Question

I have 2 tables, ‘interests’ and ‘users_interests’.

‘users_interests’ just has userid and interestid fields.
‘interests just has an id and a name.

I simply need to find userid’s who have more than 3 interest ID’s in common. I have been told that a Self Join is involved, but I cannot seem to get this to work.

Someone said something like this could work:

SELECT 
      others.userid 
  FROM interests AS user 
  JOIN interests AS others 
      USING(interestid) 
  WHERE user.userid = 2 
  GROUP BY 
      others.userid 
  ORDER BY COUNT(*) DESC

But I’m having no luck with it.

Answer 1

SELECT ui.userid, COUNT(*) AS common_interests
FROM users_interests ui
WHERE ui.interestid IN (
    SELECT ui2.interestid FROM users_interests ui2 WHERE ui2.userid = 2
) 
AND ui.userid <> 2
GROUP BY ui.userid
HAVING common_interests > 3;

Note the occurence of the userid we’re basing our search on (2) on two places in the code