해커랭크 - Symmetric Pairs (UNION)

Symmetric Pairs - Medium 
https://www.hackerrank.com/challenges/symmetric-pairs/problem

 

SELECT X, YG
FROM Functions
WHERE X=Y
GROUP BY X,Y
HAVING COUNT(*)>=2

UNION

SELECT s1.X, s1.Y
FROM functions AS s1
     INNER JOIN functions AS s2 ON s1.X = s2.Y AND s1.Y = s2.X
WHERE s1.X < s1.Y
ORDER BY X