Help With Append Query Ms Access 2003

Hi,

I have an append query that pulls data from 2 tables joined by the "CustNum" If the order has a seperate delivery address the program saves the the delivery address in the Customers table so then i need to join the tables by "CustNum" in the customers table and the "ShipNum" in the orders table. I was thinking is it possible to use an IIF statement?

for example if the "ShipNum" = 0 then the join would be "CustNum" in the customers table and "CustNum" in the orders table and if the "ShipNum" is greater than 0 the join would be "CustNum" in the customers table and "ShipNum" in the orders table.

Ihope this makes sense and someone can help me.

Thanks.