Tài liệu Retrieving a Single Value from a Query pdf

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Recipe 9.10 Retrieving a Single Value from a Query
Problem
Given a stored procedure that returns a single value, you need the fastest way to get this
data.
Solution
Use the ExecuteScalar( ) method to return a single value from a stored procedure.
The sample code uses the ExecuteScalar( ) method to get the number of records in the
Orders table of the Northwind database.
The C# code is shown in Example 9-13
.
Example 9-13. File: ExecuteScalarForm.cs
// Namespaces, variables, and constants
using System;
using System.Configuration;
using System.Data.SqlClient;

// . . .

String sqlText = "SELECT COUNT(*) FROM Orders";

// Create the connection and the command.
SqlConnection conn = new SqlConnection(
ConfigurationSettings.AppSettings["Sql_ConnectString"]);
SqlCommand cmd = new SqlCommand(sqlText, conn);
conn.Open( );
// Execute the scalar SQL statement and store results.
int count = Convert.ToInt32(cmd.ExecuteScalar( ));

conn.Close( );

resultTextBox.Text="Count of Orders records: " + count;
Discussion
The ExecuteScalar( ) method of the Command object returns a single value from the data
source rather than a table or data stream. While the ExecuteScalar( ) method does not
result in a performance improvement when compared to retrieving a single value using an
output parameter or using a DataReader, it allows a single value to be returned with the
least code and may therefore improve readability and maintainability.
If the result set returns more than one result, the first column of the first row is returned
as a scalar value. A null reference is returned if the result set is empty or if the result set
is a Ref Cursor when using the Oracle .NET data provider.

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