Sorry, I am completely new to SQL, I tried reading DSum and Subquery but still couldn't figure it out.
I tried:
SELECT T1.Week, Sum(DSum("[Qty]","T1","[Week] = [Week] and [Week] > [Week]-3")) AS L4W
FROM T1
GROUP BY T1.Week;
This only sums up everything for every week.
I have no idea how to...