Compare similarities between two result sets

Question

I am creating a music website where I would like users to be able to find users who like approximately the same artists as they.

I have a \'like\' table that has 2 c

Answer 1

Something like this:

SELECT first_user.id_user, second_user.id_user, COUNT(first_user.id_user) AS total_matches

FROM likes AS first_user

JOIN likes AS second_user
ON second_user.id_artist = first_user.id_artist
AND second_user.id_user != first_user.id_user

GROUP BY first_user.id_user, second_user.id_user

ORDER BY total_matches DESC

LIMIT 1

Note that this isn't very efficient. One way to work around this is to make a 'cache table' containing the output of this query with the LIMIT 1 portion removed. Add some relevant indexes and do query this cache table. You could set a cron job to update this table periodically.

Example:

CREATE TABLE IF NOT EXISTS `likes` (
  `id_user` varchar(50) DEFAULT NULL,
  `id_artist` varchar(50) DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

INSERT INTO `likes` (`id_user`, `id_artist`) VALUES ('8', '39'), ('8', '37'), ('4', '37'), ('8', '24'), ('8', '7'), ('4', '28'), ('8', '28'), ('4', '27'), ('4', '11'), ('8', '49'), ('4', '7'), ('4', '40'), ('4', '29'), ('8', '22'), ('4', '29'), ('8', '11'), ('8', '28'), ('4', '7'), ('4', '31'), ('8', '42'), ('8', '25'), ('4', '25'), ('4', '17'), ('4', '32'), ('4', '46'), ('4', '19'), ('8', '34'), ('3', '32'), ('4', '21')

+---------+---------+---------------+
| id_user | id_user | total_matches |
+---------+---------+---------------+
| 8       | 4       |             7 |
+---------+---------+---------------+