Getting the lastest entry grouped by ID - sql

Question

I have a table with stock for products. The problem is that every time there is a stock change, the new value is stored, together with the new Quantity. Example:
ProductID | Quantity | LastUpdate
1 123 2019.01.01
2 234 2019.01.01
1 444 2019.01.02
2 222 2019.01.02
I therefore need to get the latest stock update for every Product and return this:
ProductID | Quantity
1 444
2 222
The following SQL works, but is slow.
SELECT ProductID, Quantity
FROM (
SELECT ProductID, Quantity
FROM Stock
WHERE LastUpdate
IN (SELECT MAX(LastUpdate) FROM Stock GROUP BY ProductID)
)
Since the query is slow and supposed to be left joined into another query, I really would like some input on how to do this better.
Is there another way?

Answer 1

Use analytic functions. row_number can be used in this case.
SELECT ProductID, Quantity
FROM (SELECT ProductID, Quantity, row_number() over(partition by ProductID order by LstUpdte desc) as rnum
FROM Stock
) s
WHERE RNUM = 1
Or with first_value.
SELECT DISTINCT ProductID, FIRST_VALUE(Quantity) OVER(partition by ProductID order by LstUpdte desc) as quantuity
FROM Stock

Answer 2

Just another option is using WITH TIES in concert with Row_Number()
Full Disclosure: Vamsi's answer will be a nudge more performant.
Example
Select Top 1 with ties *
From YourTable
Order by Row_Number() over (Partition By ProductID Order by LastUpdate Desc)
Returns
ProductID Quantity LastUpdate
1 444 2019-01-02
2 222 2019-01-02

Answer 3

So you Could use a CTE(Common Table Expression)
Base Data:
SELECT 1 AS ProductID
,123 AS Quantity
,'2019-01-01' as LastUpdate
INTO #table
UNION
SELECT 2 AS ProductID
,234 AS Quantity
,'2019-01-01' as LastUpdate
UNION
SELECT 1 AS ProductID
,444 AS Quantity
,'2019-01-02' as LastUpdate
UNION
SELECT 2 AS ProductID
,222 AS Quantity
,'2019-01-02' as LastUpdate
Here is the code using a Common Table Expression.
WITH CTE (ProductID, Quantity, LastUpdate, Rnk)
AS
(
SELECT ProductID
,Quantity
,LastUpdate
,ROW_NUMBER() OVER(PARTITION BY ProductID ORDER BY LastUpdate DESC) AS Rnk
FROM #table
)
SELECT ProductID, Quantity, LastUpdate
FROM CTE
WHERE rnk = 1
Returns
You could then Join the CTE to whatever table you need.

Answer 4

row_number() function might be the most efficient, but the big slow down in your query is the use of the IN statement when used on a subquery, it's a little bit of a tricky one but a join is faster. This query should get what you want and be much faster.
SELECT
a.ProductID
,a.Quantity
FROM stock as a
INNER JOIN (
SELECT
ProductID
,MAX(LastUpdate) as LastUpdate
FROM stock
GROUP BY ProductID
) b
ON a.ProductID = b.ProductId AND
a.LastUpdate = b.LastUpdate