql.com/doc/. Other formats also are available, including HTML, PDF, and Windows CHM versions.

The primary document is the Texinfo file. The HTML version is produced automatically using 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 pdftex.

If you have any suggestions concerning additions or corrections to this manual, please send them to the documentation team at docs@mysql.com.

This manual was initially written by David Axmark and Michael ``Monty'' Widenius. It is now maintained by the MySQL Documentation Team, consisting of Paul DuBois, Stefan Hinz, Mike Hillyer, and Jon Stephens. For the many other contributors, see section B Credits.

The copyright (2004) to this manual is owned by the Swedish company MySQL AB. MySQL and the MySQL logo are (registered) trademarks of MySQL AB. Other trademarks and registered trademarks referred to in this manual are the property of their respective owners, and are used for identification purposes only.

1.1.1 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.''
constant italic
Italic constant-width font is used to indicate variable input for which you should substitute a value of your own choosing.
`filename'
Constant-width font with surrounding quotes is used for filenames and pathnames. Example: ``The distribution is 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 in table headings and to convey especially strong emphasis.

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

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

The ``shell'' is your command interpreter. On Unix, this is typically a program such as sh or csh. On Windows, the equivalent program is command.com or cmd.exe, typically run in a console window.

When you enter a command or statement shown in an example, do not type the prompt shown in the example.

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

mysql> SELECT col_name FROM db_name.tbl_name;

This means that if you were 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.author_list;

SQL keywords are not case sensitive and may be written in uppercase or lowercase. This manual uses uppercase.

In syntax descriptions, square brackets (`[' and `]') are used to indicate optional words or clauses. For example, in the following statement, IF EXISTS is optional:

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 (`[' and `]'):

TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)

When one member from a set of choices must be chosen, the alternatives are listed within braces (`{' and `}'):

{DESCRIBE | DESC} tbl_name [col_name | wild]

An ellipsis (...) indicates the omission of a section of a statement, typically to provide a shorter version of more complex syntax. For example, INSERT ... SELECT is shorthand for the form of INSERT statement that is followed by a SELECT statement.

An ellipsis can also indicate that the preceding syntax element of a statement may be repeated. In the following example, multiple reset_option values may be given, with each of those after the first preceded by commas:

RESET reset_option [,reset_option] ...

Commands for setting shell variables are shown using Bourne shell syntax. 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

If you are using csh or tcsh, you must issue commands somewhat differently. You would execute the sequence just shown like this:

shell> setenv VARNAME value
shell> some_command

1.2 Overview of the MySQL Database Management System

MySQL, the most popular Open Source SQL database management system, is developed, distributed, and supported by MySQL AB. MySQL AB is a commercial company, founded by the MySQL developers. It is a second generation Open Source company that unites Open Source values and methodology with a successful business model.

The MySQL Web site (http://www.mysql.com/) provides the latest information about MySQL software and MySQL AB.

MySQL is a database management system.
A database is a structured collection of data. It may be anything from a simple shopping list to a picture gallery or the vast amounts of information in a corporate network. To add, access, and process data stored in a computer database, you need a database management system such as MySQL Server. Since computers are very good at handling large amounts of data, database management systems play a central role in computing, as standalone utilities or as parts of other applications.
MySQL is a relational database management system.
A relational database stores data in separate tables rather than putting all the data in one big storeroom. This adds speed and flexibility. The SQL part of ``MySQL'' stands for ``Structured Query Language.'' SQL is the most common standardized language used to access databases and is defined by the ANSI/ISO SQL Standard. The SQL standard has been evolving since 1986 and several versions exist. In this manual, ``SQL-92'' refers to the standard released in 1992, ``SQL:1999'' refers to the standard released in 1999, and ``SQL:2003'' refers to the current version of the standard. We use the phrase ``the SQL standard'' to mean the current version of the SQL Standard at any time.
MySQL software is Open Source.
Open Source means that it is possible for anyone to use and modify the software. Anybody can download the MySQL software from the Internet and use it without paying anything. If you wish, you may study the source code and change it to suit your needs. The MySQL software uses the GPL (GNU General Public License), http://www.fsf.org/licenses/, to define what you may and may not do with the software in different situations. If you feel uncomfortable with the GPL or need to embed MySQL code into a commercial application, you can buy a commercially licensed version from us. See the MySQL Licensing Overview for more information (http://www.mysql.com/company/legal/licensing/).
The MySQL Database Server is very fast, reliable, and easy to use.
If that is what you are looking for, you should give it a try. MySQL Server also has a practical set of features developed in close cooperation with our users. You can find a performance comparison of MySQL Server with other database managers on our benchmark page. See section 7.1.4 The MySQL Benchmark Suite. MySQL Server was originally developed to handle large databases much faster than existing solutions and has been successfully used in highly demanding production environments for several years. Although under constant development, MySQL Server today offers a rich and useful set of functions. Its connectivity, speed, and security make MySQL Server highly suited for accessing databases on the Internet.
MySQL Server works in client/server or embedded systems.
The MySQL Database Software is a client/server system that consists of a multi-threaded SQL server that supports different backends, several different client programs and libraries, administrative tools, and a wide range of application programming interfaces (APIs). We also provide MySQL Server as an embedded multi-threaded library that you can link into your application to get a smaller, faster, easier-to-manage product.
A large amount of contributed MySQL software is available.
It is very likely that you will find that your favorite application or language already supports the MySQL Database Server.

The official way to pronounce ``MySQL'' is ``My Ess Que Ell'' (not ``my sequel''), but we don't mind if you pronounce it as ``my sequel'' or in some other localized way.

1.2.1 History of MySQL

We started out with the intention of using mSQL to connect to our tables using our own fast low-level (ISAM) routines. 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 designed to allow third-party code that was written for use with mSQL to be ported easily for use with MySQL.

The derivation of the name MySQL is not clear. Our base directory and a large number of our libraries and tools have had the prefix ``my'' for well over 10 years. However, co-founder Monty Widenius's daughter is also named My. Which of the two gave its name to MySQL is still a mystery, even for us.

The name of the MySQL Dolphin (our logo) is ``Sakila,'' which was chosen by the founders of MySQL AB from a huge list of names suggested by users in our ``Name the Dolphin'' contest. The winning name was submitted by Ambrose Twebaze, an Open Source software developer from Swaziland, Africa. According to Ambrose, the name Sakila has its roots in SiSwati, the local language of Swaziland. Sakila is also the name of a town in Arusha, Tanzania, near Ambrose's country of origin, Uganda.

