I know how derived tables are used, but I still can’t really see any real advantages of using them.
For example, in the following article http://techahead.wordpress.com/2007/10/01/sql-derived-tables/ the author tried to show benefits of a query using derived table over a query without one with an example, where we want to generate a report that shows off the total number of orders each customer placed in 1996, and we want this result set to include all customers, including those that didn’t place any orders that year and those that have never placed any orders at all( he’s using Northwind database ).
But when I compare the two queries, I fail to see any advantages of a query using a derived table ( if nothing else, use of a derived table doesn't appear to simplify our code, at least not in this example):
Regular query:
SELECT C.CustomerID, C.CompanyName, COUNT(O.OrderID) AS TotalOrders
FROM Customers C LEFT OUTER JOIN Orders O ON
C.CustomerID = O.CustomerID AND YEAR(O.OrderDate) = 1996
GROUP BY C.CustomerID, C.CompanyName
Query using a derived table:
SELECT C.CustomerID, C.CompanyName, COUNT(dOrders.OrderID) AS TotalOrders
FROM Customers C LEFT OUTER JOIN
(SELECT * FROM Orders WHERE YEAR(Orders.OrderDate) = 1996) AS dOrders
ON
C.CustomerID = dOrders.CustomerID
GROUP BY C.CustomerID, C.CompanyName
Perhaps this just wasn’t a good example, so could you show me an example where benefits of derived table are more obvious?
thanx
