Analysis and functions of MySql foreign key constraints on CASCADE, set null, RESTRICT, and empty types

Analysis and functions of MySql foreign key constraints on CASCADE, set null, RESTRICT, and empty types

MySql foreign key constraints: NULL, RESTRICT, no action, CASCADE, set null, and other types of analysis and interpretation.

Master table, slave table

Null, RESTRICT, NO ACTION

Delete: The primary table can be deleted only when the table record does not exist. Delete the slave table. The master table remains unchanged.

UPDATE: The primary table can be updated only when the slave table record does not exist. Update slave table. The master table remains unchanged.

CASCADE

Delete: The slave table is automatically deleted when the master table is deleted. Delete the slave table. The master table remains unchanged.

UPDATE: The slave table is automatically updated when the master table is updated. Update slave table. The master table remains unchanged.

SET NULL

Delete: when the master table is deleted, the value of the slave table is automatically updated to NULL. Delete the slave table. The master table remains unchanged.

Update: when the master table is updated, the value of the slave table is automatically updated to NULL. Update slave table. The master table remains unchanged.