1.2.2 The Main Features of MySQL

The following list describes some of the important characteristics of the MySQL Database Software. See also section 1.3 MySQL Development Roadmap for more information about current and upcoming features.

Internals and Portability
Column Types
Statements and Functions
Security
Scalability and Limits
Connectivity
Localization
Clients and Tools

1.2.3 MySQL Stability

This section addresses the questions, ``How stable is MySQL Server?'' and, ``Can I depend on MySQL Server in this project?'' We will try to clarify these issues and answer some important questions that concern many potential users. The information in this section is based on data gathered from the mailing lists, which are very active in identifying problems as well as reporting types of use.

The original code stems back to the early 1980s. It provides a stable code base, and the ISAM table format used by the original storage engine remains backward-compatible. At TcX, the predecessor of MySQL AB, MySQL code has worked in projects since mid-1996, without any problems. When the MySQL Database Software initially was released to a wider public, our new users quickly found some pieces of untested code. Each new release since then has had fewer portability problems, even though each new release has also had many new features.

Each release of the MySQL Server has been usable. Problems have occurred only when users try code from the ``gray zones.'' Naturally, new users don't know what the gray zones are; this section therefore attempts to document those areas that are currently known. The descriptions mostly deal with Version 3.23, 4.0 and 4.1 of MySQL Server. All known and reported bugs are fixed in the latest version, with the exception of those listed in the bugs section, which are design-related. See section 1.5.7 Known Errors and Design Deficiencies in MySQL.

The MySQL Server design is multi-layered with independent modules. Some of the newer modules are listed here with an indication of how well-tested each of them is:

Replication (Stable)
Large groups of servers using replication are in production use, with good results. Work on enhanced replication features is continuing in MySQL 5.x.
InnoDB tables (Stable)
The InnoDB transactional storage engine has been declared stable in the MySQL 3.23 tree, starting from version 3.23.49. InnoDB is being used in large, heavy-load production systems.
BDB tables (Stable)
The Berkeley DB code is very stable, but we are still improving the BDB transactional storage engine interface in MySQL Server.
Full-text searches (Stable)
Full-text searching is widely used. Important feature enhancements were added in MySQL 4.0 and 4.1.
MyODBC 3.51 (Stable)
MyODBC 3.51 uses ODBC SDK 3.51 and is in wide production use. Some issues brought up appear to be application-related and independent of the ODBC driver or underlying database server.

1.2.4 How Big MySQL Tables Can Be

MySQL 3.22 had a 4GB (4 gigabyte) limit on table size. With the MyISAM storage engine in MySQL 3.23, the maximum table size was increased to 8 million terabytes (2 ^ 63 bytes). With this larger allowed table size, the maximum effective table size for MySQL databases now usually is determined by operating system constraints on file sizes, not by MySQL internal limits.

The InnoDB storage engine maintains InnoDB tables within a tablespace that can be created from several files. This allows a table to exceed the maximum individual file size. The tablespace can include raw disk partitions, which allows extremely large tables. The maximum tablespace size is 64TB.

The following table lists some examples of operating system file-size limits. This is only a rough guide and is not intended to be definitive. For the most up-to-date information, be sure to check the documentation specific to your operating system.

Operating System File-size Limit
Linux 2.2-Intel 32-bit 2GB (LFS: 4GB)
Linux 2.4 (using ext3 filesystem) 4TB
Solaris 9/10 16TB
NetWare w/NSS filesystem 8TB
win32 w/ FAT/FAT32 2GB/4GB
win32 w/ NTFS 2TB (possibly larger)
MacOS X w/ HFS+ 2TB

On Linux 2.2, you can get MyISAM tables larger than 2GB in size by using the Large File Support (LFS) patch for the ext2 filesystem. On Linux 2.4, patches also exist for ReiserFS to get support for big files (up to 2TB). Most current Linux distributions are based on kernel 2.4 and already include all the required LFS patches. With JFS and XFS, petabyte and larger files are possible on Linux. However, the maximum available file size still depends on several factors, one of them being the filesystem used to store MySQL tables.

For a detailed overview about LFS in Linux, have a look at Andreas Jaeger's Large File Support in Linux page at http://www.suse.de/~aj/linux_lfs.html.

Windows users please note: FAT and VFAT (FAT32) are not considered suitable for production use with MySQL. Use NTFS instead.

By default, MySQL creates MyISAM tables with an internal structure that allows a maximum size of about 4GB. You can check the maximum table size for a table with the SHOW TABLE STATUS statement or with myisamchk -dv tbl_name. See section 13.5.4 SHOW Syntax.

If you need a MyISAM table that will be larger than 4GB in size (and your operating system supports large files), the CREATE TABLE statement allows AVG_ROW_LENGTH and MAX_ROWS options. See section 13.2.6 CREATE TABLE Syntax. You can also change these options with ALTER TABLE after the table has been created, to increase the table's maximum allowable size. See section 13.2.2 ALTER TABLE Syntax.

Other ways to work around file-size limits for MyISAM tables are as follows:

1.2.5 Year 2000 Compliance

The MySQL Server itself has no problems with Year 2000 (Y2K) compliance:

The following simple demonstration illustrates that MySQL Server has no problems with DATE or DATETIME values through the year 9999, and no problems with TIMESTAMP values until after the year 2030:

mysql> DROP TABLE IF EXISTS y2k;
Query OK, 0 rows affected (0.01 sec)

mysql> CREATE TABLE y2k (date DATE,
    ->                   date_time DATETIME,
    ->                   time_stamp TIMESTAMP);
Query OK, 0 rows affected (0.01 sec)

mysql> INSERT INTO y2k VALUES
    -> ('1998-12-31','1998-12-31 23:59:59',19981231235959),
    -> ('1999-01-01','1999-01-01 00:00:00',19990101000000),
    -> ('1999-09-09','1999-09-09 23:59:59',19990909235959),
    -> ('2000-01-01','2000-01-01 00:00:00',20000101000000),
    -> ('2000-02-28','2000-02-28 00:00:00',20000228000000),
    -> ('2000-02-29','2000-02-29 00:00:00',20000229000000),
    -> ('2000-03-01','2000-03-01 00:00:00',20000301000000),
    -> ('2000-12-31','2000-12-31 23:59:59',20001231235959),
    -> ('2001-01-01','2001-01-01 00:00:00',20010101000000),
    -> ('2004-12-31','2004-12-31 23:59:59',20041231235959),
    -> ('2005-01-01','2005-01-01 00:00:00',20050101000000),
    -> ('2030-01-01','2030-01-01 00:00:00',20300101000000),
    -> ('2040-01-01','2040-01-01 00:00:00',20400101000000),
    -> ('9999-12-31','9999-12-31 23:59:59',99991231235959);
Query OK, 14 rows affected (0.01 sec)
Records: 14  Duplicates: 0  Warnings: 2

