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:
1 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):
1 /*先创建一个日志表,用于触发器触发时候填入值*/ 2 mysql> DROP TABLE IF EXISTS `TriggerLog`; 3 Query OK, 0 rows affected 4 5 mysql> CREATE TABLE `TriggerLog` 6 ( 7 `id` INT auto_increment PRIMARY KEY, 8 `trigger_time` VARCHAR(30), 9 `trigger_event` VARCHAR(30), 10 `memo` VARCHAR(200) 11 ); 12 Query OK, 0 rows affected
Insert类型触发器:
1 mysql> 2 /*这边声明SQL脚本的结束符为// */ 3 DELIMITER // 4 DROP TRIGGER IF EXISTS trig_after_insert; 5 CREATE TRIGGER trig_after_insert AFTER INSERT ON students FOR EACH ROW 6 BEGIN 7 insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ('after','insert',concat('new student info,id:',cast(new.studentid as char))); 8 END // 9 Query OK, 0 rows affected 10 11 mysql> 12 /*重置SQL脚本的结束符为; */ 13 DELIMITER ; 14 Query OK, 0 rows affected 15 16 mysql> 17 /*插入一条数据 */ 18 insert into students(studentname,score,classid) values('trigger1',100,0); 19 Query OK, 1 row affected 20 21 22 mysql> 23 /*查询日志表看是否有触发写入 */ 24 select * from `TriggerLog`; 25 +----+--------------+---------------+------------------------+ 26 | id | trigger_time | trigger_event | memo | 27 +----+--------------+---------------+------------------------+ 28 | 1 | after | insert | new student info,id:21 | 29 +----+--------------+---------------+------------------------+ 30 1 row in set
Update类型触发器:
1 mysql> 2 /*这边声明SQL脚本的结束符为// */ 3 DELIMITER // 4 DROP TRIGGER IF EXISTS trig_after_update; 5 CREATE TRIGGER trig_after_update AFTER UPDATE ON students FOR EACH ROW 6 BEGIN 7 insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ('after','update',concat('update student info,id:',cast(new.studentid as char))); 8 END // 9 Query OK, 0 rows affected 10 11 mysql> 12 /*重置SQL脚本的结束符为; */ 13 DELIMITER ; 14 Query OK, 0 rows affected 15 16 mysql> 17 /*更新数据 */ 18 update students set score=99 where studentname='trigger1'; 19 Query OK, 1 row affected 20 Rows matched: 1 Changed: 1 Warnings: 0 21 22 mysql> 23 /*查询日志表看是否更新的时候有触发写入 */ 24 select * from `TriggerLog`; 25 +----+--------------+---------------+---------------------------+ 26 | id | trigger_time | trigger_event | memo | 27 +----+--------------+---------------+---------------------------+ 28 | 1 | after | insert | new student info,id:21 | 29 | 2 | after | update | update student info,id:21 | 30 +----+--------------+---------------+---------------------------+ 31 2 rows in set
Delete类型触发器:
1 mysql> 2 /*这边声明SQL脚本的结束符为// */ 3 DELIMITER // 4 DROP TRIGGER IF EXISTS trig_after_delete; 5 CREATE TRIGGER trig_after_delete AFTER DELETE ON students FOR EACH ROW 6 BEGIN 7 insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ('after','update',concat('delete student info,id:',cast(old.studentid as char))); 8 END // 9 Query OK, 0 rows affected 10 11 mysql> 12 /*重置SQL脚本的结束符为; */ 13 DELIMITER ; 14 Query OK, 0 rows affected 15 16 mysql> 17 /* 删除数据 */ 18 delete from students where studentid=21; 19 Query OK, 1 row affected 20 21 mysql> 22 /*查询日志看删除是否有触发写入*/ 23 select * from `TriggerLog`; 24 +----+--------------+---------------+---------------------------+ 25 | id | trigger_time | trigger_event | memo | 26 +----+--------------+---------------+---------------------------+ 27 | 1 | after | insert | new student info,id:21 | 28 | 2 | after | update | update student info,id:21 | 29 | 3 | after | update | delete student info,id:21 | 30 +----+--------------+---------------+---------------------------+ 31 3 rows in set
查看触发器
查看全部触发器
1 show triggers; --语法
1 mysql> show triggers; 2 +-------------------+--------+----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+---------+--------------------------------------------+----------------+----------------------+----------------------+--------------------+ 3 | Trigger | Event | Table | Statement | Timing | Created | sql_mode | Definer | character_set_client | collation_connection | Database Collation | 4 +-------------------+--------+----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+---------+--------------------------------------------+----------------+----------------------+----------------------+--------------------+ 5 | trig_after_insert | INSERT | students | BEGIN 6 insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ('after','insert',concat('new