Simple non-repeating pairs of numbers in all combinations

combinations.sql

WITH inputs AS (
    SELECT
       row_number() over () as idx,
       letter
    FROM unnest(ARRAY['a', 'b', 'c', 'd']) letters(letter)
)
SELECT
    a.letter,
    b.letter
FROM inputs a, inputs b
WHERE a.idx < b.idx;

It works.

 letter | letter 
--------+--------
 a      | b
 a      | c
 a      | d
 b      | c
 b      | d
 c      | d
(6 rows)