Sorting Custom Paged Data (VB)

by Scott Mitchell

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In the previous tutorial we learned how to implement custom paging when presenting data on a web page. In this tutorial we see how to extend the preceding example to include support for sorting custom paging.

Introduction

Compared to default paging, custom paging can improve the performance of paging through data by several orders of magnitude, making custom paging the de facto paging implementation choice when paging through large amounts of data. Implementing custom paging is more involved than implementing default paging, however, especially when adding sorting to the mix. In this tutorial we'll extend the example from the preceding one to include support for sorting and custom paging.

Note

Since this tutorial builds upon the preceding one, before beginning take a moment to copy the declarative syntax within the <asp:Content> element from the preceding tutorial s web page (EfficientPaging.aspx) and paste it between the <asp:Content> element in the SortParameter.aspx page. Refer back to Step 1 of the Adding Validation Controls to the Editing and Inserting Interfaces tutorial for a more detailed discussion on replicating the functionality of one ASP.NET page to another.

Step 1: Reexamining the Custom Paging Technique

For custom paging to work properly, we must implement some technique that can efficiently grab a particular subset of records given the Start Row Index and Maximum Rows parameters. There are a handful of techniques that can be used to achieve this aim. In the preceding tutorial we looked at accomplishing this using Microsoft SQL Server 2005 s new ROW_NUMBER() ranking function. In short, the ROW_NUMBER() ranking function assigns a row number to each row returned by a query that is ranked by a specified sort order. The appropriate subset of records is then obtained by returning a particular section of the numbered results. The following query illustrates how to use this technique to return those products numbered 11 through 20 when ranking the results ordered alphabetically by the ProductName:

SELECT ProductID, ProductName, ...
FROM
   (SELECT ProductID, ProductName, ..., ROW_NUMBER() OVER
    (ORDER BY ProductName) AS RowRank
    FROM Products) AS ProductsWithRowNumbers
WHERE RowRank > 10 AND RowRank <= 20

This technique works well for paging using a specific sort order (ProductName sorted alphabetically, in this case), but the query needs to be modified to show the results sorted by a different sort expression. Ideally, the above query could be rewritten to use a parameter in the OVER clause, like so:

SELECT ProductID, ProductName, ...
FROM
   (SELECT ProductID, ProductName, ..., ROW_NUMBER() OVER
    (ORDER BY @sortExpression) AS RowRank
    FROM Products) AS ProductsWithRowNumbers
WHERE RowRank > 10 AND RowRank <= 20

Unfortunately, parameterized ORDER BY clauses are not allowed. Instead, we must create a stored procedure that accepts a @sortExpression input parameter, but uses one of the following workarounds:

• Write hard-coded queries for each of the sort expressions that may be used; then, use IF/ELSE T-SQL statements to determine which query to execute.
• Use a CASE statement to provide dynamic ORDER BY expressions based on the @sortExpressio n input parameter; see the Used to Dynamically Sort Query Results section in T-SQL CASE Statements for more information.
• Craft the appropriate query as a string in the stored procedure and then use the sp_executesql system stored procedure to execute the dynamic query.

Each of these workarounds has some drawbacks. The first option is not as maintainable as the other two as it requires that you create a query for each possible sort expression. Therefore, if later you decide to add new, sortable fields to the GridView you will also need to go back and update the stored procedure. The second approach has some subtleties that introduce performance concerns when sorting by non-string database columns and also suffers from the same maintainability issues as the first. And the third choice, which uses dynamic SQL, introduces the risk for a SQL injection attack if an attacker is able to execute the stored procedure passing in the input parameter values of their choosing.

While none of these approaches is perfect, I think the third option is the best of the three. With its use of dynamic SQL, it offers a level of flexibility the other two do not. Furthermore, a SQL injection attack can only be exploited if an attacker is able to execute the stored procedure passing in the input parameters of his choice. Since the DAL uses parameterized queries, ADO.NET will protect those parameters that are sent to the database through the architecture, meaning that the SQL injection attack vulnerability only exists if the attacker can directly execute the stored procedure.

To implement this functionality, create a new stored procedure in the Northwind database named GetProductsPagedAndSorted. This stored procedure should accept three input parameters: @sortExpression, an input parameter of type nvarchar(100) that specifies how the results should be sorted and is injected directly after the ORDER BY text in the OVER clause; and @startRowIndex and @maximumRows, the same two integer input parameters from the GetProductsPaged stored procedure examined in the preceding tutorial. Create the GetProductsPagedAndSorted stored procedure using the following script:

