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 ;