[MySQL光速入门]026 触发器 trigger!!!

什么是触发器

就是trigger(扳机), 一碰就动, 一触即发, 就叫触发器

跟存储过程相似, 也是主要用于增, 删, 改

事先为某张表绑定一段代码, 当表中的某些内容发生改变的时候(对表进行增删改), 系统就会自动触发代码执行

监听, 只要...就触发, 而后执行...

为何须要触发器?

有些sql语句, 或者数据库操做, 在逻辑上是紧密相关, 不可分割的, 须要一块儿执行

建立触发器

咱们须要一些测试数据

drop database if exists mail;

create database mail character set utf8;

use mail;

create table goods(
    goods_id int primary key,
    goods_name varchar(20) not null,
    goods_inventory int not null
);

create table orders(
    orders_id int primary key,
    orders_goods_id int not null,
    orders_buy_number int not null,
    foreign key(orders_goods_id) references goods(goods_id)
);



insert into goods set goods_id = 1, goods_name = 'television', goods_inventory = 20;
insert into goods set goods_id = 2, goods_name = 'bicyle', goods_inventory = 99;

insert into orders set orders_id = 1, orders_goods_id = 1, orders_buy_number = 2;
insert into orders set orders_id = 2, orders_goods_id = 2, orders_buy_number = 10;
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中文版

drop database if exists 商城数据库;

create database 商城数据库 character set utf8;

use 商城数据库;

create table 商品表(
    商品ID int primary key,
    商品名称 varchar(20) not null,
    商品库存 int not null
);

create table 订单表(
    订单ID int primary key,
    订单里的商品ID int not null,
    订单里的商品购买数量 int not null,
    foreign key(订单里的商品ID) references 商品表(商品ID)
);



insert into 商品表 set 商品ID = 1, 商品名称 = '电视机', 商品库存 = 20;
insert into 商品表 set 商品ID = 2, 商品名称 = '自行车', 商品库存 = 99;

insert into 订单表 set 订单ID = 1, 订单里的商品ID = 1, 订单里的商品购买数量 = 2;
insert into 订单表 set 订单ID = 2, 订单里的商品ID = 2, 订单里的商品购买数量 = 10;
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insert

insert以前(语法)

drop trigger if exists 插入前触发;
create trigger 插入前触发 before insert on teacher for each row
   -- sql语句, 不能是select
begin
  
end;
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insert以后(语法)

drop trigger if exists 插入后触发;
create trigger 插入后触发 after insert on teacher for each row
begin
    -- sql语句, 不能是select
end;
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咱们的需求, 当订单表里新增订单以后, 商品表里的库存须要相应减小

drop trigger if exists change_inventory;

create trigger change_inventory after insert on orders for each ROW begin 
    update goods 
    set 
        goods_inventory = goods_inventory - new.orders_buy_number 
    where 
        goods_id = new.orders_goods_id;
end;

insert into orders 
set 
    orders_id = 3, 
    orders_goods_id = 1, 
    orders_buy_number = 3;
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中文版

drop trigger if exists 更改库存;

create trigger 更改库存 after insert on 订单表 for each ROW begin 
    update 商品表 
    set 
        商品库存 = 商品库存 - new.订单表里的商品购买数量 
    where 
        商品ID = new.订单里的商品ID;
end;

insert into 订单表 
set 
    订单ID = 3, 
    订单里的商品ID = 1, 
    订单表里的商品购买数量 = 3;
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delete

若是订单取消, 商品的库存应该恢复

咱们编写一个触发器, 若是订单取消, 则电视机的库存恢复到原来的20

drop trigger if exists recover_inventory;

create trigger recover_inventory after delete on orders for each row begin 
    update goods 
    set goods_inventory = goods_inventory + old.orders_buy_number 
    where goods_id = old.orders_goods_id;
end;

delete from orders where orders_id = 3;
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中文版

drop trigger if exists 恢复库存;

create trigger 恢复库存 after delete on 订单表 for each row begin 
    update 商品表 
    set 商品库存 = 商品库存 + old.订单表里的商品购买数量 
    where 商品ID = old.订单里的商品ID;
end;

delete from 订单表 where 订单ID = 3;
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查看触发器

查看全部触发器

show triggers;
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查看触发器建立语句

show create trigger 触发器名称;
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删除&修改触发器

触发器不能修改, 只能先删除, 再修改

drop trigger 触发器名称;
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最后说明

1. 触发器中, 只能有增删改, 不能有查
2. 触发器中只能对非监听的表, 进行增删改, 避免死循环
3. 触发器的监听对象,并不都有new和old

trigger监听对象

快速跳转

• [MySQL光速入门]025 函数做业答案
• [MySQL光速入门]027 索引