WITH data1 AS(
SELECT f.x x1, f.y y1, ROW_NUMBER() OVER(ORDER BY f.x) RN1
FROM functions f
),

data2 AS(
SELECT f.x x2, f.y y2, ROW_NUMBER() OVER(ORDER BY f.x) RN2
FROM functions f
)

SELECT DISTINCT x1, y1
FROM data1
JOIN data2
    ON x1=y2 AND y1=x2 AND RN1 != RN2 AND x1 <= y1
ORDER BY x1