mysql> SELECT * FROM y2k;
+------------+---------------------+----------------+
| date       | date_time           | time_stamp     |
+------------+---------------------+----------------+
| 1998-12-31 | 1998-12-31 23:59:59 | 19981231235959 |
| 1999-01-01 | 1999-01-01 00:00:00 | 19990101000000 |
| 1999-09-09 | 1999-09-09 23:59:59 | 19990909235959 |
| 2000-01-01 | 2000-01-01 00:00:00 | 20000101000000 |
| 2000-02-28 | 2000-02-28 00:00:00 | 20000228000000 |
| 2000-02-29 | 2000-02-29 00:00:00 | 20000229000000 |
| 2000-03-01 | 2000-03-01 00:00:00 | 20000301000000 |
| 2000-12-31 | 2000-12-31 23:59:59 | 20001231235959 |
| 2001-01-01 | 2001-01-01 00:00:00 | 20010101000000 |
| 2004-12-31 | 2004-12-31 23:59:59 | 20041231235959 |
| 2005-01-01 | 2005-01-01 00:00:00 | 20050101000000 |
| 2030-01-01 | 2030-01-01 00:00:00 | 20300101000000 |
| 2040-01-01 | 2040-01-01 00:00:00 | 00000000000000 |
| 9999-12-31 | 9999-12-31 23:59:59 | 00000000000000 |
+------------+---------------------+----------------+
14 rows in set (0.00 sec)

The final two TIMESTAMP column values are zero because the year values (2040, 9999) exceed the TIMESTAMP maximum. The TIMESTAMP data type, which is used to store the current time, supports values that range from 19700101000000 to 20300101000000 on 32-bit machines (signed value). On 64-bit machines, TIMESTAMP handles values up to 2106 (unsigned value).

Although MySQL Server itself is Y2K-safe, you may run into problems if you use it with applications that are not Y2K-safe. For example, many old applications store or manipulate years using two-digit values (which are ambiguous) rather than four-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 because different applications may be written by different programmers, each of whom may use a different set of conventions and date-handling functions.

Thus, even though MySQL Server has no Y2K problems, it is the application's responsibility to provide unambiguous input. See section 11.3.4 Y2K Issues and Date Types for MySQL Server's rules for dealing with ambiguous date input data that contains two-digit year values.

1.3 MySQL Development Roadmap

This section provides a snapshot of the MySQL development roadmap, including major features implemented or planned for MySQL 4.0, 4.1, 5.0, and 5.1. The following sections provide information for each release series.

The current production release series is MySQL 4.1, which was declared stable for production use as of Version 4.1.7, released in October 2004. The previous production release series is MySQL 4.0, which was declared stable for production use as of Version 4.0.12, released in March 2003. Production status means that future 4.1 and 4.0 development is limited only to bugfixes. For the older MySQL 3.23 series, only critical bugfixes are made.

Active MySQL development currently is taking place in the MySQL 5.0 release series, this means that new features are being added there. MySQL 5.0 is available in alpha status.

Before upgrading from one release series to the next, please see the notes at section 2.10 Upgrading MySQL.

Plans for some of the most requested features are summarized in the following table.

Feature MySQL Series
Unions 4.0
Subqueries 4.1
R-trees 4.1 (for MyISAM tables)
Stored procedures 5.0
Views 5.0
Cursors 5.0
Foreign keys 5.1 (already implemented in 3.23 for InnoDB)
Triggers 5.0 and 5.1
Full outer join 5.1
Constraints 5.1

1.3.1 MySQL 4.0 in a Nutshell

MySQL Server 4.0 is available in production status.

MySQL 4.0 is available for download at http://dev.mysql.com/ and from our mirrors. MySQL 4.0 has been tested by a large number of users and is in production use at many large sites.

The major new features of MySQL Server 4.0 are geared toward our existing business and community users, enhancing the MySQL database software as the solution for mission-critical, heavy-load database systems. Other new features target the users of embedded databases.

1.3.1.1 Features Available in MySQL 4.0

Speed enhancements
Embedded MySQL Server introduced
InnoDB storage engine as standard
New functionality
Standards compliance, portability, and migration
Internationalization
Usability enhancements
In the process of implementing features for new users, we have not forgotten requests from our loyal community of existing users.

The news section of this manual includes a more in-depth list of features. See section D.3 Changes in release 4.0.x (Production).

1.3.1.2 The Embedded MySQL Server

The libmysqld embedded server library makes MySQL Server suitable for a vastly expanded realm of applications. By using this library, developers can embed MySQL Server into various applications and electronics devices, where the end user has no knowledge of there actually being an underlying database. Embedded MySQL Server is ideal for use behind the scenes in Internet appliances, public kiosks, turnkey hardware/software combination units, high performance Internet servers, self-contained databases distributed on CD-ROM, and so on.

Many users of libmysqld will benefit from the MySQL Dual Licensing. For those not wishing to be bound by the GPL, the software is also made available under a commercial license. See http://www.mysql.com/company/legal/licensing/ for more information on the licensing policy of MySQL AB. The embedded MySQL library uses the same interface as the normal client library, so it is convenient and easy to use. See section 21.2.16 libmysqld, the Embedded MySQL Server Library.

On Windows there are two different libraries:

libmysqld.lib Dynamic library for threaded applications.
mysqldemb.lib Static library for not threaded applications.

1.3.2 MySQL 4.1 in a Nutshell

MySQL Server 4.0 laid the foundation for new features implemented in MySQL 4.1, such as subqueries and Unicode support, and for the work on stored procedures being done in version 5.0. These features come at the top of the wish list of many of our customers. Already well-known for its stability, speed, and ease of use, MySQL Server is now able to fulfill the requirement checklists of very demanding buyers.

MySQL Server 4.1 is currently in production status, and binaries are available for download at http://dev.mysql.com/downloads/mysql/4.1.html. All binary releases pass our extensive test suite without any errors on the platforms on which we test. See section D.2 Changes in release 4.1.x (Production).

