Medium: Question 6

/*
Medium: Question 6
*/
SELECT X,
       Y
FROM
  (SELECT f1.X,
          f1.Y
   FROM functions AS f1
   INNER JOIN functions AS f2 ON f1.X = f2.Y
   AND f1.Y = f2.X
   WHERE f1.X = f1.Y
   GROUP BY f1.X,
            f1.Y
   HAVING count(*) > 1
   UNION ALL SELECT f1.X,
                    f1.Y
   FROM functions AS f1
   INNER JOIN functions AS f2 ON f1.X = f2.Y
   AND f1.Y = f2.X
   WHERE f1.X < f1.Y ) AS pairs
GROUP BY X,Y
ORDER BY X, Y ;