MySQL 触发器

概念

触发器(Trigger)是一个特殊的存储过程，它的执行不是由程序调用，也不是手工启动，而是由事件触发。

触发器经常用于加强数据的完整性约束和业务规则等。

语法

CREATE TRIGGER 触发器名称 BEFORE|AFTER 触发事件  ON 表名 FOR EACH ROW  BEGIN    触发器程序体  END-- 触发器名称，它和MySQL中其他对象的命名方式-- {BEFORE|AFTER} 触发器触发的时机-- {INSERT|UPDATE|DELETE} 触发器事件-- FOR EACH ROW 表示当所有匹配行的事件发生时才会触发。

创建触发器的例子

mysql> desc student;+-------+------------------+------+-----+---------+----------------+| Field | Type             | Null | Key | Default | Extra          |+-------+------------------+------+-----+---------+----------------+| id    | int(10) unsigned | NO   | PRI | NULL    | auto_increment || name  | varchar(50)      | YES  |     | NULL    |                |+-------+------------------+------+-----+---------+----------------+2 rows in set (0.02 sec)mysql> desc student_total;+-------+---------+------+-----+---------+-------+| Field | Type    | Null | Key | Default | Extra |+-------+---------+------+-----+---------+-------+| total | int(11) | YES  |     | NULL    |       |+-------+---------+------+-----+---------+-------+1 row in set (0.01 sec)mysql> delimiter &&mysql> create trigger student_insert_trigger after insert    -> on student for each row    -> BEGIN    ->   update student_total set total=total+1;    -> END&&mysql> delimiter ;

3. 查看触发器

通过show语句查看

show triggers\G

通过系统表查看

use information_shcema;select * from tirggers\Gselect * from triggers where TRIGGER_NAME = '触发器名称'\G

4. 删除触发器

drop trigger 触发器名称

5. 触发器示例

--安全地删除表drop table if exists tab1;

5.1 增加tab1表记录后自动将记录添加到tab2中

5.1.1表结构

mysql> desc tab1;+---------+-------------+------+-----+---------+-------+| Field   | Type        | Null | Key | Default | Extra |+---------+-------------+------+-----+---------+-------+| tab1_id | varchar(50) | YES  |     | NULL    |       |+---------+-------------+------+-----+---------+-------+1 row in set (0.02 sec)mysql> desc tab2;+---------+-------------+------+-----+---------+-------+| Field   | Type        | Null | Key | Default | Extra |+---------+-------------+------+-----+---------+-------+| tab2_id | varchar(50) | YES  |     | NULL    |       |+---------+-------------+------+-----+---------+-------+1 row in set (0.00 sec)

5.1.2 创建触发器

触发器中new和old的作用

1. 针对update操作，new表示的是更新后的值，old表示的是原来的数据。
2. 针对insert操作，new表示的是插入的值。
3. 针对delete操作，old表示的是删除后的值。

表名触发时机动作

-- 插入触发器mysql> delimiter $$mysql> create trigger tab1_after_insert after insert    -> on tab1 for each row    -> BEGIN    ->   insert into tab2(tab2_id) values(new.tab1_id);    -> END    -> $$Query OK, 0 rows affected (0.02 sec)mysql> delimiter ;

-- 删除触发器mysql> delimiter $$mysql> create trigger tab1_delete_after after delete    -> on tab1 for each rows    -> BEGIN    ->   delete from tab2 where tab2_id = old.tab1_id;    -> END    -> $$Query OK, 0 rows affected (0.01 sec)mysql> delimiter ;

5.2 记录学生表的更新

5.2.1 表结构

-- 学生表mysql> desc student1;+--------------+-----------------------+------+-----+---------+----------------+| Field        | Type                  | Null | Key | Default | Extra          |+--------------+-----------------------+------+-----+---------+----------------+| student_id   | int(11)               | NO   | PRI | NULL    | auto_increment || student_name | varchar(30)           | NO   |     | NULL    |                || student_sex  | enum('male','female') | YES  |     | male    |                |+--------------+-----------------------+------+-----+---------+----------------+3 rows in set (0.00 sec)-- 更新记录表mysql> desc update_student1;+---------------+---------+------+-----+---------+----------------+| Field         | Type    | Null | Key | Default | Extra          |+---------------+---------+------+-----+---------+----------------+| update_record | int(11) | NO   | PRI | NULL    | auto_increment || student_id    | int(11) | NO   |     | NULL    |                || update_date   | date    | YES  |     | NULL    |                |+---------------+---------+------+-----+---------+----------------+3 rows in set (0.00 sec)

5.2.2 创建触发器

-- 更新学生表的同时也更新记录表delimiter $$mysql> create trigger student1_update_after after update    -> on student1 for each row    -> BEGIN    ->  if new.student_id != old.student_id then    ->   update update_student1 set student_id = new.student_id, update_date = now()    ->   where student_id = old.student_id;    ->  end if;    -> END    -> $$delimiter ;

-- 删除学生表的时候也删除记录表mysql> delimiter $$mysql> create trigger student1_delete_before before delete    -> on student1 for each row    -> BEGIN    ->  delete from update_student1    ->  where student_id = old.student_id;    -> END    -> $$delimiter ;