For those wishing to use the most recent development source for MySQL 4.1, we also make our BitKeeper repositories publicly available. See section 2.8.3 Installing from the Development Source Tree.

1.3.2.1 Features Available in MySQL 4.1

This section lists features implemented in MySQL 4.1. New features that will be available in MySQL 5.0 are described in section C.1 New Features Planned for 5.0.

Support for subqueries and derived tables
Speed enhancements
New functionality
Standards compliance, portability, and migration
Internationalization and Localization
Usability enhancements

The news section of this manual includes a more in-depth list of features. See section D.2 Changes in release 4.1.x (Production).

1.3.3 MySQL 5.0: The Next Development Release

New development for MySQL is focused on the 5.0 release, featuring stored procedures, views (including updatable views), rudimentary triggers, and other new features. See section C.1 New Features Planned for 5.0.

For those wishing to take a look at the bleeding edge of MySQL development, we make our BitKeeper repository for MySQL version 5.0 publicly available. See section 2.8.3 Installing from the Development Source Tree. As of December 2003, binary builds of version 5.0 have also been available.

1.4 MySQL Information Sources

1.4.1 MySQL Mailing Lists

This section introduces the MySQL mailing lists and provides guidelines as to how the lists should be used. When you subscribe to a mailing list, you will receive all postings to the list as email messages. You can also send your own questions and answers to the list.

1.4.1.1 The MySQL Mailing Lists

To subscribe to or unsubscribe from any of the mailing lists described in this section, visit http://lists.mysql.com/. For most of them, you can select the regular version of the list where you get individual messages, or a digest version where you get one large message per day.

Please do not send messages about subscribing or unsubscribing to any of the mailing lists, because such messages are distributed automatically to thousands of other users.

Your local site may have many subscribers to a MySQL mailing list. If so, the site may have a local mailing list, so that messages sent from lists.mysql.com to your site are propagated to the local list. In such cases, please contact your system administrator to be added to or dropped from the local MySQL list.

If you wish to have traffic for a mailing list go to a separate mailbox in your mail program, set up a filter based on the message headers. You can use either the List-ID: or Delivered-To: headers to identify list messages.

The MySQL mailing lists are as follows:

announce
This list is for announcements of new versions of MySQL and related programs. This is a low-volume list to which all MySQL users should subscribe.
mysql
This is the main list for general MySQL discussion. Please note that some topics are better discussed on the more-specialized lists. If you post to the wrong list, you may not get an answer.
bugs
This list will be of interest to you if you want to stay informed about issues reported since the last release of MySQL or if you want to be actively involved in the process of bug hunting and fixing. See section 1.4.1.3 How to Report Bugs or Problems.
internals
This list is for people who work on the MySQL code. This is also the forum for discussions on MySQL development and for posting patches.
mysqldoc
This list is for people who work on the MySQL documentation: people from MySQL AB, translators, and other community members.
benchmarks
This list is for anyone interested in performance issues. Discussions concentrate on database performance (not limited to MySQL), but also include broader categories such as performance of the kernel, filesystem, disk system, and so on.
packagers
This list is for discussions on packaging and distributing MySQL. This is the forum used by distribution maintainers to exchange ideas on packaging MySQL and on ensuring that MySQL looks and feels as similar as possible on all supported platforms and operating systems.
java
This list is for discussions about the MySQL server and Java. It is mostly used to discuss JDBC drivers, including MySQL Connector/J.
win32
This list is for all topics concerning the MySQL software on Microsoft operating systems, such as Windows 9x, Me, NT, 2000, XP, and 2003.
myodbc
This list is for all topics concerning connecting to the MySQL server with ODBC.
gui-tools
This list is for all topics concerning MySQL GUI tools, including MySQL Administrator and the MySQL Control Center graphical client.
cluster
This list is for discussion of MySQL Cluster.
dotnet
This list is for discussion of the MySQL server and the .NET platform. Mostly related to the MySQL Connector/Net provider.
plusplus
This list is for all topics concerning programming with the C++ API for MySQL.
perl
This list is for all topics concerning the Perl support for MySQL with DBD::mysql.

If you're unable to get an answer to your questions from a MySQL mailing list, one option is to purchase support from MySQL AB. This will put you in direct contact with MySQL developers.

The following table shows some MySQL mailing lists in languages other than English. These lists are not operated by MySQL AB.

mysql-france-subscribe@yahoogroups.com
A French mailing list.
list@tinc.net
A Korean mailing list. Email subscribe mysql your@email.address to this list.
mysql-de-request@lists.4t2.com
A German mailing list. Email subscribe mysql-de your@email.address to this list. You can find information about this mailing list at http://www.4t2.com/mysql/.
mysql-br-request@listas.linkway.com.br
A Portuguese mailing list. Email subscribe mysql-br your@email.address to this list.
mysql-alta@elistas.net
A Spanish mailing list. Email subscribe mysql your@email.address to this list.

1.4.1.2 Asking Questions or Reporting Bugs

Before posting a bug report or question, please do the following:

If you can't find an answer in the manual or the archives, check with your local MySQL expert. If you still can't find an answer to your question, please follow the guidelines on sending mail to a MySQL mailing list, outlined in the next section, before contacting us.

1.4.1.3 How to Report Bugs or Problems

The normal place to report bugs is http://bugs.mysql.com/, which is the address for our bugs database. This database is public, and can be browsed and searched by anyone. If you log in to the system, you will also be able to enter new reports.

Writing a good bug report takes patience, but doing it right the first time saves time both for us and for yourself. A good bug report, containing a full test case for the bug, makes it very likely that we will fix the bug in the next release. This section will help you write your report correctly so that you don't waste your time doing things that may not help us much or at all.

We encourage everyone to use the mysqlbug script to generate a bug report (or a report about any problem). mysqlbug can be found in the `scripts' directory (source distribution) and in the `bin' directory under your MySQL installation directory (binary distribution). If you are unable to use mysqlbug (for example, if you are running on Windows), it is still vital that you include all the necessary information noted in this section (most importantly, a description of the operating system and the MySQL version).

The mysqlbug script helps you generate a report by determining much of the following information automatically, but if something important is missing, please include it with your message. Please read this section carefully and make sure that all the information described here is included in your report.

Preferably, you should test the problem using the latest production or development version of MySQL Server before posting. Anyone should be able to repeat the bug by just using mysql test < script_file on the included test case or by running the shell or Perl script that is included in the bug report.

