Question : How to get a formula to calcutate even if one of the fields in it is null

I've got a table that will only have records in it if the loan has specific situations for example a fee has been waived, I am using fields from this table in a simple formula:

Total Commission = (commission) + (overage) - (waivedFee1) - (WaivedFee2) etc

This works fine as long as there is a record in the table containing the Waived Fee, but if not the the total commission does not calculate.  Is there a way I can get it to see a null as a 0?

Answer : How to get a formula to calcutate even if one of the fields in it is null

update items
set items.catcode='OUTLET'
from items inner join stock on stock.number=items.item
where stock.isbn='OUTLET'