MySQL 触发器学习-markdown->html 格式测试
<!doctype html>触发器
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-->
触发器(Trigger)
目标
- 定义
- 应用场景
- 创建语法
- 创建简单触发器
定义
理解: 触发器是一类特定的事务, 可以监视某种数据操作(insert, update, delete) 并触发相关操作(insert, update, delete).
即监察增删改, 触发增删改, sql_01 ...... 触发-->sql_n
应用场景
- 当向一张表增删记录时, 需要在相关表中进行同步操作.(订单, 销量增加, 库存减少..)
- 当表上的某列数据值与其他表中的数据有联系时.(信用卡消费,额度校验)
- 跟踪某张表.(花名册更新)
创建语法4要素
- 监视地点 table
- 监视事件 insert, update, delete
- 触发时间 afer, before
- 触发事件 insert, update, delete
触发器-案例
需求:
商品表: goods
订单表: ord
当下1个订单时, 对应的商品库存减少
分析:
监视谁: ord
监视动作: insert
触发时间: after
触发事件: update
(当ord插入后, 触发update)
xxxxxxxxxx
-- triger 语法结构
create trigger t1
after
inset
on ord
for each row
begin
update goods .....
end;
x
-- 创建一个数据库 mysql_advance
create database sql_advance charset=utf8;
use sql_advance;
-- 创建表
create table goods(
gid int,
name varchar(20),
num smallint
);
create table ord(
oid int,
gid int,
much smallint
);
-- 插入数据-goods表
insert into goods values
(1, 'cat', 34),
(2, 'dog', 65),
(3, 'pig', 21);
-- 插入测试
select * from goods;
select * from ord;
-- 下订单 ord (insert)
-- 常规操作
insert into ord values (123, 1, 2);
update goods set num=num-2 where gid=1;
-- 查看goods表2类商品数量是否变化 34=>32
select * from goods;
用触发器实现
drop trigger if exists t1;
delimiter //
create trigger t1
after
insert
on ord
for each row
begin
update goods set num=num-2 where gid=1;
end //
delimiter ;
-- test
show triggers \G;
\G 能显示详细, 但navicat不支持, 终端可以的 (直接复制终端的命令行, 代码块能自动格式美化)
xxxxxxxxxx
mysql> show triggers \G;
*************************** 1. row ***************************
Trigger: t1
Event: INSERT
Table: ord
Statement: begin
update goods set num=num-2 where gid=1;
end
Timing: AFTER
Created: 2019-10-01 23:14:55.08
sql_mode:
Definer: root@localhost
character_set_client: utf8mb4
collation_connection: utf8mb4_general_ci
Database Collation: utf8_general_ci
1 row in set (0.00 sec)
ERROR:
No query specified
x
-- test
select * from goods;
select * from ord;
-- 下订单, 目前有32只, 现再买2只
insert into ord values (124, 1, 2);
-- 查看goods表是否也跟着变更了
select * from goods;
输出如下
xxxxxxxxxx
mysql> select * from goods;
+-----+------+-----+
| gid | name | num |
+-----+------+-----+
| 1 | cat | 32 |
| 2 | dog | 65 |
| 3 | pig | 21 |
+-----+------+-----+
3 rows in set (0.08 sec)
mysql> select * from ord;
+-----+-----+------+
| oid | gid | much |
+-----+-----+------+
| 123 | 2 | 2 |
| 123 | 1 | 2 |
+-----+-----+------+
2 rows in set (0.07 sec)
mysql> insert into ord values (125, 1, 2);
Query OK, 1 row affected (0.09 sec)
mysql> select * from goods;
+-----+------+-----+
| gid | name | num |
+-----+------+-----+
| 1 | cat | 30 |
| 2 | dog | 65 |
| 3 | pig | 21 |
+-----+------+-----+
3 rows in set (0.07 sec)
局限性: update goods set num=num-2 where gid=1; 应该设置为变量才灵活. 即insert(被监视的语句), 产生的数据能否在触发器中引用到?
