mysql 外键 级联更新和删除。（1) 级联更新：foreign key(fkname) references tablename(colname) on update cascade;
(2) 级联删除：foreign key(fkname) references tablename(colname) on delete cascade;
(3) 级联更新和删除：foreign key(fkname) references tablename(colname) on delete cascade on update cascade;