[AccessD] Query synatx

Dear List

I have a OrderHeader and OrderDetails tables. 
The OrderHeader contains a InvoiceType field, set to True for Invoice
and False for CreditNotes.
The OrderDetails contains a Quantity field. This is always a positive
value, even for CreditNotes.
The join between the tables is on OrderID.

What I am trying to do is create a query that will return a single value
for all the quantities on order in the OrderDetails, for a given product
number. So where the InvoiceType = False, the Quantity will have to be
Quantity * -1.

This has got to be possible, but having spent most of the day going
nowhere with this, I need help.

Any ideas would be much appreciated.

Many thanks
Tim Pain



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