mysql触发器语法规则(MySQL 触发器的使用及需要注意的地方)
mysql触发器语法规则
MySQL 触发器的使用及需要注意的地方目录
- 关于触发器
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触发器的使用
- 创建触发器
- 查看触发器
- 删除触发器
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使用时的注意点
- NEW / OLD的使用区别
- 无法触发对同表的修改
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小结
- 触发器的两种限制
关于触发器
现实开发中我们经常会遇到这种情况,比如添加、删除和修改信息的时候需要记录日志,我们就要在完成常规的数据库逻辑操作之后再去写入日志表,这样变成了两步操作,更复杂了。
又比如删除一个人员信息的时候,需要将他的购物记录、收货地址、收藏夹等都删了,这个连续的操作容易出错,一致性和完整性不好保证。这时候就要使用触发器,既可以免去一堆的业务逻辑代码,又能更好的保证数据完整性。
触发器(trigger)是一种与表有关的数据库对象:在满足定义条件时触发某种操作,并执行触发器中定义的语句集合。触发器的这种特性可以协助应用在数据库端保证数据的完整性。
它是一种与表事件相关的特殊的存储过程,当对一个表进行操作( insert,delete, update)时就会激活执行。
触发器的使用
创建触发器
创建触发器的语法如下:
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CREATE TRIGGER trigger_name trigger_time trigger_event ON t_name FOR EACH ROW trigger_stmt |
说明:
trigger_name:触发器名称
tirgger_time:触发执行时间点,数据操作前(BEFORE) 或者 数据操作后(AFTER)
trigger_event:触发事件,增(INSERT)、删(DELETE)、改(UPDATE)
t_name:指的是在 t_name 这张表上建立触发器
trigger_stmt:触发器的程序体,可以是一条SQL语句或者是用BEGIN和END包含的多条语句
触发器只能创建在永久表(Permanent)上,不能创建在临时表(Temporary)上
FOR EACH ROW 固定表达式,表示任何一条记录上的操作满足触发事件都会触发该触发器
tirgger_time有两种类型,trigger_event有三种类型,所以他们一共有6种组合:BEFORE INSERT,BEFORE DELETE,BEFORE UPDATE,AFTER INSERT,AFTER DELETE,AFTER UPDATE
示例(先创建一个日志表,用于触发器触发时候填入值):
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/*先创建一个日志表,用于触发器触发时候填入值*/ mysql> DROP TABLE IF EXISTS `TriggerLog`; Query OK, 0 rows affected mysql> CREATE TABLE `TriggerLog` ( `id` INT auto_increment PRIMARY KEY , `trigger_time` VARCHAR (30), `trigger_event` VARCHAR (30), `memo` VARCHAR (200) ); Query OK, 0 rows affected |
Insert类型触发器:
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mysql> /*这边声明SQL脚本的结束符为// */ DELIMITER // DROP TRIGGER IF EXISTS trig_after_insert; CREATE TRIGGER trig_after_insert AFTER INSERT ON students FOR EACH ROW BEGIN insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ( 'after' , 'insert' ,concat( 'new student info,id:' , cast (new.studentid as char ))); END // Query OK, 0 rows affected mysql> /*重置SQL脚本的结束符为; */ DELIMITER ; Query OK, 0 rows affected mysql> /*插入一条数据 */ insert into students(studentname,score,classid) values ( 'trigger1' ,100,0); Query OK, 1 row affected mysql> /*查询日志表看是否有触发写入 */ select * from `TriggerLog`; + ----+--------------+---------------+------------------------+ | id | trigger_time | trigger_event | memo | + ----+--------------+---------------+------------------------+ | 1 | after | insert | new student info,id:21 | + ----+--------------+---------------+------------------------+ 1 row in set |
Update类型触发器:
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mysql> /*这边声明SQL脚本的结束符为// */ DELIMITER // DROP TRIGGER IF EXISTS trig_after_update; CREATE TRIGGER trig_after_update AFTER UPDATE ON students FOR EACH ROW BEGIN insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ( 'after' , 'update' ,concat( 'update student info,id:' , cast (new.studentid as char ))); END // Query OK, 0 rows affected mysql> /*重置SQL脚本的结束符为; */ DELIMITER ; Query OK, 0 rows affected mysql> /*更新数据 */ update students set score=99 where studentname= 'trigger1' ; Query OK, 1 row affected Rows matched: 1 Changed: 1 Warnings: 0 mysql> /*查询日志表看是否更新的时候有触发写入 */ select * from `TriggerLog`; + ----+--------------+---------------+---------------------------+ | id | trigger_time | trigger_event | memo | + ----+--------------+---------------+---------------------------+ | 1 | after | insert | new student info,id:21 | | 2 | after | update | update student info,id:21 | + ----+--------------+---------------+---------------------------+ 2 rows in set |
Delete类型触发器:
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mysql> /*这边声明SQL脚本的结束符为// */ DELIMITER // DROP TRIGGER IF EXISTS trig_after_delete; CREATE TRIGGER trig_after_delete AFTER DELETE ON students FOR EACH ROW BEGIN insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ( 'after' , 'update' ,concat( 'delete student info,id:' , cast (old.studentid as char ))); END // Query OK, 0 rows affected mysql> /*重置SQL脚本的结束符为; */ DELIMITER ; Query OK, 0 rows affected mysql> /* 删除数据 */ delete from students where studentid=21; Query OK, 1 row affected mysql> /*查询日志看删除是否有触发写入*/ select * from `TriggerLog`; + ----+--------------+---------------+---------------------------+ | id | trigger_time | trigger_event | memo | + ----+--------------+---------------+---------------------------+ | 1 | after | insert | new student info,id:21 | | 2 | after | update | update student info,id:21 | | 3 | after | update | delete student info,id:21 | + ----+--------------+---------------+---------------------------+ 3 rows in set |
查看触发器
查看全部触发器
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show triggers; --语法 |
