Medium 6

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;