assign selected columns to a variable
You are right, the columns in a SELECT at the same level can't refer to each other. Use a subquery instead:

SELECT
  sum_a as a
  sum_b as b
  round(sum_a / sum_b) as a_over_b
FROM (
  SELECT 
    sum(column_a) as sum_a,
    sum(column_b) as sum_b
  From
    table
  Group by column
) as subq