I would first try refactoring your SQL to use INNER JOIN statements
instead of the comma separated lists you are currently using. I would also
not use any subqueries. Test this and see if it works for you:
SELECT SUM(li.quantity) as qtysoldytd
FROM LineItem li
INNER JOIN Sales sa
on li
I spent several minutes looking at your question and your data model and
nothing jumped out at me that precluded you from determining the quantity of
the this vendor's items sold via this data model. I might have missed
something though.
I'm a little concerned that your LineItem table appeared to
