Tài liệu Using Parameterized SQL Statements ppt

The sample code contains two event handlers and one method: Form.Load Sets up the sample by creating a DataTable containing all Customers data from Northwind.. The default view of the ta

[ Team LiB ]

Recipe 2.21 Using Parameterized SQL Statements

Problem

You want to create and execute a SQL statement having parameters that are set

dynamically

Solution

Add parameters to the Command object's Parameters collection

The sample code contains two event handlers and one method:

Form.Load

Sets up the sample by creating a DataTable containing all Customers data from Northwind The default view of the table is bound to a Customers data grid on the form The handler for the CurrentCellChanged event of the data grid is called to initialize the grid containing Orders with the data for the row selected by default in the Customers data grid

DataGrid.CurrentCellChanged

Gets the CustomerID from the data grid when the rows selected in the data grid changes and calls the LoadOrderGrid( ) method to update the Orders displayed to match the selected Customer

LoadOrderGrid( )

This method defines a parameterized SQL statement A Command is built from the statement and the single parameter, @CustomerID is created and set to the customerId argument passed into the method The Command is used by a

DataAdapter to fill a DataTable with the Orders for the specified Customer The default view of the table is bound to the Customers data grid on the form

The C# code is shown in Example 2-30

Example 2-30 File: UsingParameterizedQueriesForm.cs

// Namespaces, variables, and constants

using System;

using System.Configuration;

using System.Data;

using System.Data.SqlClient;

// Table name constants

private const String CUSTOMERS_TABLE = "Customers";

private const String ORDERS_TABLE = "Orders";

//

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

{

String sqlText = "SELECT * FROM Customers";

// Retrieve table with all customers

SqlDataAdapter da = new SqlDataAdapter(sqlText,

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

DataTable dt = new DataTable(CUSTOMERS_TABLE);

da.Fill(dt);

// Bind the default view of the Customers table to the customers grid customerDataGrid.DataSource = dt.DefaultView;

// Fire the CurrentCellChanged event to refresh the orders grid customerDataGrid_CurrentCellChanged(null, null);

}

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

{

// Get the current row in the customers grid

int row = customerDataGrid.CurrentRowIndex;

// Get the customer ID from the view

String customerId =

((DataView)customerDataGrid.DataSource)

Table.Rows[row][0].ToString( );

// Retrieve the orders for the customer

LoadOrderGrid(customerId);

}

private void LoadOrderGrid(String customerId)

{

String sqlText = "SELECT * FROM Orders " +

"WHERE CustomerID = @CustomerID";

// Create a connection and parameterized command

SqlConnection conn = new SqlConnection(

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

SqlCommand cmd = new SqlCommand(sqlText, conn);

// Add the CustomerID parameter and set its value

cmd.Parameters.Add("@CustomerID", SqlDbType.NChar, 5);

cmd.Parameters["@CustomerID"].Value = customerId;

// Get the Orders result set for the Customer

SqlDataAdapter da = new SqlDataAdapter(cmd);

DataTable dt = new DataTable(ORDERS_TABLE);

da.Fill(dt);

// Bind the default view of the orders table to the orders grid

orderDataGrid.DataSource = dt.DefaultView;

// Set the caption of the orders grid

orderDataGrid.CaptionText = "Orders [CustomerID: " + customerId + "]";

}

Discussion

Parameterized queries allow one or more parameters to be replaced at runtime using Parameter objects in the ParameterCollection class of the Command object These can also be the Command classes exposed by the DataAdapter Using parameters is both easier than and less prone to errors than dynamically building queries You're not

responsible for creating delimeters such as single quotes around strings and pound signs around dates Code is reusable and not specific to the data provider

The SQL Server data provider uses the parameter names in the query and order is not important The OLE DB data provider uses positional parameter markers, the question mark (?), and order is important Consult the documentation for other NET data

providers for information about using parameters in queries

[ Team LiB ]