什么是觸發(fā)器

就是trigger(扳機(jī)), 一碰就動(dòng), 一觸即發(fā), 就叫觸發(fā)器
跟存儲(chǔ)過程類似, 也是主要用于增, 刪, 改
事先為某張表綁定一段代碼, 當(dāng)表中的某些內(nèi)容發(fā)生改變的時(shí)候(對(duì)表進(jìn)行增刪改), 系統(tǒng)就會(huì)自動(dòng)觸發(fā)代碼執(zhí)行
監(jiān)聽, 只要...就觸發(fā), 然后執(zhí)行...

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為什么需要觸發(fā)器?

有些sql語句, 或者數(shù)據(jù)庫操作, 在邏輯上是緊密相關(guān), 不可分割的, 需要一起執(zhí)行

創(chuàng)建觸發(fā)器

創(chuàng)建觸發(fā)器.png

我們需要一些測(cè)試數(shù)據(jù)

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;

商城數(shù)據(jù)庫.png

insert

insert之前(語法)

drop trigger if exists 插入前觸發(fā);
create trigger 插入前觸發(fā) before insert on teacher for each row
   -- sql語句, 不能是select
begin
  
end;

insert之后(語法)

drop trigger if exists 插入后觸發(fā);
create trigger 插入后觸發(fā) after insert on teacher for each row
begin
    -- sql語句, 不能是select
end;

我們的需求, 當(dāng)訂單表里新增訂單之后, 商品表里的庫存需要相應(yīng)減少

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

如果訂單取消, 商品的庫存應(yīng)該恢復(fù)

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我們編寫一個(gè)觸發(fā)器, 如果訂單取消, 則電視機(jī)的庫存恢復(fù)到原來的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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查看觸發(fā)器

查看所有觸發(fā)器

show triggers;

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查看觸發(fā)器創(chuàng)建語句

show create trigger 觸發(fā)器名稱;

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刪除&修改觸發(fā)器

觸發(fā)器不能修改, 只能先刪除, 再修改

drop trigger 觸發(fā)器名稱;

最后說明

1. 觸發(fā)器中, 只能有增刪改, 不能有查
2. 觸發(fā)器中只能對(duì)非監(jiān)聽的表, 進(jìn)行增刪改, 避免死循環(huán)
3. 觸發(fā)器的監(jiān)聽對(duì)象,并不都有new和old

trigger監(jiān)聽對(duì)象

監(jiān)聽對(duì)象.png