All bugs posted in the bugs database at http://bugs.mysql.com/ will be corrected or documented in the next MySQL release. If only minor code changes are needed to correct a problem, we may also post a patch that fixes the problem.

If you have found a sensitive security bug in MySQL, you can send email to security@mysql.com.

If you have a repeatable bug report, please report it to the bugs database at http://bugs.mysql.com/. Note that even in this case it's good to run the mysqlbug script first to find information about your system. Any bug that we are able to repeat has a high chance of being fixed in the next MySQL release.

To report other problems, you can use one of the MySQL mailing lists.

Remember that it is possible for us to respond to a message containing too much information, but not to one containing too little. People often omit facts because they think they know the cause of a problem and assume that some details don't matter. A good principle is this: If you are in doubt about stating something, state it. It is faster and less troublesome to write a couple more lines in your report than to wait longer for the answer if we must ask you to provide information that was missing from the initial report.

The most common errors made in bug reports are (a) not including the version number of the MySQL distribution used, and (b) not fully describing the platform on which the MySQL server is installed (including the platform type and version number). This is highly relevant information, and in 99 cases out of 100, the bug report is useless without it. Very often we get questions like, ``Why doesn't this work for me?'' Then we find that the feature requested wasn't implemented in that MySQL version, or that a bug described in a report has already been fixed in newer MySQL versions. Sometimes the error is platform-dependent; in such cases, it is next to impossible for us to fix anything without knowing the operating system and the version number of the platform.

If you compiled MySQL from source, remember also to provide information about your compiler, if it is related to the problem. Often people find bugs in compilers and think the problem is MySQL-related. Most compilers are under development all the time and become better version by version. To determine whether your problem depends on your compiler, we need to know what compiler you use. Note that every compiling problem should be regarded as a bug and reported accordingly.

It is most helpful when a good description of the problem is included in the bug report. That is, give a good example of everything you did that led to the problem and describe, in exact detail, the problem itself. The best reports are those that include a full example showing how to reproduce the bug or problem. See section E.1.6 Making a Test Case If You Experience Table Corruption.

If a program produces an error message, it is very important to include the message in your report. If we try to search for something from the archives using programs, it is better that the error message reported exactly matches the one that the program produces. (Even the lettercase should be observed.) You should never try to reproduce from memory what the error message was; instead, copy and paste the entire message into your report.

If you have a problem with Connector/ODBC (MyODBC), please try to generate a trace file and send it with your report. See section 22.1.1.9 How to Report MyODBC Problems or Bugs.

Please remember that many of the people who will read your report will do so using an 80-column display. When generating reports or examples using the mysql command-line tool, you should therefore use the --vertical option (or the \G statement terminator) for output that would exceed the available width for such a display (for example, with the EXPLAIN SELECT statement; see the example later in this section).

Please include the following information in your report:

If you are a support customer, please cross-post the bug report to mysql-support@mysql.com for higher-priority treatment, as well as to the appropriate mailing list to see whether someone else has experienced (and perhaps solved) the problem.

For information on reporting bugs in MyODBC, see section 22.1.1.9 How to Report MyODBC Problems or Bugs.

For solutions to some common problems, see section A Problems and Common Errors.

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 so that others may have the benefit of responses you received that helped you solve your problem.

1.4.1.4 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; don't feel obliged to quote the entire original message.

Please don't post mail messages from your browser with HTML mode turned on. Many users don't read mail with a browser.

1.4.2 MySQL Community Support on IRC (Internet Relay Chat)

In addition to the various MySQL mailing lists, you can find experienced community people on IRC (Internet Relay Chat). These are the best networks/channels currently known to us:

If you are looking for IRC client software to connect to an IRC network, take a look at X-Chat (http://www.xchat.org/). X-Chat (GPL licensed) is available for Unix as well as for Windows platforms.

1.4.3 MySQL Community Support at the MySQL Forums

The latest community support resource are the forums at http://forums.mysql.com.

There are a variety of forums available, grouped in the following general categories:

1.5 MySQL Standards Compliance

This section describes how MySQL relates to the ANSI/ISO SQL standards. MySQL Server has many extensions to the SQL standard, and here you will find out what they are and how to use them. You will also find information about functionality missing from MySQL Server, and how to work around some differences.

The SQL standard has been evolving since 1986 and several versions exist. In this manual, ``SQL-92'' refers to the standard released in 1992, ``SQL:1999'' refers to the standard released in 1999, and ``SQL:2003'' refers to the current version of the standard. We use the phrase ``the SQL standard'' to mean the current version of the SQL Standard at any time.

Our goal is to not restrict MySQL Server usability for any usage without a very good reason for doing so. Even if we don't have the resources to perform development for every possible use, we are always willing to help and offer suggestions to people who are trying to use MySQL Server in new territories.

One of our main goals with the product is to continue to work toward compliance with the SQL standard, but without sacrificing speed or reliability. We are not afraid to add extensions to SQL or support for non-SQL features if this greatly increases the usability of MySQL Server for a large segment of our user base. The HANDLER interface in MySQL Server 4.0 is an example of this strategy. See section 13.1.3 HANDLER Syntax.

We will continue to support transactional and non-transactional databases to satisfy both mission-critical 24/7 usage and heavy Web or logging usage.

MySQL Server was originally designed to work with medium size databases (10-100 million rows, or about 100MB per table) on small computer systems. Today MySQL Server handles terabyte-size databases, but the code can also be compiled in a reduced version suitable for hand-held and embedded devices. The compact design of the MySQL server makes development in both directions possible without any conflicts in the source tree.

Currently, we are not targeting realtime support, although MySQL replication capabilities already offer significant functionality.

Database cluster support now exists through third-party clustering solutions as well as the integration of our acquired NDB Cluster technology into a new storage engine, available from version 4.1.2. See section 16 MySQL Cluster.

We are also looking at providing XML support in the database server.

1.5.1 What Standards MySQL Follows

We are aiming toward supporting the full ANSI/ISO SQL standard, but without making concessions to speed and quality of the code.

ODBC levels 0-3.51.

1.5.2 Selecting SQL Modes

The MySQL server can operate in different SQL modes, and can apply these modes differentially for different clients. This allows an application to tailor server operation to its own requirements.

Modes define what SQL syntax MySQL should support and what kind of validation checks it should perform on the data. This makes it easier to use MySQL in a lot of different environments and to use MySQL together with other database servers.

