Question

I have a MySQL query that I used to use to return mutual friends between two users, but now that I am recoding my website, I am trying to simplify this code, or at least make it better.

So here’s my code below to check mutual friends:

SELECT a.friendID
FROM
(SELECT CASE WHEN userID = $id
    THEN userID2 
    ELSE userID 
    END AS friendID 
FROM friends 
WHERE (userID = $id OR userID2 = $id) 
    AND state='1'
) a

 JOIN
(SELECT CASE WHEN userID = $session
    THEN userID2 
    ELSE userID 
    END AS friendID 
FROM friends 
WHERE (userID = $session OR userID2 = $session) 
    AND state='1'
) b

ON b.friendID = a.friendID

My table is set up like this:

   userID -- userID2 -- state
      1 ------- 2 ------- 1
      2 ------- 3 ------- 1
      1 ------- 3 ------- 0

(sorry, I don’t know how to do the pretty database structure design, so if someone could edit that for me…)

but for the above, when user 1 is on user 3’s profile, since user 1 and user 2 are friends, and user 2 and user 3 are friends, but user 1 and user 3 are not, it should return user 2 as a mutual friend. (state 1 means friendship accepted, state 0 means friendship pending, so only if state is 1 should it be counted as a friend)

Also note that userID and userID2 can be in any order, it depends on who requests who as a friend, so like the above query does, I need to also have the “friendID” returned, as the above query does right.

Answer 1

SELECT t.f FROM friends JOIN (

  SELECT IF(userID = $id, userID2, userID) f
  FROM   friends
  WHERE  state AND $id IN (userID, userID2)

) t ON t.f IN (userID, userID2)
WHERE  state AND $session IN (userID, userID2)

See it on sqlfiddle.