student info,id:',cast(new.studentid as char))); 7 END | AFTER | NULL | STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION | root@localhost | utf8 | utf8_general_ci | latin1_swedish_ci | 8 | trig_after_update | UPDATE | students | BEGIN 9 insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ('after','update',concat('update student info,id:',cast(new.studentid as char))); 10 END | AFTER | NULL | STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION | root@localhost | utf8 | utf8_general_ci | latin1_swedish_ci | 11 | trig_after_delete | DELETE | students | BEGIN 12 insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ('after','update',concat('delete student info,id:',cast(old.studentid as char))); 13 END | AFTER | NULL | STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION | root@localhost | utf8 | utf8_general_ci | latin1_swedish_ci | 14 +-------------------+--------+----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+---------+--------------------------------------------+----------------+----------------------+----------------------+--------------------+ 15 3 rows in set
查看触发器的创建语句
1 show create trigger trigger_name; --语法
1 mysql> show create trigger trig_after_insert; 2 +-------------------+--------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+--------------------+ 3 | Trigger | sql_mode | SQL Original Statement | character_set_client | collation_connection | Database Collation | 4 +-------------------+--------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+--------------------+ 5 | trig_after_insert | STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION | CREATE DEFINER=`root`@`localhost` TRIGGER trig_after_insert AFTER INSERT ON students FOR EACH ROW 6 BEGIN 7 insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ('after','insert',concat('new student info,id:',cast(new.studentid as char))); 8 END | utf8 | utf8_general_ci | latin1_swedish_ci | 9 +-------------------+--------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+--------------------+ 10 1 row in set
删除触发器
1 drop trigger trigger_name; --语法
1 mysql> drop trigger trig_after_insert; 2 Query OK, 0 rows affected 3 4 mysql> show create trigger trig_after_insert; 5 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
1 new.cname --新增行(或修改前的行)的某列数据 2 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:
1 mysql> 2 /*这边声明SQL脚本的结束符为// */ 3 DELIMITER // 4 DROP TRIGGER IF EXISTS trig_after_update; 5 CREATE TRIGGER trig_after_update AFTER UPDATE ON students FOR EACH ROW 6 BEGIN 7 insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ('after','update',concat('from:',old.studentname,',',old.score,' ','to:',new.studentname,',',new.score)); 8 END // 9 Query OK, 0 rows affected 10 11 mysql> 12 /*重置SQL脚本的结束符为; */ 13 DELIMITER ; 14 Query OK, 0 rows affected 15 16 mysql> 17 /*更新成绩和名称 */ 18 update students set score=106,studentname='trigger2' where studentid=17; 19 Query OK, 1 row affected 20 Rows matched: 1 Changed: 1 Warnings: 0 21 22 mysql> 23 /*根据old和new 对比更新前后的值 */ 24 select * from `TriggerLog`; 25 +----+--------------+---------------+--------------------------------------+ 26 | id | trigger_time | trigger_event | memo | 27 +----+--------------+---------------+--------------------------------------+ 28 | 1 | after | insert | new student info,id:21 | 29 | 2 | after | update | update student info,id:21 | 30 | 3 | after | update | delete student info,id:21 | 31 | 4 | after | update | from:test2,101.00 to:trigger2,106.00 | 32 +----+--------------+---------------+--------------------------------------+ 33 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
1 mysql> 2 /*这边声明SQL脚本的结束符为// */ 3 DELIMITER // 4 DROP TRIGGER IF EXISTS trig_after_insert; 5 CREATE TRIGGER trig_after_insert AFTER INSERT ON students FOR EACH ROW 6 BEGIN 7 update students set score = score+1 where studentid= new.studentid; 8 END // 9 Query OK, 0 rows affected 10 11 mysql> 12 /*重置SQL脚本的结束符为; */ 13 DELIMITER ; 14 Query OK, 0 rows affected 15 16 mysql> 17 /*插入一条数据之后因为触发了对本表的修改,所以报错了 */ 18 insert into students(studentname,score,classid) values('trigger2',101,0); 19 1442 - Can't update table 'students' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.
