mysql触发器控制商品库存不可小于0

//注意要把商品库存字段设置为有符号
CREATE TRIGGER tg_goods_after_update after update ON goods FOR EACH ROW 
BEGIN
      if NEW.goods_number < 0 then
            SIGNAL SQLSTATE 'HY000'  SET mysql_errno = 10000, message_text = '库存数量不可小于0';
      end if;
END;