You can set the default SQL mode by starting mysqld with the --sql-mode="modes" option. Beginning with MySQL 4.1, you can also change the mode after startup time by setting the sql_mode variable with a SET [SESSION|GLOBAL] sql_mode='modes' statement.

For more information on setting the server mode, see section 5.2.2 The Server SQL Mode.

1.5.3 Running MySQL in ANSI Mode

You can tell mysqld to use the ANSI mode with the --ansi startup option. See section 5.2.1 mysqld Command-Line Options.

Running the server in ANSI mode is the same as starting it with these options (specify the --sql_mode value on a single line):

--transaction-isolation=SERIALIZABLE
--sql-mode=REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,
IGNORE_SPACE,ONLY_FULL_GROUP_BY

In MySQL 4.1, you can achieve the same effect with these two statements (specify the sql_mode value on a single line):

SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SET GLOBAL sql_mode = 'REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,
IGNORE_SPACE,ONLY_FULL_GROUP_BY';

See section 1.5.2 Selecting SQL Modes.

In MySQL 4.1.1, the sql_mode options shown can be also be set with this statement:

SET GLOBAL sql_mode='ansi';

In this case, the value of the sql_mode variable will be set to all options that are relevant for ANSI mode. You can check the result like this:

mysql> SET GLOBAL sql_mode='ansi';
mysql> SELECT @@global.sql_mode;
        -> 'REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,
            IGNORE_SPACE,ONLY_FULL_GROUP_BY,ANSI';

1.5.4 MySQL Extensions to Standard SQL

MySQL Server 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 write code that includes MySQL extensions, but is still portable, by using comments of the form /*! ... */. In this case, MySQL Server will parse and execute the code within the comment as it would any other MySQL statement, but other SQL servers will ignore the extensions. For example:

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

If you add a version number after the `!' character, the syntax within the comment will be executed only if the MySQL version is equal to or newer than the specified version number:

CREATE /*!32302 TEMPORARY */ TABLE t (a INT);

This means that if you have Version 3.23.02 or newer, MySQL Server will use the TEMPORARY keyword.

The following descriptions list MySQL extensions, organized by category.

Organization of data on disk
MySQL Server maps each database to a directory under the MySQL data directory, and tables within a database to filenames in the database directory. This has a few implications: Database, table, index, column, or alias names may begin with a digit (but may not consist solely of digits).
General language syntax
SQL statement syntax
Column types
Functions and operators

For a prioritized list indicating when new extensions will be added to MySQL Server, you should consult the online MySQL TODO list at http://dev.mysql.com/doc/mysql/en/TODO.html. That is the latest version of the TODO list in this manual. See section C MySQL and the Future (the TODO).

1.5.5 MySQL Differences from Standard SQL

We try to make MySQL Server follow the ANSI SQL standard and the ODBC SQL standard, but MySQL Server performs operations differently in some cases:

1.5.5.1 Subqueries

MySQL 4.1 supports subqueries and derived tables. A ``subquery'' is a SELECT statement nested within another statement. A ``derived table'' (an unnamed view) is a subquery in the FROM clause of another statement. See section 13.1.8 Subquery Syntax.

For MySQL versions older than 4.1, most subqueries can be rewritten using joins or other methods. See section 13.1.8.11 Rewriting Subqueries as Joins for Earlier MySQL Versions for examples that show how to do this.

1.5.5.2 SELECT INTO TABLE

MySQL Server doesn't support the Sybase SQL extension: SELECT ... INTO TABLE .... Instead, MySQL Server supports the standard SQL syntax INSERT INTO ... SELECT ..., which is basically the same thing. See section 13.1.4.1 INSERT ... SELECT Syntax.

INSERT INTO tbl_temp2 (fld_id)
    SELECT tbl_temp1.fld_order_id
    FROM tbl_temp1 WHERE tbl_temp1.fld_order_id > 100;

Alternatively, you can use SELECT INTO OUTFILE ... or CREATE TABLE ... SELECT.

From version 5.0, MySQL supports SELECT ... INTO with user variables. The same syntax may also be used inside stored procedures using cursors and local variables. See section 19.1.6.3 SELECT ... INTO Statement.

1.5.5.3 Transactions and Atomic Operations

MySQL Server (version 3.23-max and all versions 4.0 and above) supports transactions with the InnoDB and BDB transactional storage engines. InnoDB provides full ACID compliance. See section 14 MySQL Storage Engines and Table Types.

The other non-transactional storage engines in MySQL Server (such as MyISAM) follow a different paradigm for data integrity called ``atomic operations.'' In transactional terms, MyISAM tables effectively always operate in AUTOCOMMIT=1 mode. Atomic operations often offer comparable integrity with higher performance.

With MySQL Server supporting both paradigms, you can decide whether your applications are best served by the speed of atomic operations or the use of transactional features. This choice can be made on a per-table basis.

As noted, the trade-off for transactional versus non-transactional table types lies mostly in performance. Transactional tables have significantly higher memory and diskspace requirements, and more CPU overhead. On the other hand, transactional table types such as InnoDB also offer many significant features. MySQL Server's modular design allows the concurrent use of different storage engines to suit different requirements and deliver optimum performance in all situations.

But how do you use the features of MySQL Server to maintain rigorous integrity even with the non-transactional MyISAM tables, and how do these features compare with the transactional table types?

  1. If your applications are written in a way that is dependent on being able to call ROLLBACK rather than COMMIT in critical situations, transactions are more convenient. Transactions also ensure that unfinished updates or corrupting activities are not committed to the database; the server is given the opportunity to do an automatic rollback and your database is saved. If you use non-transactional tables, MySQL Server in almost all cases allows you to resolve potential problems by including simple checks before updates and by running simple scripts that check the databases for inconsistencies and automatically repair or warn if such an inconsistency occurs. Note that just by using the MySQL log or even adding one extra log, you can normally fix tables perfectly with no data integrity loss.
  2. More often than not, critical transactional updates can be rewritten to be atomic. Generally speaking, all integrity problems that transactions solve can be done with LOCK TABLES or atomic updates, ensuring that you never will get an automatic abort from the server, which is a common problem with transactional database systems.
  3. Even a transactional system can lose data if the server goes down. The difference between different systems lies in just how small the time-lag is where they could lose data. No system is 100% secure, only ``secure enough.'' Even Oracle, reputed to be the safest of transactional database systems, is reported to sometimes lose data in such situations. To be safe with MySQL Server, whether or not using transactional tables, you only need to have backups and have binary logging turned on. With this you can recover from any situation that you could with any other transactional database system. It is always good to have backups, regardless of which database system you use.

The transactional paradigm has its benefits and its drawbacks. Many users and application developers depend on the ease with which they can code around problems where an abort appears to be, or is necessary. However, even if you are new to the atomic operations paradigm, or more familiar with transactions, do consider the speed benefit that non-transactional tables can offer on the order of three to five times the speed of the fastest and most optimally tuned transactional tables.

In situations where integrity is of highest importance, MySQL Server offers transaction-level reliability and integrity even for non-transactional tables. If you lock tables with LOCK TABLES, all updates will stall until any integrity checks are made. If you obtain a READ LOCAL lock (as opposed to a write lock) for a table that allows concurrent inserts at the end of the table, reads are allowed, as are inserts by other clients. The new inserted records will not be seen by the client that has the read lock until it releases the lock. With INSERT DELAYED, you can queue inserts into a local queue, until the locks are released, without having the client wait for the insert to complete. See section 13.1.4.2 INSERT DELAYED Syntax.

``Atomic,'' in the sense that we mean it, is nothing magical. It only means that you can be sure that while each specific update is running, no other user can interfere with it, and there will never be an automatic rollback (which can happen with transactional tables if you are not very careful). MySQL Server also guarantees that there will not be any dirty reads.

Following are some techniques for working with non-transactional tables:

1.5.5.4 Stored Procedures and Triggers

Stored procedures are implemented in MySQL version 5.0. See section 19 Stored Procedures and Functions.

Triggers are currently being implemented, with basic functionality in MySQL 5.0, with further development planned for MySQL 5.1.

1.5.5.5 Foreign Keys

In MySQL Server 3.23.44 and up, the InnoDB storage engine supports checking of foreign key constraints, including CASCADE, ON DELETE, and ON UPDATE. See section 15.7.4 FOREIGN KEY Constraints.

For storage engines other than InnoDB, MySQL Server parses the FOREIGN KEY syntax in CREATE TABLE statements, but does not use or store it. In the future, the implementation will be extended to store this information in the table specification file so that it may be retrieved by mysqldump and ODBC. At a later stage, foreign key constraints will be implemented for MyISAM tables as well.

Foreign key enforcement offers several benefits to database developers:

Do keep in mind that these benefits come at the cost of additional overhead for the database server to perform the necessary checks. Additional checking by the server affects performance, which for some applications may be sufficiently undesirable as to be avoided if possible. (Some major commercial applications have coded the foreign-key logic at the application level for this reason.)

MySQL gives database developers the choice of which approach to use. If you don't need foreign keys and want to avoid the overhead associated with enforcing referential integrity, you can choose another table type instead, such as MyISAM. (For example, the MyISAM storage engine offers very fast performance for applications that perform only INSERT and SELECT operations, because the inserts can be performed concurrently with retrievals. See section 7.3.2 Table Locking Issues.)

If you choose not to take advantage of referential integrity checks, keep the following considerations in mind:

Be aware that the use of foreign keys can in some instances lead to problems:

Note that foreign keys in SQL are used to check and enforce referential integrity, not to join tables. If you want to get results from multiple tables from a SELECT statement, you do this by performing a join between them:

SELECT * FROM t1, t2 WHERE t1.id = t2.id;

See section 13.1.7.1 JOIN Syntax. See section 3.6.6 Using Foreign Keys.

The FOREIGN KEY syntax without ON DELETE ... is often used by ODBC applications to produce automatic WHERE clauses.

1.5.5.6 Views

Views (including updatable views) are being implemented in the 5.0 version of MySQL Server. They are already available in binary releases from 5.0.1 and up. See section 13.2.7 CREATE VIEW Syntax.

Views are useful for allowing users to access a set of relations (tables) as if it were a single table, and limiting their access to just that. Views can also be used to restrict access to rows (a subset of a particular table). For access control to columns, you can also use the sophisticated privilege system in MySQL Server. See section 5.5 The MySQL Access Privilege System.

In designing an implementation of views, our ambitious goal, as much as is possible within the confines of SQL, has been full compliance with ``Codd's Rule #6'' for relational database systems: ``All views that are theoretically updatable, should in practice also be updatable.''

1.5.5.7 `--' as the Start of a Comment

Some other SQL databases use `--' to start comments. MySQL Server uses `#' as the start comment character. You can also use the C comment style /* this is a comment */ with MySQL Server. See section 9.5 Comment Syntax.

MySQL Server 3.23.3 and above support the `--' comment style, provided the comment is followed by a space (or by a control character such as a newline). The requirement for a space is to prevent 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 account SET credit=credit-!payment!

Think about what happens if the value of payment is a negative value such as -1:

UPDATE account SET credit=credit--1

credit--1 is a legal expression in SQL, but if -- is interpreted as the start of a comment, part of the expression is discarded. The result is a statement that has a completely different meaning than intended:

UPDATE account SET credit=credit

The statement produces no change in value at all! This illustrates that allowing comments to start with `--' can have serious consequences.

Using our implementation of this method of commenting in MySQL Server 3.23.3 and up, credit--1 is actually safe.

Another safe feature is that the mysql command-line client removes all lines that start with `--'.

The following information is relevant only if you are running a MySQL version earlier than 3.23.3:

If you have an SQL program in a text file that contains `--' comments, you should use the replace utility as follows to convert the comments to use `#' characters:

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

instead of the usual:

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

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

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

Change them back with this command:

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

1.5.6 How MySQL Deals with Constraints

MySQL allows you to work both with transactional tables that allow rollback and with non-transactional tables that do not. Because of this, constraint handling is a bit different in MySQL than in other databases. We must handle the case when you have inserted or updated a lot of rows in a non-transactional table for which changes cannot be rolled back when an error occurs.

The basic philosophy is that MySQL Server tries to produce an error for anything that it can detect while parsing a statement to be executed, and tries to recover from any errors that occur while executing the statement. We do this in most cases, but not yet for all. See section C.3 New Features Planned for the Near Future.

The options MySQL has when an error occurs are to stop the statement in the middle or to recover as well as possible from the problem and continue. By default, the server follows the latter course. This means, for example, that the server may coerce illegal values to the closest legal values.

