How to enforce the FOREIGN KEY constraint in MySQL?
This index might be silently dropped later if you create another index that can be used to enforce the foreign key constraint. index_name, if given, is used as described previously. InnoDB permits a foreign key to reference any index column or group of columns.
Can a column have a foreign key reference to itself?
(A column cannot have a foreign key reference to itself.) In these cases, a “child table record” refers to a dependent record within the same table. MySQL requires indexes on foreign keys and referenced keys so that foreign key checks can be fast and not require a table scan.
How to use compound primary key as foreign key?
To use a compound Primary Key as Foreign Key, you’ll have to add the same number of columns (that compose the PK) with same datatypes to the child table and then use the combination of these columns in the FOREIGN KEY definition: Thanks for contributing an answer to Stack Overflow!
Can a table in a foreign key relationship be changed?
A table in a foreign key relationship cannot be altered to use another storage engine. To change the storage engine, you must drop any foreign key constraints first. A foreign key constraint cannot reference a virtual generated column.
How to use cascaded foreign key actions in MySQL?
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. 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.
How to drop a FOREIGN KEY constraint in ALTER TABLE?
You can drop a foreign key constraint using the following ALTER TABLE syntax: If the FOREIGN KEY clause defined a CONSTRAINT name when you created the constraint, you can refer to that name to drop the foreign key constraint. Otherwise, a constraint name was generated internally, and you must use that value.
Can a foreign key be self referential in ALTER TABLE?
The foreign key can be self referential (referring to the same table). When you add a foreign key constraint to a table using ALTER TABLE, remember to first create an index on the column (s) referenced by the foreign key. You can drop a foreign key constraint using the following ALTER TABLE syntax: