(一)、触发器的语法:

CREATE TRIGGER 触发器名

after / before (触时间)

insert / update / delete (触发条件)

ON

表名

FOR EACH ROW

BEGIN

触发事件

END;

(二)、触发器实验：

第一个触发器：

当用户购买商品时（也就是订单表增加订单数据时），商品表的该商品会自动减去用户购买的数量。

CREATE TRIGGER addOrder BEFORE insert ON ord FOR EACH ROW BEGIN

DECLARE goodsNum int;

select number into goodsNum from goods where gid=new.gid;

IF new.much > goodsNum THEN set new.much = goodsNum;

END IF;

IF new.much < 0 THEN set new.much = 0;

END IF;

update goods set number = number - new.much where gid = new.gid;

END;

第二个触发器：

当用户删除订单时，商品表的该商品会自动增加用户删除的订单数量。

CREATE TRIGGER delOrder before delete ON ord FOR EACH ROW

BEGIN

update goods set number = number + old.much where gid = old.gid;

END;

第三个触发器：

当用户修改订单数量时，商品表的该商品会自动减去或增加数量。

CREATE TRIGGER upOrder before update ON ord FOR EACH ROW

BEGIN

DECLARE goodsCount INT;

SELECT number INTO goodsCount FROM goods WHERE gid = new.gid;

IF new.much > goodsCount THEN set new.much = goodsCount;

END IF;

IF

new.much < 0 THEN set new.much = 0;

END IF;

update goods set number = number+old.much-new.much where gid=new.gid;

END;