WITH rns AS (
    SELECT   x, y, ROW_NUMBER() OVER(ORDER BY x) as rn
    FROM     functions f
)
SELECT   DISTINCT f1.x, f1.y
FROM     rns f1
         LEFT JOIN rns f2 ON f1.x = f2.y AND f1.y = f2.x AND f1.rn <> f2.rn
WHERE    f2.x IS NOT NULL AND f1.x <= f1.y
ORDER BY f1.x