以eschop的商品表，跟订单表为例：
新建商品表
create table goods(
    id int auto_increment primary key, #商品id
    name varchar(30) not null default '',#商品名
    num tinyint not null default 0    #商品数量
)engine  myisam default charset utf8;
新建订单变
create table indent(
    oid int auto_increment primary key,    #订单id
    gid int not null default 0,    #商品id
    much tinyint not null default 0    #购买数量
)engine myisam default charset utf8;
mysql> desc goods;
+-------+-------------+------+-----+---------+----------------+
| Field | Type        | Null | Key | Default | Extra          |
+-------+-------------+------+-----+---------+----------------+
| id    | int(11)     | NO   | PRI | NULL    | auto_increment |
| name  | varchar(30) | NO   |     |         |                |
| num   | tinyint(4)  | NO   |     | 0       |                |
+-------+-------------+------+-----+---------+----------------+
3 rows in set
mysql> desc indent;
+-------+------------+------+-----+---------+----------------+
| Field | Type       | Null | Key | Default | Extra          |
+-------+------------+------+-----+---------+----------------+
| oid   | int(11)    | NO   | PRI | NULL    | auto_increment |
| gid   | int(11)    | NO   |     | 0       |                |
| much  | tinyint(4) | NO   |     | 0       |                |
+-------+------------+------+-----+---------+----------------+
插入演示数据:
insert into goods values(1,'三星手机',12),(2,'ipad电脑',19),(3,'摩托罗拉mp3',38);
mysql> select * from goods;
+----+-------------+-----+
| id | name        | num |
+----+-------------+-----+
|  1 | 三星手机    |  12 |
|  2 | ipad电脑    |  19 |
|  3 | 摩托罗拉mp3 |  38 |
+----+-------------+-----+
手工给订单表添加购买记录:
insert into indent(gid,much)values(3,2);
mysql> select * from indent;
+-----+-----+------+
| oid | gid | much |
+-----+-----+------+
|   1 |   3 |    2 |
+-----+-----+------+
1 row in set
手工给商品表减少商品信息:
update goods set num=num-2 where id=3;
mysql> select * from goods;
+----+-------------+-----+
| id | name        | num |
+----+-------------+-----+
|  1 | 三星手机    |  12 |
|  2 | ipad电脑    |  19 |
|  3 | 摩托罗拉mp3 |  36 |
+----+-------------+-----+
3 rows in set
修改mysql的结束符:
mysql> delimiter $
-------------------------------------------
创建触发器
create trigger tg1
after insert  #在插入之后触发
on indent
for each row  #固定写法
begin
update goods set num=num-1 where id=3;
end
$
------------------------------------------
模拟用户下订单流程
商品表:
+----+-------------+-----+
| id | name        | num |
+----+-------------+-----+
|  1 | 三星手机    |  12 |
|  2 | ipad电脑    |  19 |
|  3 | 摩托罗拉mp3 |  36 |
+----+-------------+-----+
订单表:
+-----+-----+------+
| oid | gid | much |