Or maybe this?:
SELECT
t.ProductID
,t.Order_Day
,t.Order_Quantity
FROM (
SELECT
ProductID
,Order_Day
,Order_Quantity
,ROW_NUMBER() OVER(PARTITION BY ProductID ORDER BY Order_Quantity DESC) AS row_num
FROM (
SELECT
SUM(OrderQuantity) AS Order_Quantity
,ProductID
,DATEADD(DAY, DATEDIFF(DAY, 0, OrderDate), 0) AS Order_Day
FROM Transactions
GROUP BY ProductID, DATEADD(DAY, DATEDIFF(DAY, 0, OrderDate), 0)
) AS daily_totals
) AS t
WHERE t.row_num = 1
SQL DBA,SQL Server MVP(07, 08, 09) "It's a dog-eat-dog world, and I'm wearing Milk-Bone underwear." "Norm", on "Cheers". Also from "Cheers", from "Carla": "You need to know 3 things about Tortelli men: Tortelli men draw women like flies; Tortelli men treat women like flies; Tortelli men's brains are in their flies".