小结
1、触发器可以通过数据库中的关联表实现级联更改,即一张表数据的改变会影响其他表的数据,如数据修改,数据统计,数据复制等。
2、可以保证数据安全,并进行安全校验,限制用户具有操作数据库的某种权利。
3、可以对复杂逻辑的实现进行数据完整性检查和约束。
4、使用触发器的要应需而用,如果过分依赖触发器,会影响数据库的结构,增加数据库的执行和维护成本。
5、触发器有BEFORE触发器和AFTER触发器的却别,所以它的执行步骤是先执行 BEFORE触发器,再执行业务脚本,最后执行AFTER触发器。需要注意的是 某一个步骤错误后就不再执行了, 如果是事务表,会被回滚,如果是非事务表,那么就无法回滚了,数据可能有不一致性。
触发器的两种限制
1、触发程序不能调用将数据返回客户端的存储程序,也不能使用采用CALL语句的动态SQL语句,但是允许存储程序通过参数将数据返回触发程序,也就是存储过程或者函数通过OUT或者INOUT类型的参数将数据返回触发器是可以的,但是不能调用直接返回数据的过程。
2、不能再触发器中使用以显示或隐式方式开始或结束事务的语句,如START TRANS-ACTION,COMMIT或ROLLBACK。
,
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:
1 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):
1 /*先创建一个日志表,用于触发器触发时候填入值*/ 2 mysql> DROP TABLE IF EXISTS `TriggerLog`; 3 Query OK, 0 rows affected 4 5 mysql> CREATE TABLE `TriggerLog` 6 ( 7 `id` INT auto_increment PRIMARY KEY, 8 `trigger_time` VARCHAR(30), 9 `trigger_event` VARCHAR(30), 10 `memo` VARCHAR(200) 11 ); 12 Query OK, 0 rows affected
Insert类型触发器:
1 mysql> 2 /*这边声明SQL脚本的结束符为// */ 3 DELIMITER // 4 DROP TRIGGER IF EXISTS trig_after_insert; 5 CREATE TRIGGER trig_after_insert AFTER INSERT ON students FOR EACH ROW 6 BEGIN 7 insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ('after','insert',concat('new student info,id:',cast(new.studentid as char))); 8 END // 9 Query OK, 0 rows affected 10 11 mysql> 12 /*重置SQL脚本的结束符为; */ 13 DELIMITER ; 14 Query OK, 0 rows affected 15 16 mysql> 17 /*插入一条数据 */ 18 insert into students(studentname,score,classid) values('trigger1',100,0); 19 Query OK, 1 row affected 20 21 22 mysql> 23 /*查询日志表看是否有触发写入 */ 24 select * from `TriggerLog`; 25 +----+--------------+---------------+------------------------+ 26 | id | trigger_time | trigger_event | memo | 27 +----+--------------+---------------+------------------------+ 28 | 1 | after | insert | new student info,id:21 | 29 +----+--------------+---------------+------------------------+ 30 1 row in set
Update类型触发器:
1 mysql> 2 /*这边声明SQL脚本的结束符为// */ 3 DELIMITER // 4 DROP TRIGGER IF EXISTS trig_after_update; 5 CREATE TRIGGER trig_after_update AFTER UPDATE ON students FOR EACH ROW 6 BEGIN 7 insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ('after','update',concat('update student info,id:',cast(new.studentid as char))); 8 END // 9 Query OK, 0 rows affected 10 11 mysql> 12 /*重置SQL脚本的结束符为; */ 13 DELIMITER ; 14 Query OK, 0 rows affected 15 16 mysql> 17 /*更新数据 */ 18 update students set score=99 where studentname='trigger1'; 19 Query OK, 1 row affected 20 Rows matched: 1 Changed: 1 Warnings: 0 21 22 mysql> 23 /*查询日志表看是否更新的时候有触发写入 */ 24 select * from `TriggerLog`; 25 +----+--------------+---------------+---------------------------+ 26 | id | trigger_time | trigger_event | memo | 27 +----+--------------+---------------+---------------------------+ 28 | 1 | after | insert | new student info,id:21 | 29 | 2 | after | update | update student info,id:21 | 30 +----+--------------+---------------+---------------------------+ 31 2 rows in set
Delete类型触发器:
