[Hacker Rank] Symmetric Pairs

https://www.hackerrank.com/challenges/symmetric-pairs/problem?isFullScreen=true&h_r=next-challenge&h_v=zen 

Symmetric Pairs | HackerRank

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You are given a table, Functions, containing two columns: X and Y.

Two pairs (X1, Y1) and (X2, Y2) are said to be symmetric pairs if X1 = Y2 and X2 = Y1.

Write a query to output all such symmetric pairs in ascending order by the value of X. List the rows such that X1 ≤ Y1.

Sample Input

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[My Answer]

SELECT F1.x, F1.y
FROM FUNCTIONS AS F1
INNER JOIN 
FUNCTIONS AS F2 
ON F1.x=F2.y AND F1.y=F2.x
GROUP BY F1.x, F1.y
HAVING count(*)>=2 or F1.x < F1.y 
ORDER BY F1.x

Union으로 풀는 방식은 이해가 잘 되지 않는다,, 그냥 having으로 조건 주는게 더 편했다