The MySQL implementation of foreign key constraints differs from the SQL standard in the following key respects:
If there are several rows in the parent table with the same referenced key value,
InnoDBperforms a foreign key check as if the other parent rows with the same key value do not exist. For example, if you define a
RESTRICTtype constraint, and there is a child row with several parent rows,
InnoDBdoes not permit the deletion of any of the parent rows.
ON UPDATE CASCADEor
ON UPDATE SET NULLrecurses to update the same table it has previously updated during the same cascade, it acts like
RESTRICT. This means that you cannot use self-referential
ON UPDATE CASCADEor
ON UPDATE SET NULLoperations. This is to prevent infinite loops resulting from cascaded updates. A self-referential
ON DELETE SET NULL, on the other hand, is possible, as is a self-referential
ON DELETE CASCADE. Cascading operations may not be nested more than 15 levels deep.
In an SQL statement that inserts, deletes, or updates many rows, foreign key constraints (like unique constraints) are checked row-by-row. When performing foreign key checks,
InnoDBsets shared row-level locks on child or parent records that it must examine. MySQL checks foreign key constraints immediately; the check is not deferred to transaction commit. According to the SQL standard, the default behavior should be deferred checking. That is, constraints are only checked after the entire SQL statement has been processed. This means that it is not possible to delete a row that refers to itself using a foreign key.
No storage engine, including
InnoDB, recognizes or enforces the
MATCHclause used in referential-integrity constraint definitions. Use of an explicit
MATCHclause does not have the specified effect, and it causes
ON UPDATEclauses to be ignored. Specifying the
MATCHshould be avoided.
MATCHclause in the SQL standard controls how
NULLvalues in a composite (multiple-column) foreign key are handled when comparing to a primary key in the referenced table. MySQL essentially implements the semantics defined by
MATCH SIMPLE, which permits a foreign key to be all or partially
NULL. In that case, a (child table) row containing such a foreign key can be inserted even though it does not match any row in the referenced (parent) table. (It is possible to implement other semantics using triggers.)
MySQL requires that the referenced columns be indexed for performance reasons. However, MySQL does not enforce a requirement that the referenced columns be
UNIQUEor be declared
FOREIGN KEYconstraint that references a non-
UNIQUEkey is not standard SQL but rather an
NDBstorage engine, on the other hand, requires an explicit unique key (or primary key) on any column referenced as a foreign key.
The handling of foreign key references to nonunique keys or keys that contain
NULLvalues is not well defined for operations such as
DELETE CASCADE. You are advised to use foreign keys that reference only
MySQL parses but ignores “inline
REFERENCESspecifications” (as defined in the SQL standard) where the references are defined as part of the column specification. MySQL accepts
REFERENCESclauses only when specified as part of a separate
FOREIGN KEYspecification. For storage engines that do not support foreign keys (such as
MyISAM), MySQL Server parses and ignores foreign key specifications.
For information about foreign key constraints, see Section 188.8.131.52, “FOREIGN KEY Constraints”.