Fail to create relationship between two tables - mysql

Question

I have two tables :
CREATE TABLE `Users` (
`user_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`first_name` varchar(50) NOT NULL DEFAULT '',
`last_name` varchar(50) NOT NULL DEFAULT '',
`login` varchar(50) NOT NULL DEFAULT '',
`password` varchar(50) NOT NULL DEFAULT '',
PRIMARY KEY (`user_id`)
) ENGINE=InnoDB AUTO_INCREMENT=15 DEFAULT CHARSET=utf8;
AND
CREATE TABLE `Books` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`title` varchar(50) NOT NULL DEFAULT '',
`author` varchar(50) NOT NULL DEFAULT '',
`year` int(4) NOT NULL,
`available` int(3) NOT NULL DEFAULT '0',
`availabledate` date NOT NULL,
`user_id` int(11) unsigned NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;
I am trying to create a relationship between those two, so that one user may have multiple books (user_id) but whatever I'm doing I'm getting errors. Either
Cannot add or update a child row: a foreign key constraint fails
(bookstore.,
CONSTRAINT books_fk FOREIGN KEY (user_id) REFERENCES users
(user_id) ON DELETE CASCADE ON UPDATE CASCADE)
or before I didn't use unsigned int in the Books table and I said that default value is 0 (which I would prefere but I don't think I can do that?) In that case I got error 150.

Answer 1

I'd recommend, to change your database schema. Why?
Can a book exist without having a user? If yes, you shouldn't have a foreign key from books referencing users. Can a user exist without having a book? If yes, you shouldn't have a foreign key from users referencing books.
Can a user have multiple books? And a book multiple users? If yes, you have a m:n relationship. This means you need a bridge table.
In your tables you don't need foreign keys:
CREATE TABLE `Users` (
`user_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`first_name` varchar(50) NOT NULL DEFAULT '',
`last_name` varchar(50) NOT NULL DEFAULT '',
`login` varchar(50) NOT NULL DEFAULT '',
`password` varchar(50) NOT NULL DEFAULT '',
PRIMARY KEY (`user_id`)
) ENGINE=InnoDB AUTO_INCREMENT=15 DEFAULT CHARSET=utf8;
CREATE TABLE `Books` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`title` varchar(50) NOT NULL DEFAULT '',
`author` varchar(50) NOT NULL DEFAULT '',
`year` int(4) NOT NULL,
`available` int(3) NOT NULL DEFAULT '0',
`availabledate` date NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;
And a bridge table would look like this:
CREATE TABLE books_users (
book_id int(11) unsigned NOT NULL,
user_id int(11) unsigned NOT NULL,
PRIMARY KEY (book_id, user_id),
KEY idx_user_id (user_id),
FOREIGN KEY fk_books (book_id) REFERENCES Books(id),
FOREIGN KEY fk_users (user_id) REFERENCES Users(user_id)
) ENGINE=InnoDB;
This solves both problems and is common practice.
To query both users and books in one query, you join them like this:
SELECT
whatever
FROM
Books b
INNER JOIN books_users bu ON b.id = bu.book_id
INNER JOIN users u ON bu.user_id = u.user_id
WHERE user_id = 1 /*for example*/
;
If you want to insert something in the tables, just do the insert and get the id which was generated for the row with SELECT LAST_INSERT_ID();, then insert this id in the books_users bridge table.
Updates don't affect anything, you can simply perform them on users or books. If you really really have to update the auto_increment column (which usually isn't needed and not recommended) you can add ON UPDATE CASCADE after the foreign keys in the books_users table.

Answer 2

Change these lines and try again
change user_id int(11) unsigned NOT NULL AUTO_INCREMENT to user_id
int(11) NOT NULL AUTO_INCREMENT
and
id int(11) unsigned NOT NULL AUTO_INCREMENT, to id int(11) NOT
NULL AUTO_INCREMENT,
and
user_id int(11) unsigned NOT NULL, to user_id int(11) NOT NULL,
Finally try
CREATE TABLE `Users` (
`user_id` int(11) NOT NULL AUTO_INCREMENT,
`first_name` varchar(50) NOT NULL DEFAULT '',
`last_name` varchar(50) NOT NULL DEFAULT '',
`login` varchar(50) NOT NULL DEFAULT '',
`password` varchar(50) NOT NULL DEFAULT '',
PRIMARY KEY (`user_id`)
) ENGINE=InnoDB AUTO_INCREMENT=15 DEFAULT CHARSET=utf8;
and
CREATE TABLE `Books` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`title` varchar(50) NOT NULL DEFAULT '',
`author` varchar(50) NOT NULL DEFAULT '',
`year` int(4) NOT NULL,
`available` int(3) NOT NULL DEFAULT '0',
`availabledate` date NOT NULL,
`user_id` int(11) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

Answer 3

If you are using Mysql then you must use InnoDB database engine to enable relationship between two tables
MyISAM will not allow this relationship
alter table `users` add constraint `FK_users` FOREIGN KEY (`user_id`) REFERENCES `books` (`user_id`) ON DELETE NO ACTION ON UPDATE NO ACTION
or you can use this query for cascade edit delete
alter table `users` add constraint `FK_users` FOREIGN KEY (`user_id`) REFERENCES `books` (`user_id`) ON DELETE CASCADE ON UPDATE CASCADE
This is your new DDL for two tables try this
CREATE TABLE IF NOT EXISTS `books` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`title` varchar(50) NOT NULL DEFAULT '',
`author` varchar(50) NOT NULL DEFAULT '',
`year` int(4) NOT NULL,
`available` int(3) NOT NULL DEFAULT '0',
`availabledate` date NOT NULL,
`user_id` int(11) unsigned NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=3 ;
-- --------------------------------------------------------
--
-- Table structure for table `users`
--
CREATE TABLE IF NOT EXISTS `users` (
`user_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`first_name` varchar(50) NOT NULL DEFAULT '',
`last_name` varchar(50) NOT NULL DEFAULT '',
`login` varchar(50) NOT NULL DEFAULT '',
`password` varchar(50) NOT NULL DEFAULT '',
PRIMARY KEY (`user_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=15 ;
--
-- Constraints for dumped tables
--
--
-- Constraints for table `users`
--
ALTER TABLE `users`
ADD CONSTRAINT `FK_users` FOREIGN KEY (`user_id`) REFERENCES `books` (`id`) ON DELETE CASCADE ON UPDATE CASCADE;