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使用MySQL 触发器时需要注意哪些事项

发表于:2022-12-07 作者:安全数据网编辑
编辑最后更新 2022年12月07日,本篇文章给大家分享的是有关使用MySQL 触发器时需要注意哪些事项,小编觉得挺实用的,因此分享给大家学习,希望大家阅读完这篇文章后可以有所收获,话不多说,跟着小编一起来看看吧。触发器的使用创建触发器创

本篇文章给大家分享的是有关使用MySQL 触发器时需要注意哪些事项,小编觉得挺实用的,因此分享给大家学习,希望大家阅读完这篇文章后可以有所收获,话不多说,跟着小编一起来看看吧。

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 affectedmysql> 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 ROWBEGIN 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 affectedmysql>/*重置SQL脚本的结束符为; */DELIMITER ;Query OK, 0 rows affectedmysql>/*插入一条数据 */insert into students(studentname,score,classid) values('trigger1',100,0);Query OK, 1 row affectedmysql>/*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 ROWBEGIN 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 affectedmysql>/*重置SQL脚本的结束符为; */DELIMITER ;Query OK, 0 rows affectedmysql>/*更新数据 */update students set score=99 where studentname='trigger1';Query OK, 1 row affectedRows matched: 1 Changed: 1 Warnings: 0mysql>/*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 ROWBEGIN 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 affectedmysql>/*重置SQL脚本的结束符为; */DELIMITER ;Query OK, 0 rows affectedmysql>/* 删除数据 */delete from students where studentid=21;Query OK, 1 row affectedmysql>/*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 ROWBEGIN 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

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:

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 ROWBEGIN 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 affectedmysql>/*重置SQL脚本的结束符为; */DELIMITER ;Query OK, 0 rows affectedmysql>/*更新成绩和名称 */update students set score=106,studentname='trigger2' where studentid=17;Query OK, 1 row affectedRows matched: 1 Changed: 1 Warnings: 0mysql>/*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 ROWBEGIN update students set score = score+1 where studentid= new.studentid;END //Query OK, 0 rows affectedmysql>/*重置SQL脚本的结束符为; */DELIMITER ;Query OK, 0 rows affectedmysql>/*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。

以上就是使用MySQL 触发器时需要注意哪些事项,小编相信有部分知识点可能是我们日常工作会见到或用到的。希望你能通过这篇文章学到更多知识。更多详情敬请关注行业资讯频道。

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