mysql触发器的一个小例子

触发器的一个小例子

触发器是一个事务.
当某时间点;某个表执行了某个操作,将引起的一系列事件.这一整套事务.便是一个触发器.

2.语法:

create trigger trigger_name
    trigger_time
    trigger_event on tbl_name
    for each row
    trigger_stmt

trigger_name：标识触发器名称，用户自行指定； trigger_time：标识触发时机，取值为 before 或 after； trigger_event：标识触发事件，取值为 insert、update 或 delete； tbl_name：标识建立触发器的表名，即在哪张表上建立触发器； trigger_stmt：触发器程序体，可以是一句sql语句，或者用 begin 和 end 包含的多条语句。

模拟事件(当订单发生变化时;商品也要随之发生变化)

首先创建订单表

create table goods_order(order_id int ,goods_id int,purchase_number int);

创建商品表

create table goods(goods_id int,goods_name varchar(20),goods_number int);

给商品表放一些数据

insert into goods values (1,'cat',23);
insert into goods values (2,'dog',23);
insert into goods values (3,'pig',23);
insert into goods values (4,'horse',23);
insert into goods values (5,'cow',23);
insert into goods values (6,'bird',23);
insert into goods values (7,'fish',23);

有人订了6头猪

insert into goods_order values (1,3,6);

引起的事务:商品猪的数量减少6头.

update goods t set t.goods_number=t.goods_number-6 where t.goods_id=3;

将以上事务替换为trigger

需要优化的事项:将3和6改换成变量,通过插入订单的记录中的数据来赋值 定义trigger时,出现的两个语句结束符; 如何解决这个冲突问题? 自定义一个结束符$,解决语句结束符冲突问题

delimiter $;   
create trigger make_an_order
after
insert 
on goods_order
for each row
begin 
update goods set goods_number=goods_number-6 where goods_id=3;
end$

--查看trigger
show triggers$
show triggers \g

如何使用触发器?
当insert + goods_order就会自动触发.
好,试水试水~

insert into goods_order values (1,3,6);

结果:
        mysql> insert into goods_order values (1,3,6)$
        query ok, 1 row affected (0.00 sec)

        mysql> select * from goods;
            -> \c
        mysql> select * from goods$
        +----------+------------+--------------+
        | goods_id | goods_name | goods_number |
        +----------+------------+--------------+
        |        1 | cat        |           23 |
        |        2 | dog        |           23 |
        |        3 | pig        |           11 |
        |        4 | horse      |           23 |
        |        5 | cow        |           23 |
        |        6 | bird       |           23 |
        |        7 | fish       |           23 |
        +----------+------------+--------------+
        nice

接下来就要处理调变量的问题

mysql把insert的记录定义为new mysql把delete的记录定义为old 通过new,old记录来获取目标字段则可以优化触发器make_an_order

create trigger make_an_nice_order
after
insert 
on goods_order
for each row
begin 
update goods set goods_number=goods_number-new.purchase_number where goods_id=new.goods_id;
end$

执行上面的语句会发生错误,因为两个触发器在同一时间(after)对同一个人(table goods_order)执行同一件事(insert),那么当执行insert事件时,会触发哪一个触发器呢,第一个?第二个?还是两个都触发?这里两个触发器之间明显存在冲突.因此,需要先把第一个触发器删掉.

drop trigger make_an_order$

接着,继续试水~

insert into goods_order values (1,2,4)$

结果:
        mysql> insert into goods_order values (1,2,4)$
        query ok, 1 row affected (0.01 sec)

        mysql> select * from goods$
        +----------+------------+--------------+
        | goods_id | goods_name | goods_number |
        +----------+------------+--------------+
        |        1 | cat        |           23 |
        |        2 | dog        |           19 |
        |        3 | pig        |           11 |
        |        4 | horse      |           23 |
        |        5 | cow        |           23 |
        |        6 | bird       |           23 |
        |        7 | fish       |           23 |
        +----------+------------+--------------+
        7 rows in set (0.00 sec)

        mysql> select * from goods_order$
        +----------+----------+-----------------+
        | order_id | goods_id | purchase_number |
        +----------+----------+-----------------+
        |        1 |        3 |               6 |
        |        1 |        3 |               6 |
        |        1 |        2 |               4 |
        +----------+----------+-----------------+
        3 rows in set (0.00 sec)
        恩~,完美

关于update

多讲无益,来一发试试水验验货
假设有那么一个功能需求:你的购物车中订单可以调节商品的数量
需要满足的条件:必须是同一种商品
那么原来的订单记录就是old
后来的订单记录就是new
他们之间的差值就是(new.purchase_number-old.purchase_number)
以实际的数字举个栗子
原先下的订单数量为4
后来更改数量为6
差值=6-4

create trigger update_orders
after
update 
on goods_order
for each row
begin 
update goods set goods_number=goods_number-(new.purchase_number-old.purchase_number) where goods_id=new.goods_id;
end$

update goods_order t set t.purchase_number=3 where t.goods_id=2$

结果:
        mysql> select * from goods$
        +----------+------------+--------------+
        | goods_id | goods_name | goods_number |
        +----------+------------+--------------+
        |        1 | cat        |           23 |
        |        2 | dog        |           19 |
        |        3 | pig        |           11 |
        |        4 | horse      |           23 |
        |        5 | cow        |           23 |
        |        6 | bird       |           23 |
        |        7 | fish       |           23 |
        +----------+------------+--------------+
        7 rows in set (0.00 sec)

        mysql> update goods_order t set t.purchase_number=3 where t.goods_id=2$
        query ok, 1 row affected (0.00 sec)
        rows matched: 1  changed: 1  warnings: 0

        mysql> select * from goods$
        +----------+------------+--------------+
        | goods_id | goods_name | goods_number |
        +----------+------------+--------------+
        |        1 | cat        |           23 |
        |        2 | dog        |           20 |
        |        3 | pig        |           11 |
        |        4 | horse      |           23 |
        |        5 | cow        |           23 |
        |        6 | bird       |           23 |
        |        7 | fish       |           23 |
        +----------+------------+--------------+
        7 rows in set (0.00 sec)
        恩,完美~