MYSQL select mutual friends

Question

My \'friends\' table has the following columns: id, userID, userID2, state

userID and userID2 don\'t have a specific order of being put into the database.

I

Answer 1

After the explanations (and actually reading the "mutual" part):

SELECT a.friendID
FROM
      ( SELECT CASE WHEN userID = $id
                      THEN userID2 
                      ELSE userID 
               END AS friendID 
        FROM friends 
        WHERE userID = $id OR userID2 = $id
      ) a
  JOIN
      ( SELECT CASE WHEN userID = $session
                      THEN userID2 
                      ELSE userID 
               END AS friendID 
        FROM friends 
        WHERE userID = $session OR userID2 = $session
      ) b
    ON b.friendID = a.friendID