CREATE PROCEDURE dbo.GetProductsPagedAndSorted
(
    @sortExpression nvarchar(100),
    @startRowIndex int,
    @maximumRows int
)
AS
-- Make sure a @sortExpression is specified
IF LEN(@sortExpression) = 0
    SET @sortExpression = 'ProductID'
-- Issue query
DECLARE @sql nvarchar(4000)
SET @sql = 'SELECT ProductID, ProductName, SupplierID, CategoryID, QuantityPerUnit,
                   UnitPrice, UnitsInStock, UnitsOnOrder, ReorderLevel, Discontinued,
                   CategoryName, SupplierName
            FROM (SELECT ProductID, ProductName, p.SupplierID, p.CategoryID,
                         QuantityPerUnit, UnitPrice, UnitsInStock, UnitsOnOrder,
                         ReorderLevel, Discontinued,
                  c.CategoryName, s.CompanyName AS SupplierName,
                   ROW_NUMBER() OVER (ORDER BY ' + @sortExpression + ') AS RowRank
            FROM Products AS p
                    INNER JOIN Categories AS c ON
                        c.CategoryID = p.CategoryID
                    INNER JOIN Suppliers AS s ON
                        s.SupplierID = p.SupplierID) AS ProductsWithRowNumbers
            WHERE     RowRank > ' + CONVERT(nvarchar(10), @startRowIndex) +
                ' AND RowRank <= (' + CONVERT(nvarchar(10), @startRowIndex) + ' + '
                + CONVERT(nvarchar(10), @maximumRows) + ')'
-- Execute the SQL query
EXEC sp_executesql @sql

The stored procedure starts by ensuring that a value for the @sortExpression parameter has been specified. If it is missing, the results are ranked by ProductID. Next, the dynamic SQL query is constructed. Note that the dynamic SQL query here differs slightly from our previous queries used to retrieve all rows from the Products table. In prior examples, we obtained each product s associated category s and supplier s names using a subquery. This decision was made back in the Creating a Data Access Layer tutorial and was done in lieu of using JOIN s because the TableAdapter cannot automatically create the associated insert, update, and delete methods for such queries. The GetProductsPagedAndSorted stored procedure, however, must use JOIN s for the results to be ordered by the category or supplier names.

This dynamic query is built up by concatenating the static query portions and the @sortExpression, @startRowIndex, and @maximumRows parameters. Since @startRowIndex and @maximumRows are integer parameters, they must be converted into nvarchars in order to be correctly concatenated. Once this dynamic SQL query has been constructed, it is executed via sp_executesql.

Take a moment to test this stored procedure with different values for the @sortExpression, @startRowIndex, and @maximumRows parameters. From the Server Explorer, right-click on the stored procedure name and choose Execute. This will bring up the Run Stored Procedure dialog box into which you can enter the input parameters (see Figure 1). To sort the results by the category name, use CategoryName for the @sortExpression parameter value; to sort by the supplier s company name, use CompanyName. After providing the parameters values, click OK. The results are displayed in the Output window. Figure 2 shows the results when returning products ranked 11 through 20 when ordering by the UnitPrice in descending order.

Figure 1: Try Different Values for the Stored Procedure s Three Input Parameters

Figure 2: The Stored Procedure s Results are Shown in the Output Window (Click to view full-size image)

Note

When ranking the results by the specified ORDER BY column in the OVER clause, SQL Server must sort the results. This is a quick operation if there is a clustered index over the column(s) the results are being ordered by or if there is a covering index, but can be more costly otherwise. To improve performance for sufficiently large queries, consider adding a non-clustered index for the column by which the results are ordered by. Refer to Ranking Functions and Performance in SQL Server 2005 for more details.

Step 2: Augmenting the Data Access and Business Logic Layers

With the GetProductsPagedAndSorted stored procedure created, our next step is to provide a means to execute that stored procedure through our application architecture. This entails adding an appropriate method to both the DAL and BLL. Let s start by adding a method to the DAL. Open the Northwind.xsd Typed DataSet, right-click on the ProductsTableAdapter, and choose the Add Query option from the context menu. As we did in the preceding tutorial, we want to configure this new DAL method to use an existing stored procedure - GetProductsPagedAndSorted, in this case. Start by indicating that you want the new TableAdapter method to use an existing stored procedure.

Figure 3: Choose to Use an Existing Stored Procedure

To specify the stored procedure to use, select the GetProductsPagedAndSorted stored procedure from the drop-down list in the next screen.

Figure 4: Use the GetProductsPagedAndSorted Stored Procedure

This stored procedure returns a set of records as its results so, in the next screen, indicate that it returns tabular data.

Figure 5: Indicate that the Stored Procedure Returns Tabular Data

Finally, create DAL methods that use both the Fill a DataTable and Return a DataTable patterns, naming the methods FillPagedAndSorted and GetProductsPagedAndSorted, respectively.

Figure 6: Choose the Methods Names

Now that we ve extended the DAL, we re ready to turn to the BLL. Open the ProductsBLL class file and add a new method, GetProductsPagedAndSorted. This method needs to accept three input parameters sortExpression, startRowIndex, and maximumRows and should simply call down into the DAL s GetProductsPagedAndSorted method, like so:

<System.ComponentModel.DataObjectMethodAttribute( _
    System.ComponentModel.DataObjectMethodType.Select, False)> _
Public Function GetProductsPagedAndSorted(ByVal sortExpression As String, _
    ByVal startRowIndex As Integer, ByVal maximumRows As Integer) _
    As Northwind.ProductsDataTable
    Return Adapter.GetProductsPagedAndSorted(sortExpression, startRowIndex, maximumRows)
End Function

Step 3: Configuring the ObjectDataSource to Pass in the SortExpression Parameter

Having augmented the DAL and BLL to include methods that utilize the GetProductsPagedAndSorted stored procedure, all that remains is to configure the ObjectDataSource in the SortParameter.aspx page to use the new BLL method and to pass in the SortExpression parameter based on the column that the user has requested to sort the results by.

