SQL and unique n-column combinations

Your query for 2 columns could be rewritten like this:

SELECT
  c1.n,
  c2.n
FROM combinations c1
  INNER JOIN combinations c2 ON c1.n < c2.n

For 3 columns you would then need to make some additions (highlighted in bold):

SELECT
  c1.n,
  c2.n,
  c3.n
FROM combinations c1
  INNER JOIN combinations c2 ON c1.n < c2.n
  INNER JOIN combinations c3 ON c2.n < c3.n

I'm sure you can now easily guess how to scale this for more columns.