use this query:
SELECT
Item_Name,
YEAR(DatePurchased)*100+MONTH(DatePurchased) AS YearMonth,
Sum(QtyInStock) as TotQtyInStock
FROM Stock
GROUP BY Item_Name,
YEAR(DatePurchased)*100+MONTH(DatePurchased)
Then you'd have this:
Item_Name YearMonth TotQntyInStock
-------------------------------------------------------------
abc 201201 5
abc 201202 2
abc 201203 1
xyz 201201 1
xyz 201205 5
etc...
--------------------------------------------------------------