Start by changing the ObjectDataSource s SelectMethod from GetProductsPaged to GetProductsPagedAndSorted. This can be done through the Configure Data Source wizard, from the Properties window, or directly through the declarative syntax. Next, we need to provide a value for the ObjectDataSource s SortParameterName property. If this property is set, the ObjectDataSource attempts to pass in the GridView s SortExpression property to the SelectMethod. In particular, the ObjectDataSource looks for an input parameter whose name is equal to the value of the SortParameterName property. Since the BLL s GetProductsPagedAndSorted method has the sort expression input parameter named sortExpression, set the ObjectDataSource s SortExpression property to sortExpression .

After making these two changes, the ObjectDataSource s declarative syntax should look similar to the following:

<asp:ObjectDataSource ID="ObjectDataSource1" runat="server"
    OldValuesParameterFormatString="original_{0}" TypeName="ProductsBLL"
    SelectMethod="GetProductsPagedAndSorted" EnablePaging="True"
    SelectCountMethod="TotalNumberOfProducts" SortParameterName="sortExpression">
</asp:ObjectDataSource>

Note

As with the preceding tutorial, ensure that the ObjectDataSource does not include the sortExpression, startRowIndex, or maximumRows input parameters in its SelectParameters collection.

To enable sorting in the GridView, simply check the Enable Sorting checkbox in the GridView s smart tag, which sets the GridView s AllowSorting property to true and causing the header text for each column to be rendered as a LinkButton. When the end user clicks on one of the header LinkButtons, a postback ensues and the following steps transpire:

1. The GridView updates its SortExpression property to the value of the SortExpression of the field whose header link was clicked
2. The ObjectDataSource invokes the BLL s GetProductsPagedAndSorted method, passing in the GridView s SortExpression property as the value for the method s sortExpression input parameter (along with the appropriate startRowIndex and maximumRows input parameter values)
3. The BLL invokes the DAL s GetProductsPagedAndSorted method
4. The DAL executes the GetProductsPagedAndSorted stored procedure, passing in the @sortExpression parameter (along with the @startRowIndex and @maximumRows input parameter values)
5. The stored procedure returns the appropriate subset of data to the BLL, which returns it to the ObjectDataSource; this data is then bound to the GridView, rendered into HTML, and sent down to the end user

Figure 7 shows the first page of results when sorted by the UnitPrice in ascending order.

Figure 7: The Results are Sorted by the UnitPrice (Click to view full-size image)

While the current implementation can correctly sort the results by product name, category name, quantity per unit, and unit price, attempting to order the results by the supplier name results in a runtime exception (see Figure 8).

Figure 8: Attempting to Sort the Results by the Supplier Results in the Following Runtime Exception

This exception occurs because the SortExpression of the GridView s SupplierName BoundField is set to SupplierName. However, the supplier s name in the Suppliers table is actually called CompanyName we have been aliased this column name as SupplierName. However, the OVER clause used by the ROW_NUMBER() function cannot use the alias and must use the actual column name. Therefore, change the SupplierName BoundField s SortExpression from SupplierName to CompanyName (see Figure 9). As Figure 10 shows, after this change the results can be sorted by the supplier.

Figure 9: Change the SupplierName BoundField s SortExpression to CompanyName

Figure 10: The Results Can Now Be Sorted by Supplier (Click to view full-size image)

Summary

The custom paging implementation we examined in the preceding tutorial required that the order by which the results were to be sorted be specified at design time. In short, this meant that the custom paging implementation we implemented could not, at the same time, provide sorting capabilities. In this tutorial we overcame this limitation by extending the stored procedure from the first to include a @sortExpression input parameter by which the results could be sorted.

After creating this stored procedure and creating new methods in the DAL and BLL, we were able to implement a GridView that offered both sorting and custom paging by configuring the ObjectDataSource to pass in the GridView s current SortExpression property to the BLL SelectMethod.

Happy Programming!

About the Author

Scott Mitchell, author of seven ASP/ASP.NET books and founder of 4GuysFromRolla.com, has been working with Microsoft Web technologies since 1998. Scott works as an independent consultant, trainer, and writer. His latest book is Sams Teach Yourself ASP.NET 2.0 in 24 Hours. He can be reached at mitchell@4GuysFromRolla.com. or via his blog, which can be found at http://ScottOnWriting.NET.

Special Thanks To

This tutorial series was reviewed by many helpful reviewers. Lead reviewer for this tutorial was Carlos Santos. Interested in reviewing my upcoming MSDN articles? If so, drop me a line at mitchell@4GuysFromRolla.com.

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