Actually this DELETE rule is referential constraint.
The DELETE rule defines the effects that deleting a row in the referenced table has on the referencing table The DELETE rule is also used when column attributes are defined.
No DELETE rule: deleting a row in the referenced table will fail if matching rows exist.
ON DELETE CASCADE: if a row in the referenced table is deleted, all of the matching rows are deleted.
ON DELETE RESTRICT: deleting a row in the referenced table will fails if matching rows exist.
ON DELETE SET NULL: if a row in the referenced table is deleted, a NULL value is assigned to each referencing column of every matching row.
None of these referencing tables may be a NOT NULL column.
