Symmetric Pairs — HackerRank Advanced SQL (cool)

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/* 
Enter your query here.
*/

SELECT a.x,
a.y
FROM functions a
INNER JOIN functions b
ON ( a.x = b.y
AND b.x = a.y )
WHERE a.x <= a.y
AND a.x != b.x
GROUP BY x,
y
UNION
SELECT x,
y
FROM functions p
GROUP BY x,
y
HAVING Count(*) = 2
ORDER BY x,
y
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