问题
In foreign key options on update and on delete. What does each field [Restrict, Cascade, Set Null, No Action] do?
回答1:
If you take one by one :
For both update and delete :
if you try to update / delete the parent row :
Restrict : Nothing gonna be delete if there is a child row
Cascade : the child row will be delete / update too
Set Null : the child column will be set to null if you delete the parent
No action : The child row will not be concern of the delete / update
回答2:
The table containing the foreign key is called the referencing or child table, and the table containing the candidate key is called the referenced or parent table.
Set NULL : Sets the column value to NULL when you delete the parent table row.
CASCADE : CASCADE will propagate the change when the parent changes. If you delete a row, rows in constrained tables that reference that row will also be deleted, etc.
RESTRICT : RESTRICT causes you can not delete a given parent row if a child row exists that references the value for that parent row.
NO ACTION : NO ACTION and RESTRICT are very much alike. when an UPDATE or DELETE statement is executed on the referenced table, the DBMS verifies at the end of the statement execution that none of the referential relationships are violated. in short child row no concern if parent row delete or update.
回答3:
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 using ON UPDATE and ON DELETE subclauses of the FOREIGN KEY clause. MySQL supports five options regarding the action to be taken, listed here:
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.
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 InnoDB rejects table definitions containing ON DELETE SET DEFAULT or ON UPDATE SET DEFAULT clauses.
For an ON DELETE or ON UPDATE that is not specified, the default action is always RESTRICT.
Copied above text from: https://dev.mysql.com/doc/refman/5.5/en/create-table-foreign-keys.html
来源：https://stackoverflow.com/questions/16163301/mysql-workbench-foreign-key-options-restrict-cascade-set-null-no-action-wh