Beginning with MySQL 5.0.2, several SQL mode options are available to provide greater control over how accepting to be of bad data values and whether to continue executing a statement or abort it when errors occur. Using these options, you can configure MySQL Server to act in a more traditional fashion that is like other DBMSs that reject improper input. The SQL mode can be set at runtime, which enables individual clients to select the behavior most appropriate for their requirements. See section 5.2.2 The Server SQL Mode.

The following sections describe what happens for the different types of constraints.

1.5.6.1 PRIMARY KEY and UNIQUE Index Constraints

Normally, an error occurs when you try to INSERT or UPDATE a row that causes a primary key, unique key, or foreign key violation. If you are using a transactional storage engine such as InnoDB, MySQL automatically rolls back the statement. If you are using a non-transactional storage engine, MySQL stops processing the statement at the row for which the error occurred and leaves any remaining rows unprocessed.

If you wish to ignore such key violations, MySQL supports an IGNORE keyword for INSERT and UPDATE. In this case, MySQL ignores any key violations and continues processing with the next row. See section 13.1.4 INSERT Syntax. See section 13.1.10 UPDATE Syntax.

You can get information about the number of rows actually inserted or updated with the mysql_info() C API function. See section 21.2.3.31 mysql_info(). In MySQL 4.1 and up, you also can use the SHOW WARNINGS statement. See section 13.5.4.20 SHOW WARNINGS Syntax.

For the moment, only InnoDB tables support foreign keys. See section 15.7.4 FOREIGN KEY Constraints. Foreign key support in MyISAM tables is scheduled for implementation in MySQL 5.1.

1.5.6.2 Constraints on Invalid Data

Before MySQL 5.0.2, MySQL is forgiving of illegal or improper data values and coerces them to legal values for data entry. In MySQL 5.0.2 and up, that remains the default behavior, but you can select more traditional treatment of bad values such that the server rejects them and aborts the statement in which they occur. This section describes the default (forgiving) behavior of MySQL, as well as the newer strict SQL mode and how it differs.

The following holds true when you are not using strict mode. If you insert an ``incorrect'' value into a column, such as a NULL into a NOT NULL column or a too-large numeric value into a numeric column, MySQL sets the column to the ``best possible value'' instead of producing an error:

The reason for the preceding rules is that we can't check these conditions until the statement has begun executing. We can't just roll back if we encounter a problem after updating a few rows, because the storage engine may not support rollback. The option of terminating the statement is not that good; in this case, the update would be ``half done,'' which is probably the worst possible scenario. In this case, it's better to ``do the best you can'' and then continue as if nothing happened.

In MySQL 5.0.2 and up, you can select stricter treatment of input values by using the STRICT_TRANS_TABLES or STRICT_ALL_TABLES SQL modes. See section 5.2.2 The Server SQL Mode.

STRICT_TRANS_TABLES works like this: For transactional storage engines, bad data values occurring anywhere in the statement causes the to abort and roll back. For non-transactional storage engines, the statement aborts if the error occurs in the first row to be inserted or updated. (In this case, the statement can be regarded to leave the table unchanged, just as for a transactional table.) Errors in rows after the first do not abort the statement. Instead, bad data values are adjusted and result in warnings rather than errors. In other words, with STRICT_TRANS_TABLES, a wrong value causes MySQL to roll back, if it can, all updates done so far.

For stricter checking, enable STRICT_ALL_TABLES. This is the same as STRICT_TRANS_TABLES except that for non-transactional storage engines, errors abort the statement even for bad data in rows following the first row. This means that if an error occurs partway through a multiple-row insert or update for a non-transactional table, a partial update results. Earlier rows are inserted or updated, but those from the point of the error on are not. To avoid this for non-transactional tables, either use single-row statements or else use STRICT_TRANS_TABLES if conversion warnings rather than errors are acceptable. To avoid problems in the first place, do not use MySQL to check column content. It is safest (and often faster) to let the application ensure that it passes only legal values to the database.

With either of the strict mode options, you can cause errors to be treated as warnings by using INSERT IGNORE or UPDATE IGNORE.

1.5.6.3 ENUM and SET Constraints

ENUM and SET columns provide an efficient way to define columns that can contain only a given set of values. However, before MySQL 5.0.2, ENUM and SET are not real constraints. This is for the same reasons that NOT NULL is not honored. See section 1.5.6.2 Constraints on Invalid Data.

ENUM columns always have a default value. If you don't specify a default value, then it will be NULL for columns that can have NULL, otherwise the first enumeration value is used as the default value.

If you insert an incorrect value into an ENUM column or if you force a value into an ENUM column with IGNORE, it is set to the reserved enumeration value of 0, which is displayed as an empty string in string context. See section 11.4.4 The ENUM Type.

If you insert an incorrect value into a SET column, the incorrect value is ignored. For example, if the column can contain the values 'a', 'b', and 'c', an attempt to assign 'a,x,b,y' results in a value of 'a,b'. See section 11.4.5 The SET Type.

As of MySQL 5.0.2, you can configure the server to use strict SQL mode. See section 5.2.2 The Server SQL Mode. When strict mode is not enabled, values entered into ENUM and SET columns are handled as just described for MySQL 4.x. However, if strict mode is enabled, the definition of a ENUM or SET column does act as a constraint on values entered into the column. An error occurs for values that do not satisify these conditions:

Errors for invalid values can be suppressed in strict mode if you use INSERT IGNORE or UPDATE IGNORE. In this case, a warning is generated rather than an error. For ENUM, the value is inserted as the error member (0). For SET, the value is inserted as given except that any invalid substrings are deleted. For example, 'a,x,b,y' results in a value of 'a,b', as described earlier.

1.5.7 Known Errors and Design Deficiencies in MySQL

1.5.7.1 Errors in 3.23 Fixed in a Later MySQL Version

The following known errors or bugs are not fixed in MySQL 3.23 because fixing them would involve changing a lot of code that could introduce other even worse bugs. The bugs are also classified as ``not fatal'' or ``bearable.''

1.5.7.2 Errors in 4.0 Fixed in a Later MySQL Version

The following known errors or bugs are not fixed in MySQL 4.0 because fixing them would involve changing a lot of code that could introduce other even worse bugs. The bugs are also classified as ``not fatal'' or ``bearable.''

1.5.7.3 Open Bugs and Design Deficiencies in MySQL

The following problems are known and fixing them is a high priority:

The following problems are known and will be fixed in due time:

The following are known bugs in earlier versions of MySQL:

For information about platform-specific bugs, see the installation and porting instructions in section 2.12 Operating System-Specific Notes and section E Porting to Other Systems.


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