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;