数据库之 表与表之间的关系

表1 foreign key 表2
则表1的多条记录对应表2的一条记录，即多对一

利用foreign key的原理我们可以制作两张表的多对多，一对一关系
多对多：
    表1的多条记录可以对应表2的一条记录
    表2的多条记录也可以对应表1的一条记录

一对一：
    表1的一条记录唯一对应表2的一条记录，反之亦然

分析时，我们先从按照上面的基本原理去套，然后再翻译成真实的意义，就很好理解了

书要关联出版社
被关联的表
create  table press(
id int primary key auto_increment,
name char(20)
);
关联的表
create table book(
book_id int primary key auto_increment,
book_name varchar(20),
book_price int,
press_id int,
constraint Fk_pressid_id foreign key(press_id) references press(id)
on delete cascade
on update cascade
);

插记录
insert into press(name) values(‘新华出版社‘),
                              (‘海燕出版社‘),
                              (‘摆渡出版社‘),
                              (‘大众出版社‘);
insert into book(book_name,book_price,press_id) values(‘Python爬虫‘,100,1),
                                                       (‘Linux‘,80,1),
                                                       (‘操作系统‘,70,2),
                                                       (‘数学‘,50,2),
                                                       (‘英语‘,103,3),
                                                       (‘网页设计‘,22,3);

===========例子一：用户表和管理员表=========
先建被关联的表
create table user(
id int primary key auto_increment, #主键自增
name char(10)
);
在建关联表
create table admin(
id int primary key auto_increment,
user_id int unique,
password varchar(16),
foreign key(user_id) references user(id)
on delete cascade
on update cascade
);
insert into user(name) values(‘susan1‘),
                             (‘susan2‘),
                             (‘susan3‘),
                             (‘susan4‘),
                             (‘susan5‘),
                             (‘susan6‘);
insert into admin(user_id,password) values(4,‘sds156‘),
                                          (2,‘531561‘),
                                          (6,‘f3swe‘);

========例子二：学生表和客户表=========
create table customer(
id int primary key auto_increment,
name varchar(10),
qq int unique,
phone int unique
);
create table student1(
sid int primary key auto_increment,
course char(20),
class_time time,
cid int unique,
foreign key(cid) references customer(id)
on delete cascade
on update cascade
);
insert into customer(name,qq,phone) values(‘小小‘,13564521,11111111),
                                          (‘嘻哈‘,14758254,22222222),
                                          (‘王维‘,44545522,33333333),
                                          (‘胡军‘,545875212,4444444),
                                          (‘李希‘,145578543,5555555),
                                          (‘李迪‘,754254653,8888888),
                                          (‘艾哈‘,74545145,8712547),
                                          (‘啧啧‘,11147752,7777777);
insert into student1(course,class_time,cid) values(‘python‘,‘08:30:00‘,3),
                                                 (‘python‘,‘08:30:00‘,4),
                                                 (‘linux‘,‘08:30:00‘,1),
                                                 (‘linux‘,‘08:30:00‘,7);

========书和作者，另外在建一张表来存书和作者的关系
#被关联的
create table book1(
id int primary key auto_increment,
name varchar(10),
price float(3,2)
);
#========被关联的
create table author(
id int primary key auto_increment,
name char(5)
);
#========关联的
create table author2book(
id int primary key auto_increment,
book_id int not null,
author_id int not null,
unique(book_id,author_id),
foreign key(book_id) references book1(id)
on delete cascade
on update cascade,
foreign key(author_id) references author(id)
on delete cascade
on update cascade
);
#========插入记录
insert into book1(name,price) values(‘九阳神功‘,9.9),
                                    (‘葵花宝典‘,9.5),
                                    (‘辟邪剑谱‘,5),
                                    (‘降龙十巴掌‘,7.3);
insert into author(name) values(‘egon‘),(‘e1‘),(‘e2‘),(‘e3‘),(‘e4‘);
insert into author2book(book_id,author_id) values(1,1),
                                                 (1,4),
                                                 (2,1),
                                                 (2,5),
                                                 (3,2),
                                                 (3,3),
                                                 (3,4),
                                                 (4,5);