CREATE TABLE foo(a,b,c)
AS VALUES
    ( 1, 20, 1 ),
    ( 3, 21, 2 ),
    ( 1, 22, 3 ),
    ( 2, 22, 4 ),
    ( 3, 21, 5 ),
    ( 1, 20, 6 );

SELECT greatest(a,b), least(a,b)
FROM foo
GROUP BY greatest(a,b), least(a,b);