第26课 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
跟存储过程类似, 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, 然后执行...

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为什么需要触发器?

有些sql语句, 或者数据库操作, 在逻辑上是紧密相关, 不可分割的, 需要一起执行

创建触发器

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我们需要一些测试数据

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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insert

insert之前(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;

insert之后(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;

我们的需求, 当订单表里新增订单之后, 商品表里的库存需要相应减少

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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delete

如果订单取消, 商品的库存应该恢复

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我们编写一个触发器, 如果订单取消, 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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查看触发器

查看所有触发器

show triggers;

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查看触发器创建语句

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;

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

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