触发器引用行变量
下订单: 关键词: NEW
insert 操作时, 看作是NEW一个新行, new关键字, 即取到该行(类似对象)
xxxxxxxxxx
drop trigger if exists t2;
delimiter //
create trigger t2
after
insert
on ord
for each row
begin
update goods set num=num-NEW.much where gid=NEW.gid;
end //
delimiter ;
-- 查看已有 triggers: show triggers;
-- 删除已有 triggers: drop trigger [if exists] triggerName
-- test
select * from goods;
select * from ord;
-- 分别去购买1,2,3号商品, 对应的goods表也会发生变化
insert into ord values (128, 1, 3);
insert into ord values (130, 2, 5);
insert into ord values (131, 3, 1)
-- 查看数量是否改变
select * from goods;
效果:
mysql> select * from goods;
select * from ord;
+-----+------+-----+
| gid | name | num |
+-----+------+-----+
| 1 | cat | 26 |
| 2 | dog | 65 |
| 3 | pig | 21 |
+-----+------+-----+
3 rows in set (0.06 sec)
+-----+-----+------+
| oid | gid | much |
+-----+-----+------+
| 123 | 2 | 2 |
| 123 | 1 | 2 |
| 125 | 1 | 2 |
| 126 | 2 | 5 |
| 127 | 1 | 10 |
+-----+-----+------+
5 rows in set (0.08 sec)
mysql> insert into ord values (128, 1, 3);
Query OK, 1 row affected (0.07 sec)
mysql> select * from goods;
+-----+------+-----+
| gid | name | num |
+-----+------+-----+
| 1 | cat | 23 |
| 2 | dog | 65 |
| 3 | pig | 21 |
+-----+------+-----+
3 rows in set (0.07 sec)
mysql> insert into ord values (130, 2, 5);
Query OK, 1 row affected (0.05 sec)
mysql> select * from goods;
+-----+------+-----+
| gid | name | num |
+-----+------+-----+
| 1 | cat | 23 |
| 2 | dog | 60 |
| 3 | pig | 21 |
+-----+------+-----+
3 rows in set (0.08 sec)
删订单: 关键词: OLD , 引用delete的行
曾经的一行old
x
drop trigger if exists t3;
delimiter //
create trigger t3
after
delete
on ord
for each row
begin
-- 删掉一条订单后, 对应的goods表应该对应增加该订单的数量
update goods set num=num+OLD.much where gid=OLD.gid;
end //
delimiter ;
效果如下
mysql> drop trigger if exists t3;
delimiter //
create trigger t3
after
delete
on ord
for each row
begin
-- 删掉一条订单后, 对应的goods表应该对应增加该订单的数量
update goods set num=num+OLD.much where gid=OLD.gid;
end //
delimiter ;
Query OK, 0 rows affected (0.00 sec)
Query OK, 0 rows affected (0.11 sec)
mysql> select * from ord;
+-----+-----+------+
| oid | gid | much |
+-----+-----+------+
| 123 | 2 | 2 |
| 123 | 1 | 2 |
| 125 | 1 | 2 |
| 126 | 2 | 5 |
| 127 | 1 | 10 |
| 128 | 1 | 3 |
| 130 | 2 | 5 |
| 131 | 3 | 1 |
+-----+-----+------+
8 rows in set (0.09 sec)
mysql> select * from goods where gid=2;
+-----+------+-----+
| gid | name | num |
+-----+------+-----+
| 2 | dog | 60 |
+-----+------+-----+
1 row in set (0.07 sec)
mysql> delete from ord where oid=123;
Query OK, 2 rows affected (0.10 sec)
mysql> select * from goods;
+-----+------+-----+
| gid | name | num |
+-----+------+-----+
| 1 | cat | 25 |
| 2 | dog | 62 |
| 3 | pig | 20 |
+-----+------+-----+
3 rows in set (0.09 sec)
xxxxxxxxxx
drop trigger if exists t3;
delimiter //
create trigger t3
after
delete
on ord
for each row
begin
update goods set num=num+old.much where gid=old.gid
end //
delimiter ;
改订单 (数量) update : 结合old和new, 改之前是old, 改之后是new.
