mysql外键的三种关系实例详解

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);

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 |+----+-----------------+----------+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      |+----+--------------------------------+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 |+----+-----------+---------+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;