MYSQL sélectionne des amis communs

Après les explications (et en fait la lecture de la partie "mutuelle") :

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