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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 14.4.5.2 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:
ON DELETE
is the only referential integrity capability an
application needs, note that as of MySQL Server 4.0, you can use
multiple-table DELETE
statements to delete rows from many
tables with a single statement. See section DELETE
.
ON DELETE
is to add
the appropriate DELETE
statement 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 in some instances lead to problems:
FOREIGN KEY
Constraints.
As of MySQL 4.1.1,
mysqldump
generates dump files that take advantage of this
capability automatically when reloaded.)
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 table1,table2 WHERE table1.id = table2.id; |
See section JOIN
. 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.
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