CREATE OR REPLACE FUNCTION combinations(n, k) as table (
  WITH RECURSIVE cte(current, remaining) AS (
    -- start with empty combination and the full list
    SELECT
      [],
      range(1, n + 1)
    UNION ALL
    -- recursive case: add one item to the current combination
    SELECT
      current || remaining[j:j],
      remaining[j+1:]
    FROM cte, range(1, length(remaining)+1) _(j)
    WHERE length(current) < k
  )
  SELECT current AS combination
  FROM cte
  WHERE length(current) = k
);

## Example:
FROM combinations(5,3)
ORDER BY combination;