Add two select columns and show them as another select column

Hi there,

I have a select statement and I want to add to sum of of the columns and show the result as another column.
for e.g.

select
   st_com_frm_sup_qty,
   st_mfg_frm_sup_qty,
   (st_com_frm_sup_qty+st_mfg_frm_sup_qty) as sum_from_sup_to_com_and_mfg
from

Select allOpen in new window

But the issue I have is if any on the first 2 value is NULL the result of the third col is NULL