HackerRank MySQL - Symmetric Pairs

You are given a table, Functions, containing two columns: X and Y.

Two pairs (X1, Y1) and (X2, Y2) are said to be symmetric pairs if X1 = Y2 and X2 = Y1.

Write a query to output all such symmetric pairs in ascending order by the value of X.

SELECT X,
       Y
FROM FUNCTIONS F1
WHERE EXISTS
    (SELECT *
     FROM FUNCTIONS F2
     WHERE F2.Y = F1.X
       AND F2.X = F1.Y
       AND F2.X > F1.X)
  AND (X != Y)
UNION
SELECT X,
       Y
FROM FUNCTIONS F1
WHERE X = Y
  AND (
         (SELECT COUNT(*)
          FROM FUNCTIONS
          WHERE X = F1.X
            AND Y = F1.X) > 1)
ORDER BY X;