WITH TBL AS(
    SELECT * FROM Functions WHERE x = y
    GROUP BY x, y HAVING COUNT(X) = 2
    UNION
    SELECT TOP 50 PERCENT * FROM Functions F
    WHERE x <> y 
    AND x IN (SELECT y FROM Functions WHERE x <> F.x)
    AND y IN (SELECT x FROM Functions WHERE y <> F.y)
)
SELECT * FROM TBL ORDER BY x