SQL GROUP BY on a sub query

Question

I have a query that will return results from 2 tables into 1 using a UNION ALL, which all works as I need it to. However I need to run a GROUP BY and an ORDER BY on the returned dataset however I am getting many errors and I'm not sure how to solve it.

Here is my Query:

SELECT ProductID, Quantity 
FROM BasketItems 
UNION ALL 
SELECT ProductID, Quantity 
FROM OrderItems

This will return a results set such as this:

ProductID  Quantity  
15         2
20         2
15         1
8          5
5          1

I then want to run a GROUP BY on the ProductID field and then finally an ORDER BY DESC on the Quantity field. So in the final output, this particular results set will finally result in this:

ProductID
8
15
20
5

I can then run queries on this result set as I usually do

EDIT:

As stated above, but maybe not implied enough is that I will need to run queries on the returned results, which isn't working as you cannot run a query on a set of results that have an ORDER BY clause (so far as I gathered from the error list)

If you want more information on the problem, here it is:

From this results set, I want to get the products from the product table that they relate to

SELECT * FROM Products WHERE ID IN (
    SELECT ProductID
    FROM
    (
        SELECT ProductID, Quantity  
        FROM BasketItems  
        UNION ALL  
        SELECT ProductID, Quantity  
        FROM OrderItems 
    ) v
    GROUP BY ProductID
    ORDER BY SUM(Quantity) DESC
) 

However, I get this error: The ORDER BY clause is invalid in views, inline functions, derived tables, subqueries, and common table expressions, unless TOP, OFFSET or FOR XML is also specified.

The output of products need to be in the order that they are returned in the sub query (By quantity)

Answer 1

SELECT Products.*
FROM Products
     INNER JOIN 
(
    SELECT ProductID, Sum(Quantity) as QuantitySum
    from
    (
        SELECT ProductID, Quantity  
        FROM BasketItems  
        UNION ALL  
        SELECT ProductID, Quantity  
        FROM OrderItems 
    ) v
    GROUP BY ProductID
) ProductTotals
    ON Products.ID = ProductTotals.ProductID
ORDER BY QuantitySum DESC

Answer 2

will this work?

SELECT ProductID
    from
    (
        SELECT ProductID, Quantity  
        FROM BasketItems  
        UNION ALL  
        SELECT ProductID, Quantity  
        FROM OrderItems 
    ) temp
    GROUP BY temp.ProductID
    ORDER BY SUM(temp.Quantity) desc