Tài liệu Counting Records pdf

[ Team LiB ]


Recipe 10.5 Counting Records
Problem
You want to determine how many rows that meet certain criteria are in a table.
Solution
Use the ExecuteScalar( ) method of the Command object to determine the number of
records in the table.
The sample code executes the COUNT function on the results of a query returning rows
from the Orders table in the Northwind sample database, where the rows match a user-
specified CustomerID.
The C# code is shown in Example 10-5
.
Example 10-5. File: CountRecordForm.cs
// Namespaces, variables, and constants
using System;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;

// . . .

// Create the connection.
SqlConnection conn = new SqlConnection(
ConfigurationSettings.AppSettings["Sql_ConnectString"]);

// Build the query to count, including CustomerID criteria if specified.
String selectText = "SELECT COUNT(*) FROM Orders";
if(customerIdTextBox.Text.Trim( ) != "")
selectText += " WHERE CustomerId='" + customerIdTextBox.Text + "'";

// Create the command to count the records.
SqlCommand cmd = new SqlCommand(selectText, conn);
// Execute the command, storing the results.
conn.Open( );
int recordCount = (int)cmd.ExecuteScalar( );
conn.Close( );
Discussion
The ExecuteScalar( ) method of the Command object returns a single value from a query
rather than a table or a data stream. If the query returns a result set, this method returns
the value of the first column of the first row.
The number of records matching certain criteria can be determined by executing a SQL
statement that returns the COUNT(*) aggregate function and including a WHERE clause
that specifies the criteria. Use the ExecuteScalar( ) method to execute the statement and
return the count. Cast the result to an integer data type.
This technique can also be used with other aggregate functions to determine values such
as averages or sums. For more information about aggregate functions, see Microsoft SQL
Server Books Online.

[ Team LiB ]