I have five fields as below within a Query, is it possible in the sixth field to put an expresion:-

sum StkQty - AlloQty = result
and if
StkPartN0 is equal to previous lines StkPartN0, sum result - line 2 AlloQty etc.
This would give me a result of
22 -10 on line 1 = 12
12 - 1 on line 2 = 11
11 - 1 on line 3 = 10

StkPartN0 StkPartDesc StkQty AlloQty Duedate
1B4819B GM3/D 2ND WHEEL 77T 18074SDO-B 22 10 03/02/2006
1B4819B GM3/D 2ND WHEEL 77T 18074SDO-B 22 1 15/02/2006
1B4819B GM3/D 2ND WHEEL 77T 18074SDO-B 22 1 10/03/2006


Apoligies if this is basic stuff