CREATE TABLE t1 (
          id int(11) NOT NULL AUTO_INCREMENT,
          t2_id int(11),
          PRIMARY KEY (id)
        ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
        CREATE TABLE t2 (
          id int(11) NOT NULL AUTO_INCREMENT,
          PRIMARY KEY (id)
        ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
        CREATE TRIGGER `t2_BEFORE_DELETE` before delete ON `t2` FOR EACH ROW update t1 set t2_id = null where t1.t2_id = OLD.id;

      CREATE TABLE t1 (
        id int(11) NOT NULL AUTO_INCREMENT,
        t2_id int(11),
        PRIMARY KEY (id)
      ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
      CREATE TABLE t2 (
        id int(11) NOT NULL AUTO_INCREMENT,
        PRIMARY KEY (id)
      ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
      CREATE TRIGGER `t2_BEFORE_DELETE` before delete ON `t2` FOR EACH ROW update t1 set t2_id = null from dual where t1.t2_id = OLD.id;