When an UPDATE or DELETE operation affects a key value in the parent table that has matching rows in the child table, the result depends on the referential action specified by ON UPDATE and ON DELETE subclauses of the FOREIGN KEY clause. Referential actions include:
- CASCADE: Delete or update the row from the parent table and automatically delete or update the matching rows in the child table. Both ON DELETE CASCADE and ON UPDATE CASCADE are supported. Between two tables, do not define several ON UPDATE CASCADE clauses that act on the same column in the parent table or in the child table.
Note: Cascaded foreign key actions do not activate triggers.
- SET NULL: Delete or update the row from the parent table and set the foreign key column or columns in the child table to NULL. Both ON DELETE SET NULL and ON UPDATE SET NULL clauses are supported.
If you specify a SET NULL action, make sure that you have not declared the columns in the child table as NOT NULL.
- RESTRICT: Rejects the delete or update operation for the parent table. Specifying RESTRICT (or NO ACTION) is the same as omitting the ON DELETE or ON UPDATE clause.
- NO ACTION: A keyword from standard SQL. In MySQL, equivalent to RESTRICT. The MySQL Server rejects the delete or update operation for the parent table if there is a related foreign key value in the referenced table. Some database systems have deferred checks, and NO ACTION is a deferred check. In MySQL, foreign key constraints are checked immediately, so NO ACTION is the same as RESTRICT.
- SET DEFAULT: This action is recognized by the MySQL parser, but both InnoDB and NDB reject table definitions containing ON DELETE SET DEFAULT or ON UPDATE SET DEFAULT clauses.
For storage engines that support foreign keys, MySQL rejects any INSERT or UPDATE operation that attempts to create a foreign key value in a child table if there is no matching candidate key value in the parent table.
For an ON DELETE or ON UPDATE that is not specified, the default action is always RESTRICT.
For NDB tables, ON UPDATE CASCADE is not supported where the reference is to the parent table’s primary key.
InnoDB performs cascading operations using a depth-first search algorithm on the records of the index that corresponds to the foreign key constraint.