Tài liệu Filtering for Null Values pptx

[ Team LiB ]Recipe 3.12 Filtering for Null Values Problem You want to filter a DataView for rows that have null values.. The sample code contains two event handlers: Form.Load Sets up

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Recipe 3.12 Filtering for Null Values

Problem

You want to filter a DataView for rows that have null values

Solutions

Use the IS NULL clause with the RowFilters property of the DataView

The sample code contains two event handlers:

Form.Load

Sets up the sample by creating a DataTable containing the Orders table from the Northwind sample database The default view of the table is bound to the data grid

on the form

Filter Button.Click

Toggles on and off the filter on the data view that selects only rows with a null ShipRegion

The C# code is shown in Example 3-12

Example 3-12 File: FilterNullValuesForm.cs

// Namespaces, variables, and constants

using System;

using System.Configuration;

using System.Data;

using System.Data.SqlClient;

// Table name constants

private const String ORDERS_TABLE = "Orders";

// Field name constants

private const String SHIPREGION_FIELD = "ShipRegion";

//

private void FilterNullValuesForm_Load(object sender, System.EventArgs e)

{

// Create and fill the Orders table

DataTable dt = new DataTable(ORDERS_TABLE);

SqlDataAdapter da = new SqlDataAdapter("SELECT * FROM Orders",

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

da.Fill(dt);

// Bind the default view to the grid

dataGrid.DataSource = dt.DefaultView;

}

private void filterButton_Click(object sender, System.EventArgs e)

{

String filter = SHIPREGION_FIELD + " IS NULL";

DataView dv = (DataView)dataGrid.DataSource;

if(filterButton.Text == "Apply Filter")

{

// Apply the filter

dv.RowFilter = filter;

dataGrid.CaptionText =

"Orders table: filtered for null ShipRegion field.";

filterButton.Text = "Remove Filter";

}

else

{

// Remove the filter

dv.RowFilter = "";

dataGrid.CaptionText = "Orders table: no filter.";

filterButton.Text = "Apply Filter";

}

}

Discussion

Every DataTable has a default DataView associated with it that can filter a table for records meeting specific criteria In the solution, the RowFilter property of the

DefaultView is filtered for rows containing a null ShipRegion field The result of

applying the filter is immediately reflected in any controls bound to the DataView object

in addition to the any operations performed on the records within the DataView

Alternatively, you can use the Select( ) method on the DataTable underlying the DataView to retrieve an array of DataRow objects containing only rows with the null ShipRegion using the same filter expression

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