xxxxxxxxxx
delimiter //
create trigger t4
before
update
on ord
for each row
begin
update goods set num = num + old.much - new.much where gid = old.gid;
end //
delimiter ;
-- test
select * from goods;
select * from ord;
update ord set much=2 where gid=125;
x
-- 爆仓测试
delete from ord;
select * from goods;
-- 还剩21只pig, 现在买30只
insert into ord values (1, 3, 30);
select * from goods;
new & old
需求: 将30的数量改为10
原理: 先删掉30, 再加回10, 即先old, 再new.
x
mysql> select * from ord;
+-----+-----+------+
| oid | gid | much |
+-----+-----+------+
| 1 | 3 | 30 |
+-----+-----+------+
1 row in set (0.06 sec)
-- 在update之前(before)
drop trigger if exists t4;
delimiter //
create trigger t4
before
update on ord
for each row
begin
update goods set num=num+old.much - new.much where gid=old.gid;
end //
delimiter ;
xxxxxxxxxx
-- 爆仓演示
mysql> select * from goods;
+-----+------+-----+
| gid | name | num |
+-----+------+-----+
| 1 | cat | 40 |
| 2 | dog | 72 |
| 3 | pig | 21 |
+-----+------+-----+
3 rows in set (0.05 sec)
mysql> insert into ord values (1, 3, 30);
Query OK, 1 row affected (0.05 sec)
mysql> select * from goods;
+-----+------+-----+
| gid | name | num |
+-----+------+-----+
| 1 | cat | 40 |
| 2 | dog | 72 |
| 3 | pig | -9 |
+-----+------+-----+
3 rows in set (0.07 sec)
过程:
xxxxxxxxxx
mysql> drop trigger if exists t4;
delimiter //
create trigger t4
before
update on ord
for each row
begin
update goods set num=num+old.much - new.much where gid=old.gid;
end //
delimiter ;
Query OK, 0 rows affected (0.00 sec)
Query OK, 0 rows affected (0.12 sec)
mysql> select * from goods;
+-----+------+-----+
| gid | name | num |
+-----+------+-----+
| 1 | cat | 40 |
| 2 | dog | 72 |
| 3 | pig | -9 |
+-----+------+-----+
3 rows in set (0.05 sec)
mysql> select * from ord;
+-----+-----+------+
| oid | gid | much |
+-----+-----+------+
| 1 | 3 | 30 |
+-----+-----+------+
1 row in set (0.06 sec)
-- 即 -9 + 30 -10 = 11
mysql> update ord set much=10 where oid=1;
Query OK, 1 row affected (0.05 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from goods;
+-----+------+-----+
| gid | name | num |
+-----+------+-----+
| 1 | cat | 40 |
| 2 | dog | 72 |
| 3 | pig | 11 |
+-----+------+-----+
3 rows in set (0.07 sec)
Q1: before 与 afer 的而区别在哪?
Q2: 如何预防"爆仓"?
Q3: 在购买量 much > 库存量 num时, 把much自动改为num?
x
-- 在t2的基础上, 完成 much 与 num 的判断
drop trigger t5 if exists;
delimiter //
create trigger t5
after
inset
on ord
-- 声明变量用来存储查询到的剩余库存num值
declare rNum int;
for each row
begin
-- 查询到剩余库存
select num INTO rNum from goods where gid=NEW.gid;
-- if much > num 就爆仓了呀
if NEW.much > rNum
update goods set num=num-NEW.much where gid=NEW.gid
end //
delimiter ;
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