Трюк дня. Поиск симметричных записей в MySQL. Решение

SELECT f1.x,f1.y
FROM
(SELECT x,y,ROW_NUMBER() OVER (ORDER BY x) AS id FROM items) f1
JOIN
(SELECT x,y,ROW_NUMBER() OVER (ORDER BY x) AS id FROM items) f2
ON f1.x=f2.y
AND f1.y=f2.x
AND (f1.id<f2.id)
ORDER BY f1.x;

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