Using CROSS APPLY in SQL Server 2005

By Bill Graziano on 4 May 2005 | 18 Comments | Tags: Queries, User Defined Functions


My interest in writing this article was started by an MSDN article titled SQL Server 2005: The CLR Enters the Relational Stage. The article shows how to write a function that returns the top three countries per category. That's always been something that was difficult to do in SQL so I was curious about the approach. The article started out well but I was very unhappy by the end. It's just soooo much easier to do this in SQL Server 2005 using the new CROSS APPLY clause in Transact-SQL. So I'm going to write a query to return the top 3 orders for each customer and I'm going to do it in about 10 lines of SQL. (UPDATE: An alert reader found an even better approach!)

First please take a second to read the MSDN article. Pay special attention to how much C# code is required for the solution and how flexible it is.

Note: I also want to mention here that I started out trying to replicate their solution. I tried to install the MSI but that failed with an invalid characters message. I tried to recreate the CLR code but since he mostly posted snippets and had three versions I finally gave up trying to get that to work. One thing I really wanted to do was compare performance on identical data. Unfortunately I wasn't able to do that. My example will use the AdventureWorks database

Let's start at the TOP

Along the way to making this solution work we'll discuss a few new features of SQL Server 2005. The first is that the TOP clause can now take a variable:

DECLARE @Rows INT
SET @Rows = 10

SELECT TOP ( @Rows ) *
FROM Sales.SalesOrderHeader

This will return the top 10 rows from SalesOrderHeader. You can also replace @Rows with anything that evaluates to a number. The following query looks odd but runs just fine:

SELECT TOP ( 
	SELECT COUNT(*)
	FROM Sales.Customer
		 ) *
FROM Sales.SalesOrderHeader

There are 19,185 rows in the Customer table and this query returns the top 19,185 rows from SalesOrderHeader. You can also use the TOP clause for INSERT, UPDATE and DELETE statements. If you wanted to DELETE in batches of 500 you can now do that using the TOP clause.

My Function

Next we need a function to return the TOP X rows from SalesOrderHeader based on the total sales amount. That function looks like this:

CREATE FUNCTION dbo.fn_GetTopOrders(@custid AS int, @n AS INT)
  RETURNS TABLE
AS
RETURN
  SELECT TOP(@n) *
  FROM Sales.SalesOrderHeader
  WHERE CustomerID = @custid
  ORDER BY TotalDue DESC
GO

Notice that it accepts the number of orders to return as a parameter. Also notice that I'm using SELECT * all over the place. I really encourage you to explicitly list out the columns when you write this for real. If you call this with a CustomerID and a number of rows it will return that many rows ordered by the total amount of the order in descending order. Also notice that there is an ORDER BY clause in this function. We'll talk about that in a minute.

Apply the APPLY Clause

The real magic happens when you use SQL Server 2005's new APPLY clause. The APPLY clause let's you join a table to a table-valued-function. That let's you write a query like this:

SELECT  C.CustomerID, 
	O.SalesOrderID,
	O.TotalDue
FROM 
	AdventureWorks.Sales.Customer AS C
CROSS APPLY
	AdventureWorks.dbo.fn_GetTopOrders(C.CustomerID, 3) AS O
ORDER BY 
	CustomerID ASC, TotalDue DESC

which results in this...

CustomerID  SalesOrderID TotalDue
----------- ------------ ---------------------
1           45283        37643.1378
1           46042        34722.9906
1           44501        26128.8674
2           46976        10184.0774
2           47997        5469.5941
2           57044        4537.8484
3           53616        92196.9738
3           47439        78578.9054
3           48378        56574.3871
4           47658        132199.8023
. . .

The APPLY clause acts like a JOIN without the ON clause comes in two flavors: CROSS and OUTER. The OUTER APPLY clause returns all the rows on the left side (Customers) whether they return any rows in the table-valued-function or not. The columns that the table-valued-function returns are null if no rows are returned. The CROSS APPLY only returns rows from the left side (Customers) if the table-valued-function returns rows.

Notice that I'm just passing in the CustomerID to the function. It returns the TOP 3 rows based on the amount of the order. Since I'm using CROSS APPLY a customer without orders won't appear in the list. I can also pass in a number other than 3 to easily return a different number of orders per customer. Even better I can pass in a different number of orders for each customer. So I could list the top 5 orders for one type of customer but the top 10 for another type of customer. How cool is that?!?

And it gets even better. Remember the function has an ORDER BY in it. It will always return the top orders based on the amount. However you can change the ORDER BY clause in the query that calls the function to display those rows in whatever order you want. You could easily display the top 3 orders in ascending order instead.

Performance and Conclusion

Unfortunately I wasn't able to compare the performance of my solution to MSDN's. The query plan from my solution doesn't look that bad. (And did I mention that it showed me a full query plan for the query with the UDF rather than just a UDF step? Sweet!) If anyone gets their solution running send me the code and I'll update the article.

I also think their solution returns the result as a comma separated value. Now that would be a great use for the CLR. You could easily wrap that around the results of this function for a fast, effecient procedure. And it happens to be the next article I'm working on. :) If you want to see a great article on using the CLR inside SQL Server I'd start with Using CLR Integration in SQL Server 2005. The code is from an earlier beta but it gives you a great idea of when to use CLR code.

In summary I think that 5-10 lines of SQL is much simpler than 50+ lines of C# (or VB) for this type of solution. The article provides a lot of good information on the CLR inside SQL Server. I just don't think this is the best place to use it. One of my biggest fears is that people will use the CLR incorrectly and cause performance problems. This is the first real case I've seen of this. Is this really how Microsoft wants us to use the CLR?

UPDATE

I was curious when I posted this how long it would take to find an even better solution. It didn't take long. Arnold Fribble posted it in the forums. The syntax needed a little tweaking but it appears you can do this in a single query. And how did I forget the PARTITION BY clause of the ROW_NUMBER function. Doh! Here's the cleaned up query:

SELECT 
	CustomerID,
	SalesOrderID,
	TotalDue
FROM (
	SELECT
		O.CustomerID,
		O.SalesOrderID, 
		O.TotalDue, 
		ROW_NUMBER = ROW_NUMBER() OVER (
			PARTITION BY O.CustomerID 
			ORDER BY O.TotalDue DESC)
	FROM	
		Sales.SalesOrderHeader AS O
	) AS d
WHERE
	d.ROW_NUMBER <= 3

You have to use a derived table (or a Common Table Expression) since the row number function can't be used in the WHERE clause of the query where it's defined. Now I'll have to write an article on the new ranking functions. :) Thanks Arnold!

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