数据库开发——MySQL——foreign key

五，foreign key

foreign key是跟外部表关联的字段。

foreign key理解

学生信息表有三个字段：学号、姓名、年级，学校有四个年级，但是有3万多学生，那就意味着年级这个字段的信息要重复存储，要存储的年级信息越长就越浪费资源。

可以通过定义一个年级表关联学生信息表，这样每个学生的年级就只需要存储所处年级在年级信息表中的id，这个外表关联关系，就是通过foreign key定义的。

既然学生的年级信息要关联到年级信息，所以在创建学生信息表之前年级信息表必须存在，也就是必须先创建年级信息表，并且表的类型必须是innodb存储引擎，且被关联的字段必须是另一个表中的unique字段。

create table grade(
	grade_year year primary key,
	name varchar(20) not null
	)engine=innodb;

create table student(
	id int primary key,
	name varchar(20) not null,
	grade_id year,
	constraint fk_grade_id foreign key(grade_id) references grade(grade_year)
	on delete cascade	# 同步删除
	on update cascade	# 同步更新

insert into grade values 
	(2019, "2019 grade"), 
	(2018, "2018 grade"), 
	(2017, "2017 grade"), 
	(2016, "2016 grade");

insert into student values
	(1, "Alex", 2018),
	(2, "Coco", 2019),
	(3, "alex", 2018),
	(4, "coco", 2019);

select * from student;
select * from grade;

delete from grade where grade_year=2019;
select * from student;

update grade set grade_year=2019 where grade_year=2018;
select * from student;

执行结果为：

mysql> create table grade(
    -> grade_year year primary key,
    -> name varchar(20) not null
    -> )engine=innodb;
Query OK, 0 rows affected (0.85 sec)

mysql> create table student(
    -> id int primary key,
    -> name varchar(20) not null,
    -> grade_id year,
    -> constraint fk_grade_id foreign key(grade_id) references grade(grade_year)
    -> on delete cascade
    -> on update cascade
    -> )engine=innodb;
Query OK, 0 rows affected (0.66 sec)

mysql> insert into grade values (2019, "2019 grade"), (2018, "2018 grade"), (2017, "2017 grade"), (2016, "2016 grade");
Query OK, 4 rows affected (0.15 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> insert into student values
    -> (1, "Alex", 2018),
    -> (2, "Coco", 2019),
    -> (3, "alex", 2018),
    -> (4, "coco", 2019);
Query OK, 4 rows affected (0.22 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> select * from student;
+----+------+----------+
| id | name | grade_id |
+----+------+----------+
|  1 | Alex |     2018 |
|  2 | Coco |     2019 |
|  3 | alex |     2018 |
|  4 | coco |     2019 |
+----+------+----------+
4 rows in set (0.00 sec)

mysql> select * from grade;
+------------+------------+
| grade_year | name       |
+------------+------------+
|       2016 | 2016 grade |
|       2017 | 2017 grade |
|       2018 | 2018 grade |
|       2019 | 2019 grade |
+------------+------------+
4 rows in set (0.00 sec)

mysql> delete from grade where grade_year=2019;
Query OK, 1 row affected (0.27 sec)

mysql> select * from student;
+----+------+----------+
| id | name | grade_id |
+----+------+----------+
|  1 | Alex |     2018 |
|  3 | alex |     2018 |
+----+------+----------+
2 rows in set (0.00 sec)

mysql> update grade set grade_year=2019 where grade_year=2018;
Query OK, 1 row affected (0.07 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from student;
+----+------+----------+
| id | name | grade_id |
+----+------+----------+
|  1 | Alex |     2019 |
|  3 | alex |     2019 |
+----+------+----------+
2 rows in set (0.00 sec)

如何找出两表之间的关系

分析步骤：

1、先站在左表的角度去找

是否左表的多条记录可以对应右表的一条记录，如果是，则证明左表的一个字段foreign key 右表一个字段（通常是id）

2、再站在右表的角度去找

是否右表的多条记录可以对应左表的一条记录，如果是，则证明右表的一个字段foreign key 左表一个字段（通常是id）

3、总结：

#多对一：
如果只有步骤1成立，则是左表多对一右表
如果只有步骤2成立，则是右表多对一左表

#多对多
如果步骤1和2同时成立，则证明这两张表时一个双向的多对一，即多对多,需要定义一个这两张表的关系表来专门存放二者的关系

#一对一:
如果1和2都不成立，而是左表的一条记录唯一对应右表的一条记录，反之亦然。这种情况很简单，就是在左表foreign key右表的基础上，将左表的外键字段设置成unique即可

建立两表之间的关系

三张表：出版社，作者信息，书

一对多或多对一

一对多（或多对一）：一个出版社可以出版多本书

关联方式：foreign key

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,
	foreign key(press_id) references press(id)
	on delete cascade
	on update cascade);


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');

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);

select * from press;

select * from book;

执行结果为：

mysql> create table press(
	id int primary key auto_increment,
	name varchar(20));

mysql> create table book(
    -> id int primary key auto_increment,
    -> name varchar(20),
    -> press_id int not null,
    -> foreign key(press_id) references press(id)
    -> on delete cascade
    -> on update cascade);
Query OK, 0 rows affected (0.50 sec)

mysql> 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');
Query OK, 3 rows affected (0.18 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> 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);
Query OK, 6 rows affected (0.10 sec)
Records: 6  Duplicates: 0  Warnings: 0

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)

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)
Query OK, 0 rows affected (0.62 sec)

多堆多

多对多：一个作者可以写多本书，一本书也可以有多个作者，双向的一对多，即多对多
　　
关联方式：foreign key+一张新的表

create table author(
	id int primary key auto_increment,
	name varchar(20));

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));

insert into author(name) values('Alex'),('Coco'),('BeiBei'),('Python');

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);

select * from author;
select * from author2book;
select * from book;

执行结果为：

mysql> create table author(
    -> id int primary key auto_increment,
    -> name varchar(20));
Query OK, 0 rows affected (0.52 sec)

mysql> 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));
Query OK, 0 rows affected (0.60 sec)

mysql> insert into author(name) values('Alex'),('Coco'),('BeiBei'),('Python');
Query OK, 4 rows affected (0.08 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql>
mysql> 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);
Query OK, 12 rows affected (0.06 sec)
Records: 12  Duplicates: 0  Warnings: 0

mysql> select * from author;
+----+--------+
| id | name   |
+----+--------+
|  1 | Alex   |
|  2 | Coco   |
|  3 | BeiBei |
|  4 | Python |
+----+--------+
4 rows in set (0.00 sec)

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)

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)