I have to show outstanding quantities of deliveries for orders. There is no
problem if there was a partial delivery: outstanding=ordered-delivered. In
case of orders where there was no delivery at all, the field delivered
contains no value i.e. null. Accordingly the field outstanding has no value
either although it should show the quantity ordered. How can I solve this
task? Should I have zero in the field delivered, and if yes, how?

Thank u very much for your kind assistance.