Symmetric Pairs

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

Sample Output

20 20
20 21
22 23

Solution Implementation

SELECT X, Y FROM FUNCTIONS F1
WHERE EXISTS(SELECT * FROM FUNCTIONS F2 WHERE F2.Y = F1.X
AND F2.X = F1.Y AND F2.X > F1.X) AND (X != Y)
UNION
SELECT X,Y FROM FUNCTIONS F1 WHERE X = Y AND 
((SELECT COUNT(*) FROM FUNCTIONS WHERE X = F1.X AND Y = F1.X) > 1)
ORDER BY X;