MySQL全面瓦解17:触发器相关

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。

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