MYSQL数据表建立外键

MySQL创建关联表可以理解为是两个表之间有个外键关系，但这两个表必须满足三个条件
1.两个表必须是InnoDB数据引擎
2.使用在外键关系的域必须为索引型(Index)
3.使用在外键关系的域必须与数据类型相似

例如：

1、建立s_user表

create table s_user(
       u_id int auto_increment primary key,
       u_name varchar(15),
       u_pwd varchar(15),
       u_truename varchar(20),
        u_role varchar(6),
       u_email varchar(30)
)

2、

插入几条数据

insert into s_user values
       (1,"wangc","aaaaaa","wangchao","buyer","wang@163.com"),
      (2,"huangfp","bbbbbb","huangfp","seller","huang@126.com"),
      (3,"zhang3","cccccc","zhangsan","buyer","zhang@163.com"),
      (4,"li4","dddddd","lisi","seller","li@1256.com")

3、

建立s_orderform表

create table s_orderform(

          o_id int auto_increment primary key,
         o_buyer_id int,
         o_seller_id int,
         o_totalprices double,
         o_state varchar(50),
         o_information varchar(200),
         foreign key(o_buyer_id) references s_user(u_id),      #外链到s_user表的u_id字段
         foreign key(o_seller_id) references s_user(u_id)      #外链到s_user表的u_id字段
)