With SQL Server 2005, it is now a lot easier to use paged queries than in previous versions. I will be using the NorthWind database (mostly), so you can also use the examples I have provided. I will keep the examples simple; anything complex will only cause confusion. I will start with "traditional" methods such as SELECT, TOP, and then move on to the specific SQL Server 2005 paging examples.
I was asked a question or a series of questions, "How would you do paging in a SQL? How would you do this with a lot of records, say, 10,000 or more?"
I thought about answers. To be more precise, I thought of more questions and this got me thinking, "This must be a common problem, every developer must have done or solved this. What about paging sizes and working with very large data sets? What about getting results from multiple tables?"
So, I decided to look into these questions with specific reference to SQL Server 2005. The following is by far the easiest way and should be used, but it is rarely this easy.
select * from mytable Where ID between 20 and 30
Top (returns records from the TOP of the result set) is very good at returning a set number of records from each end of a results set. The example below gets the top 10 customers by order qty. This is a very common question on forums.
TOP can also pull a percentage of records, although this isn't discussed here.
select top 10 * from customers -- This is a very basic example. select TOP 10 Customers.CustomerID, Customers.CompanyName, count(*) OrderCount from Customers inner join Orders on Orders.CustomerID = Customers.CustomerID GROUP BY Customers.CustomerID, Customers.CompanyName ORDER BY OrderCount DESC
This is really useful. When you want to pull records 11 to 20 you could use temp tables.
-- SELECT First 30 records in to Temp table SELECT TOP 30 * INTO #TEMP from Customers ORDER BY CompanyName ASC --Select Bottom 10 records in another temp table SELECT TOP 10 * INTO #TEMP2 from #Temp ORDER BY CompanyName DESC -- GET THE RECORDS SELECT * FROM #TEMP2
This is fine for the first few pages or the first few users. If you have users that want to return page after page after page, you end up getting 1000 records to return 10, which is not very efficient. You could also have placed an identity on the first temp table and used a
SELECT statement as an alternative.
Alternative to TOP
There is an alternative to
TOP, which is to use
rowcount with care, as it can lead you into all sorts of problems if it's not turned off.
SET rowcount 10 SELECT * from Customers ORDER BY CompanyName
WITH, ROW_NUMBER and OVER
This is new to SQL Server 2005 and looks really useful. Below is an example to get records 20 to 29 from a results set. It might a bit strange at first, but I will go through the query so you'll see how simple it is.
With Cust AS ( SELECT CustomerID, CompanyName, ROW_NUMBER() OVER (order by CompanyName) as RowNumber FROM Customers ) select * from Cust Where RowNumber Between 20 and 30
WITH in SQL Server 2005 specifies a temporary named result, much like a temporary table in previous versions of SQL Server. However, the import parts are the
ROW_NUMBER and the
OVER statement, which create a row number on each row based on the Company name. This is like adding an identity seed to a temp table with an order by clause.
I hope you are still with me. If not, run the code and view the results. This is really very quick for large tables; I have been impressed with the speed on tables with over 250,000 records.
Putting it All Together in a Stored Procedure
Now we will put it all together in a Stored Procedure that can be used by your application. I won't show a .NET datagrid or similar control, as that is outside the scope of this article. The stored procedure below uses flexible page sizes and page numbers, so you can select any page at random. This is quite useful if you wish to jump ahead 10 pages to find a particular record. The paging for this example starts at page 1 rather than at page 0, but this can be easily changed.
CREATE PROC GetCustomersByPage @PageSize int, @PageNumber int AS Declare @RowStart int Declare @RowEnd int if @PageNumber > 0 Begin SET @PageNumber = @PageNumber -1 SET @RowStart = @PageSize * @PageNumber + 1; SET @RowEnd = @RowStart + @PageSize - 1 ; With Cust AS ( SELECT CustomerID, CompanyName, ROW_NUMBER() OVER (order by CompanyName) as RowNumber FROM Customers ) select * from Cust Where RowNumber >= @RowStart and RowNumber <= @RowEnd end END
To run this, simply specify the page size and page number (
@PageNumber) as shown below.
exec GetCustomersByPage 10, 1