因为有foreign key的约束，使得两张表形成了三种了关系：

• 多对一
• 多对多
• 一对一

一对多或多对一

create table press(    id int primary key auto_increment,    name varchar(20));create table book(    id int primary key auto_increment,    name varchar(20),    press_id int not null,         constraint fk_book_press foreign key(press_id) references press(id)    on delete cascade    on update cascade);# 先往被关联表中插入记录insert into press(name) values('北京工业地雷出版社'),('人民音乐不好听出版社'),('知识产权没有用出版社');# 再往关联表中插入记录insert into book(name,press_id) values('九阳神功',1),('九阴真经',2),('九阴白骨爪',2),('独孤九剑',3),('降龙十巴掌',2),('葵花宝典',3);查询结果：mysql> select * from book;+----+-----------------+----------+| id | name            | press_id |+----+-----------------+----------+|  1 | 九阳神功        |        1 ||  2 | 九阴真经        |        2 ||  3 | 九阴白骨爪      |        2 ||  4 | 独孤九剑        |        3 ||  5 | 降龙十巴掌      |        2 ||  6 | 葵花宝典        |        3 |+----+-----------------+----------+6 rows in set (0.00 sec)mysql> select * from press;+----+--------------------------------+| id | name                           |+----+--------------------------------+|  1 | 北京工业地雷出版社             ||  2 | 人民音乐不好听出版社           ||  3 | 知识产权没有用出版社           |+----+--------------------------------+3 rows in set (0.00 sec)

多对多，引入第三张表

# 创建被关联表author表，之前的book表在讲多对一的关系已创建create table author(    id int primary key auto_increment,    name varchar(20));#这张表就存放了author表和book表的关系，即查询二者的关系查这表就可以了create table author2book(    id int not null unique auto_increment,    author_id int not null,    book_id int not null,    constraint fk_author foreign key(author_id) references author(id)    on delete cascade    on update cascade,    constraint fk_book foreign key(book_id) references book(id)    on delete cascade    on update cascade,    primary key(author_id,book_id));#插入四个作者，id依次排开insert into author(name) values('egon'),('alex'),('wusir'),('yuanhao');# 每个作者的代表作egon: 九阳神功、九阴真经、九阴白骨爪、独孤九剑、降龙十巴掌、葵花宝典alex: 九阳神功、葵花宝典wusir:独孤九剑、降龙十巴掌、葵花宝典yuanhao:九阳神功# 在author2book表中插入相应的数据insert into author2book(author_id,book_id) values(1,1),(1,2),(1,3),(1,4),(1,5),(1,6),(2,1),(2,6),(3,4),(3,5),(3,6),(4,1);# 现在就可以查author2book对应的作者和书的关系了mysql> select * from author2book;+----+-----------+---------+| id | author_id | book_id |+----+-----------+---------+|  1 |         1 |       1 ||  2 |         1 |       2 ||  3 |         1 |       3 ||  4 |         1 |       4 ||  5 |         1 |       5 ||  6 |         1 |       6 ||  7 |         2 |       1 ||  8 |         2 |       6 ||  9 |         3 |       4 || 10 |         3 |       5 || 11 |         3 |       6 || 12 |         4 |       1 |+----+-----------+---------+12 rows in set (0.00 sec)

一对一的情况

#例如： 一个用户只能注册一个博客#两张表： 用户表 （user）和 博客表(blog)# 创建用户表create table user(    id int primary key auto_increment,    name varchar(20));# 创建博客表create table blog(    id int primary key auto_increment,    url varchar(100),    user_id int unique,    constraint fk_user foreign key(user_id) references user(id)    on delete cascade    on update cascade);#插入用户表中的记录insert into user(name) values('alex'),('wusir'),('egon'),('xiaoma');# 插入博客表的记录insert into blog(url,user_id) values('http://www.cnblog/alex',1),('http://www.cnblog/wusir',2),('http://www.cnblog/egon',3),('http://www.cnblog/xiaoma',4);# 查询wusir的博客地址select url from blog where user_id=2;