Mysql中的触发器简单介绍及使用案例

时间:2022-08-28 16:19:42

什么是触发器 

触发器是数据库的一个程序,他是用来监听着数据表的某个行为,一旦数据表的这个行为发生了,马上执行相应的sql语句 

触发器的语法结构: 

create trigger 触发器的名称触发器事件 on 监听的表名 for each row 行为发生后执行的sql语句 

触发器事件组成:;两部分组成: 

触发器事件发生的时间-----是在监听的表的行为 after before 常用的是after 

触发器执行的内容:增删改 

创建order 表的时候,需要注意,因为order在mysql中是一个关键字排序,为了避免错误的发生,我们可以添加反引号,表明这不是一个关键字 

案例研究: 

一旦生成订单,对应的库存表要减去相应的数据 

(1)建两张表 :一个商品goods表 一个订单order表 
mysql> create table goods(goods_id int primary key auto_increment,goods_name var 
char(64),shop_price decimal(10,2),goods_number int)engine=mysiam default charset 
=utf8; 

mysql> create table `order`(goods_id int primary key auto_increment,goods_name v 
archar(64),buy_number int)engine=mysiam default charset=utf8; 

mysql> insert into goods values(null,'nokiaN85',2000,35),(null,'iphone4S',4500,3 
0),(null,'Lnmia',5000,40),(null,'samsung',4200,20); 

mysql> select*from goods; 
+----------+------------+------------+--------------+ 
| goods_id | goods_name | shop_price | goods_number | 
+----------+------------+------------+--------------+ 
| 1 | nokiaN85 | 2000.00 | 35 | 
| 2 | iphone4S | 4500.00 | 30 | 
| 3 | Lnmia | 5000.00 | 40 | 
| 4 | samsung | 4200.00 | 20 | 
+----------+------------+------------+--------------+ 

(2)创建触发器 
mysql> create trigger alter_goods_number after insert on `order` for each row up 
date goods set goods_number=goods_number-5 where goods_id=1; 

mysql> insert into `order` values(1,'nokiaN85',5); 

mysql> select*from goods; 
+----------+------------+------------+--------------+ 
| goods_id | goods_name | shop_price | goods_number | 
+----------+------------+------------+--------------+ 
| 1 | nokiaN85 | 2000.00 | 30 | 
| 2 | iphone4S | 4500.00 | 30 | 
| 3 | Lnmia | 5000.00 | 40 | 
| 4 | samsung | 4200.00 | 20 | 
+----------+------------+------------+--------------+ 


new 的使用 mysql> create trigger alter_goods_number after insert on `order` for each row up 
date goods set goods_number=goods_number-new.buy_number where goods_id=new.goods 
_id; 

mysql> insert into `order` values(4,'samsung',5); 

old 的使用 

mysql> create trigger back_goods_number after delete on `order` for each row upd 
ate goods set goods_number=goods_number+old.buy_number where goods_id=old.goods_ 
id; 

mysql> delete from `order` where goods_id=1; 

更新 (update将之前下的订单撤销,再重新下订单) 

mysql> create trigger update_goods_number after update on `order` for each row u 
pdate goods set goods_number=goods_number+old.buy_number-new.buy_number where go 
ods_id=new.goods_id; 

mysql> update `order` set buy_number = 10 ;