SQL - Subquery in Aggregate Function

Question

I'm using the northwind database to refresh my SQL skills by creating some more or less complex queries. Unfortunately I could not find a solution for my last use case: "Get the sum of the five greatest orders for every category in year 1997."

The tables involved are:

Orders(OrderId, OrderDate)
Order Details(OrderId, ProductId, Quantity, UnitPrice)
Products(ProductId, CategoryId)
Categories(CategoryId, CategoryName)

I have tried the following query

SELECT c.CategoryName, SUM(
  (SELECT TOP 5 od2.UnitPrice*od2.Quantity 
   FROM [Order Details] od2, Products p2
   WHERE od2.ProductID = p2.ProductID
   AND c.CategoryID = p2.CategoryID
   ORDER BY 1 DESC))
FROM [Order Details] od, Products p, Categories c, Orders o 
WHERE od.ProductID = p. ProductID
AND p.CategoryID = c.CategoryID
AND od.OrderID = o.OrderID
AND YEAR(o.OrderDate) = 1997
GROUP BY c.CategoryName

Well... It turned out that subqueries are not allowed in aggregate functions. I've read other posts about this issue but could not find a solution for my specific use case. Hope you can help me out...

Answer 1

Subqueries are not generally allowed in aggregate functions. Instead, move the aggregate inside the subquery. In this case, you'll need an extra level of subquery because of the top 5:

SELECT c.CategoryName,
  (select sum(val)
   from (SELECT TOP 5 od2.UnitPrice*od2.Quantity as val
         FROM [Order Details] od2, Products p2
         WHERE od2.ProductID = p2.ProductID
         AND c.CategoryID = p2.CategoryID
         ORDER BY 1 DESC
        ) t
  )
FROM [Order Details] od, Products p, Categories c, Orders o 
WHERE od.ProductID = p. ProductID
AND p.CategoryID = c.CategoryID
AND od.OrderID = o.OrderID
AND YEAR(o.OrderDate) = 1997
GROUP BY c.CategoryName, c.CategoryId

Answer 2

Use CTE with ROW_NUMBER ranking function instead of excessive subquery.

 ;WITH cte AS
 (
  SELECT c.CategoryName, od2.UnitPrice, od2.Quantity,
         ROW_NUMBER() OVER(PARTITION BY c.CategoryName ORDER BY od2.UnitPrice * od2.Quantity DESC) AS rn
  FROM [Order Details] od JOIN Products p ON od.ProductID = p.ProductID
                          JOIN Categories c ON p.CategoryID = c.CategoryID
                          JOIN Orders o ON od.OrderID = o.OrderID
  WHERE o.OrderDate >= DATEADD(YEAR, DATEDIFF(YEAR, 0, '19970101'), 0)
    AND o.OrderDate < DATEADD(YEAR, DATEDIFF(YEAR, 0, '19970101')+1, 0)
  )
  SELECT CategoryName, SUM(UnitPrice * Quantity) AS val
  FROM cte
  WHERE rn < 6
  GROUP BY CategoryName

Answer 3

Its definitely a sub query problem here is an excellent article on this (originally written for Access but the syntax is identical), also orderdate = 1997 will give order date for 1 jan 1997' -- you need datepart(year, orderdate) = 1997, once you have the (up to five) rows returned for each category you can then encapsulate the rows returned and agregate them

Answer 4

I ran into a very similar problem with an Access subquery where the records were sorted by date. When I used the "Last" aggregate function I found it passed through all of the subqueries and retrieved the last row of data from the Access table, and not the sorted query as intended. Although I could have rewritten the query to use the aggregate function within the first set of parenthesis (as was previously suggested) I found it easier to save query results as a table in the database sorted in the order I wanted and then use the "Last" aggregate function to retrieve the values I wanted. I'll run an update query in the future to keep results current. Not efficient but effective.