sql - How can i select first record of each distinct couple of 2 columns

Question

I have a table where i can have records like this:

ID	ColA	ColB
1	10	11
2	11	10
3	12	10
4	10	12

Answer 1

You may use ROW_NUMBER here along with least/greatest logic:

WITH cte AS (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY
                                     CASE WHEN ColA <= ColB THEN ColA ELSE ColB END,
                                     CASE WHEN ColA > ColB  THEN ColA ELSE ColB END
                                 ORDER BY ID) rn
    FROM yourTable
)

SELECT ID, ColA, ColB
FROM cte
WHERE rn = 1;

Demo

Explanation:

The two CASE expressions which appear in the partition clause of ROW_NUMBER find the smallest and greatest of the two columns ColA and ColB, for each record. So, we partition on the pair of least/greatest of the two columns, and then retain the record for each pair having the smallest ID value.