MySQL :: MySQL 5.7 Reference Manual :: 1.6.3.2 FOREIGN KEY Constraints (original) (raw)

1.6.3.2 FOREIGN KEY Constraints

Foreign keys let you cross-reference related data across tables, andforeign key constraints help keep this spread-out data consistent.

MySQL supports ON UPDATE and ON DELETE foreign key references inCREATE TABLE andALTER TABLE statements. The available referential actions are RESTRICT (the default), CASCADE, SET NULL, and NO ACTION.

SET DEFAULT is also supported by the MySQL Server but is currently rejected as invalid byInnoDB. Since MySQL does not support deferred constraint checking, NO ACTION is treated as RESTRICT. For the exact syntax supported by MySQL for foreign keys, seeSection 13.1.18.5, “FOREIGN KEY Constraints”.

MATCH FULL, MATCH PARTIAL, and MATCH SIMPLE are allowed, but their use should be avoided, as they cause the MySQL Server to ignore any ON DELETE orON UPDATE clause used in the same statement. MATCH options do not have any other effect in MySQL, which in effect enforces MATCH SIMPLE semantics full-time.

MySQL requires that foreign key columns be indexed; if you create a table with a foreign key constraint but no index on a given column, an index is created.

You can obtain information about foreign keys from the Information SchemaKEY_COLUMN_USAGE table. An example of a query against this table is shown here:

mysql> SELECT TABLE_SCHEMA, TABLE_NAME, COLUMN_NAME, CONSTRAINT_NAME
     > FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
     > WHERE REFERENCED_TABLE_SCHEMA IS NOT NULL;
+--------------+---------------+-------------+-----------------+
| TABLE_SCHEMA | TABLE_NAME    | COLUMN_NAME | CONSTRAINT_NAME |
+--------------+---------------+-------------+-----------------+
| fk1          | myuser        | myuser_id   | f               |
| fk1          | product_order | customer_id | f2              |
| fk1          | product_order | product_id  | f1              |
+--------------+---------------+-------------+-----------------+
3 rows in set (0.01 sec)

Information about foreign keys on InnoDB tables can also be found in theINNODB_SYS_FOREIGN andINNODB_SYS_FOREIGN_COLS tables, in the INFORMATION_SCHEMA database.

InnoDB and NDB tables support foreign keys.