多对多

1、什么是多对多
两张表之间是一个双向的多对一关系，称之为多对多
如何实现？
建立第三张表，该表中有一个字段fk左表的id，还有一个字段是fk右表的id


create table author(
    id int primary key auto_increment,
    name char(16)
);

create table book(
    id int primary key auto_increment,
    bname char(16),
    price int
);

insert into author(name) values
('egon'),
('alex'),
('wxx')
;
insert into book(bname,price) values
('python从入门到入土',200),
('葵花宝典切割到精通',800),
('九阴真经',500),
('九阳神功',100)
;


create table author2book(
    id int primary key auto_increment,
    author_id int,
    book_id int,
    foreign key(author_id) references author(id)
    on update cascade
    on delete cascade,
    foreign key(book_id) references book(id)
    on update cascade
    on delete cascade
);

insert into author2book(author_id,book_id) values
(1,3),
(1,4),
(2,2),
(2,4),
(3,1),
(3,2),
(3,3),
(3,4);