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)