13.4.5.2、on delete cascade; ``` mysql对有些约束的修改时不支持的，所以我们可以先删除，再添加 alter table t_student drop foreign key fk_classes_id; alter table t_student add constraint fk_classes_id_1 foreign key(classes_id) references t_classes(classes_id) on delete cascade; delete from t_classes where classes_id = 20; ```  我们只删除了父表中的数据，但是子表也会中的数据也会删除。