Please provide table structure, sample data and expected result set based on the sample as described in the link in my signature.

Reason: It looks like you don't need the cross join at all.

Basis idea: using a CTE/subquery with the Group By function on your table dbo.tblPurchase to get the weeks where a purchase has been made and do an right outer join to a calendar table. If you don't have a calendar table by now you might want to look into it. It also could be created on the fly within the CTE, if this query is not heavily used.