complex sql query comparing data and creating a list - mysql

Question

I've users table:
CREATE TABLE `users` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`email` char(255) NOT NULL DEFAULT '',
`password` char(12) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `email` (`email`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;
books table:
CREATE TABLE `books` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`book` char(55) NOT NULL DEFAULT '',
`user_id` int(11) unsigned NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `book` (`book`),
KEY `user_id` (`user_id`),
CONSTRAINT `books_ibfk_1` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON DELETE NO ACTION ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
And read table:
CREATE TABLE `read` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`user_id` int(11) unsigned NOT NULL,
`book_id` int(11) unsigned NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `No duplicates` (`user_id`,`book_id`),
KEY `book_id` (`book_id`),
CONSTRAINT `connections_ibfk_1` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `connections_ibfk_2` FOREIGN KEY (`book_id`) REFERENCES `books` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
I'd like for user_id = 1, to create list of other user_id's - matching point is the common books they have read. So if user_id = 1 and user_id = 2 have in common 5 books then user_id should be on that list. I'm not so good at sql, so any recommendation on how to achieve this, even little optimisation tip will be much appreciated.

Answer 1

Possible solution - gives users that read 5 or more common books
SELECT r2.user_id
FROM `read` r1
JOIN `read` r2
ON r1.user_id <> r2.user_id AND r1.book_id = r2.book_id
WHERE r1.user_id = 1
GROUP BY r2.user_id
HAVING count(*) >= 5
If users must have exactly 5 common books (no less and no more), a clause with HAVING must be changed into:
HAVING count(*) = 5
Demo: --> http://www.sqlfiddle.com/#!2/7a9b7/1
A simple change to the query gives pairs of users that read >= 5 common books:
SELECT r1.user_id user1, r2.user_id user2
FROM `read` r1
JOIN `read` r2
ON r1.user_id < r2.user_id AND r1.book_id = r2.book_id
-- WHERE r1.user_id = 1
GROUP BY r1.user_id, r2.user_id
HAVING count(*) >= 5;
Demo: --> http://www.sqlfiddle.com/#!2/7a9b7/3
---- EDIT ----
To order users by number of matches use this query:
SELECT r1.user_id user1, r2.user_id user2,
count(*) number_of_matches
FROM `read` r1
JOIN `read` r2
ON r1.user_id < r2.user_id AND r1.book_id = r2.book_id
-- WHERE r1.user_id = 1
GROUP BY r1.user_id, r2.user_id
HAVING count(*) >= 5
ORDER BY number_of_matches DESC
-- you may also use:
-- ORDER BY COUNT(*) DESC
;
demo --> http://www.sqlfiddle.com/#!2/7a9b7/8