1 mysql> 2 /*这边声明SQL脚本的结束符为// */ 3 DELIMITER // 4 DROP TRIGGER IF EXISTS trig_after_delete; 5 CREATE TRIGGER trig_after_delete AFTER DELETE ON students FOR EACH ROW 6 BEGIN 7 insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ('after','update',concat('delete student info,id:',cast(old.studentid as char))); 8 END // 9 Query OK, 0 rows affected 10 11 mysql> 12 /*重置SQL脚本的结束符为; */ 13 DELIMITER ; 14 Query OK, 0 rows affected 15 16 mysql> 17 /* 删除数据 */ 18 delete from students where studentid=21; 19 Query OK, 1 row affected 20 21 mysql> 22 /*查询日志看删除是否有触发写入*/ 23 select * from `TriggerLog`; 24 +----+--------------+---------------+---------------------------+ 25 | id | trigger_time | trigger_event | memo | 26 +----+--------------+---------------+---------------------------+ 27 | 1 | after | insert | new student info,id:21 | 28 | 2 | after | update | update student info,id:21 | 29 | 3 | after | update | delete student info,id:21 | 30 +----+--------------+---------------+---------------------------+ 31 3 rows in set
查看触发器
查看全部触发器
1 show triggers; --语法
1 mysql> show triggers; 2 +-------------------+--------+----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+---------+--------------------------------------------+----------------+----------------------+----------------------+--------------------+ 3 | Trigger | Event | Table | Statement | Timing | Created | sql_mode | Definer | character_set_client | collation_connection | Database Collation | 4 +-------------------+--------+----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+---------+--------------------------------------------+----------------+----------------------+----------------------+--------------------+ 5 | trig_after_insert | INSERT | students | BEGIN 6 insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ('after','insert',concat('new student info,id:',cast(new.studentid as char))); 7 END | AFTER | NULL | STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION | root@localhost | utf8 | utf8_general_ci | latin1_swedish_ci | 8 | trig_after_update | UPDATE | students | BEGIN 9 insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ('after','update',concat('update student info,id:',cast(new.studentid as char))); 10 END | AFTER | NULL | STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION | root@localhost | utf8 | utf8_general_ci | latin1_swedish_ci | 11 | trig_after_delete | DELETE | students | BEGIN 12 insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ('after','update',concat('delete student info,id:',cast(old.studentid as char))); 13 END | AFTER | NULL | STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION | root@localhost | utf8 | utf8_general_ci | latin1_swedish_ci | 14 +-------------------+--------+----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------+---------+--------------------------------------------+----------------+----------------------+----------------------+--------------------+ 15 3 rows in set
查看触发器的创建语句
1 show create trigger trigger_name; --语法
