mysql外鍵的三種關(guān)系實(shí)例詳解

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
);

# 先往被關(guān)聯(lián)表中插入記錄
insert into press(name) values
('北京工業(yè)地雷出版社'),
('人民音樂不好聽出版社'),
('知識(shí)產(chǎn)權(quán)沒有用出版社')
;
# 再往關(guān)聯(lián)表中插入記錄
insert into book(name,press_id) values
('九陽神功',1),
('九陰真經(jīng)',2),
('九陰白骨爪',2),
('獨(dú)孤九劍',3),
('降龍十巴掌',2),
('葵花寶典',3)
;

mysql> select * from book;
+----+-----------------+----------+
| id | name      | press_id |
+----+-----------------+----------+
| 1 | 九陽神功    |    1 |
| 2 | 九陰真經(jīng)    |    2 |
| 3 | 九陰白骨爪   |    2 |
| 4 | 獨(dú)孤九劍    |    3 |
| 5 | 降龍十巴掌   |    2 |
| 6 | 葵花寶典    |    3 |
+----+-----------------+----------+
rows in set (0.00 sec)
mysql> select * from press;
+----+--------------------------------+
| id | name              |
+----+--------------------------------+
| 1 | 北京工業(yè)地雷出版社       |
| 2 | 人民音樂不好聽出版社      |
| 3 | 知識(shí)產(chǎn)權(quán)沒有用出版社      |
+----+--------------------------------+
rows in set (0.00 sec)

# 創(chuàng)建被關(guān)聯(lián)表author表，之前的book表在講多對(duì)一的關(guān)系已創(chuàng)建
create table author(
  id int primary key auto_increment,
  name varchar(20)
);
#這張表就存放了author表和book表的關(guān)系，即查詢二者的關(guān)系查這表就可以了
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)
);
#插入四個(gè)作者，id依次排開
insert into author(name) values('egon'),('alex'),('wusir'),('yuanhao');
# 每個(gè)作者的代表作
egon: 九陽神功、九陰真經(jīng)、九陰白骨爪、獨(dú)孤九劍、降龍十巴掌、葵花寶典
alex: 九陽神功、葵花寶典
wusir:獨(dú)孤九劍、降龍十巴掌、葵花寶典
yuanhao:九陽神功
# 在author2book表中插入相應(yīng)的數(shù)據(jù)
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)
;

# 現(xiàn)在就可以查author2book對(duì)應(yīng)的作者和書的關(guān)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： 一個(gè)用戶只能注冊(cè)一個(gè)博客
#兩張表： 用戶表 （user）和 博客表(blog)
# 創(chuàng)建用戶表
create table user(
  id int primary key auto_increment,
  name varchar(20)
);
# 創(chuàng)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
);
#插入用戶表中的記錄
insert into user(name) values
('alex'),
('wusir'),
('egon'),
('xiaoma')
;
# 插入博客表的記錄
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)
;
# 查詢wusir的博客地址
select url from blog where user_id=2;