window function and order by

Hi,

assuming I have a query that computes a running sum like this:

  select id, buy_price, sum(buy_price) over (order by id) sum
    from fmb
   where 202300<=id and id<=202400
   order by id;

Do I need the ORDER BY clause at the end? Or does the ORDER BY in the
window function already define the order of the result set?

Thanks,
Torsten