The InnoDB storage engine supports checking
of foreign key constraints, including
CASCADE, ON DELETE, and
ON UPDATE. See
Section 13.6.5.4, “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:
Assuming proper design of the relationships, foreign key constraints make it more difficult for a programmer to introduce an inconsistency into the database.
Centralized checking of constraints by the database server makes it unnecessary to perform these checks on the application side. This eliminates the possibility that different applications may not all check the constraints in the same way.
Using cascading updates and deletes can simplify the application code.
Properly designed foreign key rules aid in documenting relationships between tables.
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 storage engine instead, such as
MyISAM. (For example, the
MyISAM storage engine offers very fast
performance for applications that perform only
INSERT and
SELECT operations. In this
case, the table has no holes in the middle and the inserts can
be performed concurrently with retrievals. See
Section 7.10.3, “Concurrent Inserts”.)
If you choose not to take advantage of referential integrity checks, keep the following considerations in mind:
In the absence of server-side foreign key relationship checking, the application itself must handle relationship issues. For example, it must take care to insert rows into tables in the proper order, and to avoid creating orphaned child records. It must also be able to recover from errors that occur in the middle of multiple-record insert operations.
If
ON DELETEis the only referential integrity capability an application needs, you can achieve a similar effect as of MySQL Server 4.0 by using multiple-tableDELETEstatements to delete rows from many tables with a single statement. See Section 12.2.2, “DELETESyntax”.A workaround for the lack of
ON DELETEis to add the appropriateDELETEstatements to your application when you delete records from a table that has a foreign key. In practice, this is often as quick as using foreign keys and is more portable.
Be aware that the use of foreign keys can sometimes lead to problems:
Foreign key support addresses many referential integrity issues, but it is still necessary to design key relationships carefully to avoid circular rules or incorrect combinations of cascading deletes.
It is not uncommon for a DBA to create a topology of relationships that makes it difficult to restore individual tables from a backup. (MySQL alleviates this difficulty by enabling you to temporarily disable foreign key checks when reloading a table that depends on other tables. See Section 13.6.5.4, “
FOREIGN KEYConstraints”. As of MySQL 4.1.1, mysqldump generates dump files that take advantage of this capability automatically when they are reloaded.)
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 INNER JOIN t2 ON t1.id = t2.id;
See Section 12.2.9.1, “JOIN Syntax”, and
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.