Tài liệu Filtering for Null Values pptx

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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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