我将实现一个书店数据库。我已经创建了表book、author和publisher。我想建立以下两个关系。

Book is written by Author.
Book is published by Publisher.

为了实现这些关系，我编写了一些SQL语句，如：

create table book(
            ISBN varchar(30) NOT NULL,
            title varchar(30) not null,
            author varchar(30) not null,
            stock Int,
            price Int,
            category varchar(30), 
            PRIMARY KEY ( ISBN )
);

create table author(
            author_id int not null auto_increment,
            author_name varchar(15) NOT NULL,
            address varchar(50) not null,
            ISBN varchar(30) not null,
            primary key (author_id)
);

alter table author add constraint ISBN foreign key (ISBN) references book (ISBN);

create table publisher(
            publisher_id int not null auto_increment,
            publisher_name varchar(15) NOT NULL,
            address varchar(50) not null,
            ISBN varchar(30) not null,
            primary key (publisher_id)
);

alter table publisher add constraint ISBN foreign key (ISBN) references book (ISBN);

当MySQL shell执行最后一条alter语句时，我得到了这个错误。

ERROR 1022 (23000): Can't write; duplicate key in table '#sql-2b8_2'

原来，外键不能指定两次吗？有什么不好？提前谢谢。