Thanks for the advice, but I already tried that. I managed to resolve
this on my own by adding another layer of nesting. To restate the
problem, there are items, orders and, customers. I am selecting
orders that contain a certain item# in a certain date range (the inner
most select) and then trying to count (among these orders) the number
of unique orders by customer that have a different item#. My query
could successsfully sum the the quantity and cost of this different
item# for each customer but couldn't give me a unique count of orders
that included the item -- instead the count counted the number of items
meeting the criteria, not unique orders.

I solved it by changing the above query to sum up the quantity, cost
etc. by order (not by customer) and then nest all of it within another
select statement that summed those order subtotals by customer, which
could then properly count the number of orders. Whew!