문제 link
https://www.hackerrank.com/challenges/symmetric-pairs/problem
Code link
https://github.com/JamesJung01/Hackerrank_sql/blob/main/02_Medium/Symmetric_Pairs.sql
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(f1.X) > 1 or f1.X < f1.Y ORDER BY f1.X