I've decided that my colleague's approach (which does not rely on a
subquery) is the most efficient. I can't really think of a reason why the
subquery needs to hang around in my SQL.
My colleague had written
Select TOP 5 CompanyName, Sum(Quantity * UnitPrice) as Sales
FROM Customers
JOIN Orders
ON Customers.CustomerID = Orders.CustomerID
JOIN [Order Details]
ON [Order Details].OrderID = Orders.OrderID
GROUP BY CompanyName
ORDER BY Sales Desc
Which, on further reflection, feels more efficient than :
Select Top 5 CompanyName, Sales
FROM (
SELECT CompanyName, Sum(Quantity*UnitPrice) as Sales
FROM Customers
JOIN Orders
ON Customers.CustomerID = Orders.CustomerID
JOIN [Order Details]
ON [Order Details].OrderID = Orders.OrderID
GROUP BY CompanyName
) As CustomerSales
ORDER BY Sales Desc