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mysql> show triggers; + -------------------+--------+----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+---------+--------------------------------------------+----------------+----------------------+----------------------+--------------------+ | Trigger | Event | Table | Statement | Timing | Created | sql_mode | Definer | character_set_client | collation_connection | Database Collation | + -------------------+--------+----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+---------+--------------------------------------------+----------------+----------------------+----------------------+--------------------+ | trig_after_insert | INSERT | students | BEGIN insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ( 'after' , 'insert' ,concat( 'new student info,id:' , cast (new.studentid as char ))); END | AFTER | NULL | STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION | root@localhost | utf8 | utf8_general_ci | latin1_swedish_ci | | trig_after_update | UPDATE | students | BEGIN insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ( 'after' , 'update' ,concat( 'update student info,id:' , cast (new.studentid as char ))); END | AFTER | NULL | STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION | root@localhost | utf8 | utf8_general_ci | latin1_swedish_ci | | trig_after_delete | DELETE | students | BEGIN insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ( 'after' , 'update' ,concat( 'delete student info,id:' , cast (old.studentid as char ))); END | AFTER | NULL | STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION | root@localhost | utf8 | utf8_general_ci | latin1_swedish_ci | + -------------------+--------+----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+---------+--------------------------------------------+----------------+----------------------+----------------------+--------------------+ 3 rows in set |
查看触发器的创建语句
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show create trigger trigger_name; --语法 |
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mysql> show create trigger trig_after_insert; + -------------------+--------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+--------------------+ | Trigger | sql_mode | SQL Original Statement | character_set_client | collation_connection | Database Collation | + -------------------+--------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+--------------------+ | trig_after_insert | STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION | CREATE DEFINER=`root`@`localhost` TRIGGER trig_after_insert AFTER INSERT ON students FOR EACH ROW BEGIN insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ( 'after' , 'insert' ,concat( 'new student info,id:' , cast (new.studentid as char ))); END | utf8 | utf8_general_ci | latin1_swedish_ci | + -------------------+--------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+--------------------+ 1 row in set |
删除触发器
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drop trigger trigger_name; --语法 |
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mysql> drop trigger trig_after_insert; Query OK, 0 rows affected mysql> show create trigger trig_after_insert; 1360 - Trigger does not exist |
使用时的注意点
NEW / OLD的使用区别
触发器针对的是数据库中的每一行记录,每行数据在操作前后都会有一个对应的状态,触发器将没有操作之前的状态保存到 old 关键字中,将操作后的状态保存到 new 中
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new.cname --新增行(或修改前的行)的某列数据 old.cname --删除行(或修改后的行)的某列数据 |
需要注意的是,old 和 new 不是所有触发器都有
触发器类型 | new和old的使用 |
---|---|
INSERT型触发器 | 没有 old,只有 new,new 表示将要(插入前)或者已经增加(插入后)的数据 |
UPDATE型触发器 | 既有 old 也有 new,old 表示更新之前的数据,new 表示更新之后的数据 |
DELETE型触发器 | 没有 new,只有 old,old 表示将要(删除前)或者已经被删除(删除后)的数据 |
其实我们上面已经用过 new/old 来创建触发器并获取数据,我们根据这个表格重新来修改下那个更新触发器(trig_after_update),输出修改前和修改后的内容比较:
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mysql> /*这边声明SQL脚本的结束符为// */ DELIMITER // DROP TRIGGER IF EXISTS trig_after_update; CREATE TRIGGER trig_after_update AFTER UPDATE ON students FOR EACH ROW BEGIN insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ( 'after' , 'update' ,concat( 'from:' ,old.studentname, ',' ,old.score, ' ' , 'to:' ,new.studentname, ',' ,new.score)); END // Query OK, 0 rows affected mysql> /*重置SQL脚本的结束符为; */ DELIMITER ; Query OK, 0 rows affected mysql> /*更新成绩和名称 */ update students set score=106,studentname= 'trigger2' where studentid=17; Query OK, 1 row affected Rows matched: 1 Changed: 1 Warnings: 0 mysql> /*根据old和new 对比更新前后的值 */ select * from `TriggerLog`; + ----+--------------+---------------+--------------------------------------+ | id | trigger_time | trigger_event | memo | + ----+--------------+---------------+--------------------------------------+ | 1 | after | insert | new student info,id:21 | | 2 | after | update | update student info,id:21 | | 3 | after | update | delete student info,id:21 | | 4 | after | update | from :test2,101.00 to :trigger2,106.00 | + ----+--------------+---------------+--------------------------------------+ 4 rows in set |
无法触发对同表的修改
MySQL 的触发器中不能对本表进行 insert、update 和 delete 操作,否则会报错