Hi! i need help with summing fields and expressions

[Keidy]

Hi! I'm trying to create a query using option groups, i have two options the first one is STOCK and the other is PD.
If i select stock the the query i want to create to introduce it on a form is:
I have 5 fields
-denominación
-reserva
-almacen
-cantidad
-stmax
If i select stock
then the query is
If (reserva +almacen)<cantidad then on a sub form have to appear the table with all the rows where (reserva +almacen)<cantidad.
but if i select PD only have to select on stmax the rows where value=PD
PLEASE i need help with this....