I have a drop down list that I'm using to sum up detail by specific customer.
The detail is kept on a worksheet called "Detail". The summary and drop down
list is on a worksheet called "Summary". When no customer is selected from
the list I want to be able to sum all customers. Then when a customer is
selected I only want it to sum that customer. How do I add this "All"
function to my worksheet? Here is an example of the formula I am using to sum
up detail by a customer and date. The cell "B3" is where my drop down list is.

=SUMPRODUCT(--(MONTH(Detail!$I$2:$I$43998)=MONTH($A8)),--(YEAR(Detail!$I$2:$I$43998)=YEAR($A8)),--(Detail!$B$2:$B$43998=Summary!$B$3))