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)

# 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)

#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;