MySQL 触发器的使用及需要注意的地方

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：

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）：

/*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：

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：

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：

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

show triggers; --语法 

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

show create trigger trigger_name;  --语法

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

drop trigger trigger_name;  --语法 

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

 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

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：

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

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
 update students set score = score+1 where studentid= new.studentid;
END //
Query OK, 0 rows affected

mysql>
/*重置SQL脚本的结束符为; */
DELIMITER ;
Query OK, 0 rows affected

mysql>
/*插入一条数据之后因为触发了对本表的修改，所以报错了 */
insert into students(studentname,score,classid) values('trigger2',101,0);
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。

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