105 表与表之间的关系

#一对多
# 先创建被关联表

create table dep(
id int primary key auto_increment,
name varchar(6),
comment varchar(30)
);

# 再创建关联表
create table employee(
id int primary key auto_increment,
name varchar(10),
gender varchar(5),
dep_id int,
foreign key(dep_id) references dep(id) on delete cascade on update cascade
);


#多对多： --其实就是建一个中间表，用于存放两个表的对应关系
create table author(
    id int primary key auto_increment,
    name varchar(10)
);
create table book(
    id int primary key auto_increment,
    name varchar(16)
);
create table author2book(
    id int primary key auto_increment,
    author_id int,
    book_id int,
    foreign key(author_id) references author(id) on delete cascade on update cascade,
    foreign key(book_id) references book(id) on delete cascade on update cascade
);


# 一对一：
create table customer(
    id int primary key auto_increment,
    name varchar(16),
    phone char(11)
);

create table student(
    id int primary key auto_increment,
    class varchar(10),
    course varchar(16),
    c_id int unique,
    foreign key(c_id) references customer(id) on delete cascade on update cascade
);