assign selected columns to a variable Select
  sum(column_a) as a,
  sum(column_b) as b
  round(a / b)
From
  table
Group by column


In the example above I am summing two column, and I would like to use those 2 results in calculating another result, how can I do this? round(a / b) is not going to work, is just as an example of what I want to do.
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