1 mysql> show create trigger trig_after_insert; 2 +-------------------+--------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+--------------------+ 3 | Trigger | sql_mode | SQL Original Statement | character_set_client | collation_connection | Database Collation | 4 +-------------------+--------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+--------------------+ 5 | trig_after_insert | STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION | CREATE DEFINER=`root`@`localhost` TRIGGER trig_after_insert AFTER INSERT ON students FOR EACH ROW 6 BEGIN 7 insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ('after','insert',concat('new student info,id:',cast(new.studentid as char))); 8 END | utf8 | utf8_general_ci | latin1_swedish_ci | 9 +-------------------+--------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+--------------------+ 10 1 row in set
删除触发器
1 drop trigger trigger_name; --语法
1 mysql> drop trigger trig_after_insert; 2 Query OK, 0 rows affected 3 4 mysql> show create trigger trig_after_insert; 5 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
1 new.cname --新增行(或修改前的行)的某列数据 2 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:
1 mysql> 2 /*这边声明SQL脚本的结束符为// */ 3 DELIMITER // 4 DROP TRIGGER IF EXISTS trig_after_update; 5 CREATE TRIGGER trig_after_update AFTER UPDATE ON students FOR EACH ROW 6 BEGIN 7 insert into `TriggerLog`(`trigger_time`,`trigger_event`,`memo`) values ('after','update',concat('from:',old.studentname,',',old.score,' ','to:',new.studentname,',',new.score)); 8 END // 9 Query OK, 0 rows affected 10 11 mysql> 12 /*重置SQL脚本的结束符为; */ 13 DELIMITER ; 14 Query OK, 0 rows affected 15 16 mysql> 17 /*更新成绩和名称 */ 18 update students set score=106,studentname='trigger2' where studentid=17; 19 Query OK, 1 row affected 20 Rows matched: 1 Changed: 1 Warnings: 0 21 22 mysql> 23 /*根据old和new 对比更新前后的值 */ 24 select * from `TriggerLog`; 25 +----+--------------+---------------+--------------------------------------+ 26 | id | trigger_time | trigger_event | memo | 27 +----+--------------+---------------+--------------------------------------+ 28 | 1 | after | insert | new student info,id:21 | 29 | 2 | after | update | update student info,id:21 | 30 | 3 | after | update | delete student info,id:21 | 31 | 4 | after | update | from:test2,101.00 to:trigger2,106.00 | 32 +----+--------------+---------------+--------------------------------------+ 33 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
1 mysql> 2 /*这边声明SQL脚本的结束符为// */ 3 DELIMITER // 4 DROP TRIGGER IF EXISTS trig_after_insert; 5 CREATE TRIGGER trig_after_insert AFTER INSERT ON students FOR EACH ROW 6 BEGIN 7 update students set score = score+1 where studentid= new.studentid; 8 END // 9 Query OK, 0 rows affected 10 11 mysql> 12 /*重置SQL脚本的结束符为; */ 13 DELIMITER ; 14 Query OK, 0 rows affected 15 16 mysql> 17 /*插入一条数据之后因为触发了对本表的修改,所以报错了 */ 18 insert into students(studentname,score,classid) values('trigger2',101,0); 19 1442 - Can't update table 'students' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.
小结
1、触发器可以通过数据库中的关联表实现级联更改,即一张表数据的改变会影响其他表的数据,如数据修改,数据统计,数据复制等。
2、可以保证数据安全,并进行安全校验,限制用户具有操作数据库的某种权利。
3、可以对复杂逻辑的实现进行数据完整性检查和约束。
4、使用触发器的要应需而用,如果过分依赖触发器,会影响数据库的结构,增加数据库的执行和维护成本。
5、触发器有BEFORE触发器和AFTER触发器的却别,所以它的执行步骤是先执行 BEFORE触发器,再执行业务脚本,最后执行AFTER触发器。需要注意的是 某一个步骤错误后就不再执行了, 如果是事务表,会被回滚,如果是非事务表,那么就无法回滚了,数据可能有不一致性。
触发器的两种限制
1、触发程序不能调用将数据返回客户端的存储程序,也不能使用采用CALL语句的动态SQL语句,但是允许存储程序通过参数将数据返回触发程序,也就是存储过程或者函数通过OUT或者INOUT类型的参数将数据返回触发器是可以的,但是不能调用直接返回数据的过程。
2、不能再触发器中使用以显示或隐式方式开始或结束事务的语句,如START TRANS-ACTION,COMMIT或ROLLBACK。