In a query:

SELECT A, B, IIf(IsNull([b])=True,0,[a]-[b]) AS C
FROM Table1;

"in English"  if the field b is null then return 0, otherwise return field
a - field b.
 Query1 A B C
     1 1 0
     2 0 2
     3
    0
     4 5 -1

    0

Ed Warren