Updating Product Order is not working - sql

Question

I have Products table with (Name, ParentID, Order) columns. I have a insert statement which keep the inserted child products. After the insert I need to update the order,
I have the following SQL,
UPDATE Products
SET [Order] = (SELECT ISNULL(MAX([Order]), 0) + 1 FROM Products WHERE ParentID = CP.ParentID)
FROM Products P
INNER JOIN #InsertedChildProduct CP ON (CP.ID = P.ID)
The problem is that I am updating the order of products that are just inserted, but [Order] is not working. If I have,
Products
--------
ParentID Order
----------------
1 1
1 2
and let say I have inserted 2 child products then the table should be,
Products
--------
ParentID Order
----------------
1 1
1 2
1 3
1 4
But I am seeing,
Products
--------
ParentID Order
----------------
1 1
1 2
1 3
1 3

Answer 1

You can try this, taken from the answers on here:
declare #MaxNumber int
set #MaxNumber = 0
UPDATE Products
SET [Order] = #MaxNumber, #MaxNumber = (SELECT ISNULL(MAX([Order]), 0)
FROM Products
WHERE ParentID = CP.ParentID) + 1
FROM Products P
INNER JOIN #InsertedChildProduct CP ON (CP.ID = P.ID)

Answer 2

Try like this instead, you need to gt the MAX() first in inner query
UPDATE P
SET [Order] = X.newval
FROM Products P
JOIN
(
SELECT ID, (ISNULL(MAX([Order]), 0) + 1) as newval
FROM Products P
JOIN #InsertedChildProduct ip
on P.ParentID = ip.ParentID
group by ID
) X
ON X.ID = P.ID

Answer 3

Thought about this a bunch of different ways and can't see how this would work without imposing order on both datasets, even if arbitrary. Here's one way to do it (Fiddle - make sure to build the schema first, then run the code): http://www.sqlfiddle.com/#!3/d34df/3)
WITH cteRN_c
AS
(
SELECT ROW_NUMBER() OVER (PARTITION BY ID ORDER BY ID) AS RN_c,
ID
FROM #InsertedChildProduct
),
cteRN_p
AS
(
SELECT ROW_NUMBER() OVER (PARTITION BY ParentID ORDER BY [Order]) AS RN_p,
ParentID,
[Order]
FROM Products
WHERE [Order] IS NULL
)
UPDATE p
SET [Order] = (SELECT ISNULL(MAX([ORDER]), 0) FROM Products WHERE ParentID = p.ParentID) + c.RN_c
FROM cteRN_p p INNER JOIN cteRN_c c
ON p.ParentID = c.ID AND
p.RN_p = c.RN_c;
We impose order by adding arbitrary row numbers to both the temp table set and the parent set, via ROW_NUMBER in CTEs. From that point, it's just a matter of joining the CTEs on the correct datapoints, and running the updates against the parent CTE. Granted, it's arbitrary which child will get numbered in which order, but at least it will happen.
Edit: Forgot the ISNULL in the MAX portion of the query - in case no children yet. Fiddle updated as well.