mysql_触发器

定义:是用户定义在关系表上的一类由时间驱动的数据对象;也是一种保障数据完整性的方法

分类:

1. insert触发器

   insert触发器代码里,可以引用一个名为NEW的虚拟表(它在内存或者缓存中),用来访问被插入的行,后面的例子我们来结合着理解下.

   在BEFORE INSERT 触发器中，NEW 中的值可以被更新。

   • AFTER,BEFORE

   mysql> create trigger qiao_test.customers_insert_trigger #在qiao_test库创建一个触发器名为customers_insert_trigger
       -> AFTER INSERT #在customers中每执行一次insert操作后
       -> on qiao_test.customers FOR EACH ROW 
       -> set @str='one customer added!'; #定义变量的值.
    
   mysql> insert into qiao_test.customers
       -> values(0,'qiaoying','m','wanshoulu','zhongguo');
   Query OK, 1 row affected (0.00 sec)
   
   mysql> select @str;
   +---------------------+
   | @str                |
   +---------------------+
   | one customer added! |
   +---------------------+
   
   mysql> drop trigger qiao_test.customers_insert_trigger;
   Query OK, 0 rows affected (0.00 sec)
   #删除该触发器
   

   意思就是每次向该表插入一行数据时,执行完insert操作之后,将这个变量的值进行定义.

   而for each row语法:个人理解为该表中的不管哪一行做(insert,delete,update)操作,都会出发该触发器,该语法不是必须的.

   mysql> create trigger qiao_test.customers_insert_trigger
       -> AFTER INSERT
       -> on qiao_test.customers
       -> FOR EACH ROW
       -> set @str=new.cust_id;
   #每执行insert操作之后,将str变量的值设置为new表里的cust_id的值.new表就是一个虚表,它会将我们执行insert的行读取;
   
   mysql> insert into qiao_test.customers
       -> values(0,'yi','m','cuiwei','zhongguo');
   Query OK, 1 row affected (0.01 sec)
   #我这里该字段设置了自增张.
   
   mysql> select * from qiao_test.customers where cust_name='yi';
   +---------+-----------+----------+----------+-----------+
   | cust_id | cust_name | cust_sex | cust_add | cust_city |
   +---------+-----------+----------+----------+-----------+
   |     105 | yi        | m        | cuiwei   | zhongguo  |
   +---------+-----------+----------+----------+-----------+
   1 row in set (0.00 sec)
   
   mysql> select @str;
   +------+
   | @str |
   +------+
   |  105 |
   +------+
   1 row in set (0.00 sec)
   

2. delete触发器

   在DELETE 触发器代码内，可引用一个名为OLD（不区分大小写）的虚拟表，来访问被删除
   的行。
   OLD 中的值全部是只读的，不能被更新。

   mysql> create trigger qiao_test.customers_delete_trigger
       -> AFTER DELETE
       -> on qiao_test.customers
       -> for each row
       -> set @str=old.cust_name;
   Query OK, 0 rows affected (0.02 sec)
   #创建delete触发器,每执行delete操作后,会将str变量的值设置为已经执行delete操作的cust_name字段的值.
   
   mysql> delete from qiao_test.customers
       -> where cust_name='yi';
   Query OK, 1 row affected (0.00 sec)
   
   mysql> select @str;
   +------+
   | @str |
   +------+
   | yi   |
   +------+
   1 row in set (0.00 sec)
   

3. update触发器

   在UPDATE 触发器代码内，可引用一个名为OLD（不区分大小写）的虚拟表，来访问UPDATE
   语句执行前的值，也可以引用一个名为NEW（不区分大小写）的虚拟表来访问更新后的值。

   update触发器相当于是执行了一个delete+insert.

   mysql> create trigger qiao_test.customers_update_trigger
       -> BEFORE UPDATE
       -> on qiao_test.customers FOR EACH ROW
       -> set new.cust_add=old.cust_city;
   Query OK, 0 rows affected (0.01 sec)
   #创建before update触发器,一般使用before的场景:设置的触发表被更新时使用before;如这个例子中我们的处发表是customers,即将要更新的也是customers表中的cust_id这一个字段.
   
   mysql> update qiao_test.customers
       -> set cust_add='pingguoyuan'
       -> where cust_name='qi';
   Query OK, 1 row affected (0.00 sec)
   Rows matched: 1  Changed: 1  Warnings: 0
   #这里使用update语句更改qi的cust_name字段的值为'pingguoyuan'
   
   mysql> select * from qiao_test.customers
       -> where cust_name='qi';
   +---------+-----------+----------+----------+-----------+
   | cust_id | cust_name | cust_sex | cust_add | cust_city |
   +---------+-----------+----------+----------+-----------+
   |     103 | qi        | f        | hangzhou | hangzhou  |
   +---------+-----------+----------+----------+-----------+
   1 row in set (0.00 sec)
   #整体下来我们可以把它理解:将old.cust_city的值赋给了new.cust_add