Tài liệu Counting Records in a DataReader ppt

[ Team LiB ] Recipe 2.7 Counting Records in a DataReader Problem You want to determine how many records there are in a DataReader. Solution Use one of the following three techniques: • Iterate over the rows in the DataReader. • Issue a COUNT(*) query as part of a batch query. Note that not all data sources support batch queries. If not, execute the statements separately one after the other for a similar result. • Use the @@ROWCOUNT function to return the number or rows in a DataReader after the DataReader has been closed. This technique is SQL Server specific. The sample code uses a single stored procedure: SP0207_GetOrders Returns a result set containing all records in the Orders table in Northwind. Also, the stored procedure returns the @@ROWCOUNT value for the query in an output parameter. The stored procedure is shown in Example 2-7 . Example 2-7. Stored procedure: SP0207_GetOrders ALTER PROCEDURE SP0207_GetOrders @RowCount int output AS set nocount on select * from Orders set @RowCount = @@ROWCOUNT RETURN The C# code is shown in Example 2-8 . Example 2-8. File: DataReaderRowCountForm.cs // Namespaces, variables, and constants using System; using System.Configuration; using System.Data; using System.Data.SqlClient; // . . . // Batch query to retrieve the COUNT of records and // all of the records in the Orders table as two result sets. String sqlText = "SELECT COUNT(*) FROM Orders; " + "SELECT * FROM Orders;"; // Create the connection. SqlConnection conn = new SqlConnection( ConfigurationSettings.AppSettings["Sql_ConnectString"]); SqlCommand cmd = new SqlCommand(sqlText, conn); conn.Open( ); // Create a DataReader on the first result set. SqlDataReader dr = cmd.ExecuteReader( ); // Get the count of records from the select count(*) statement. dr.Read( ); resultTextBox.Text = "Orders table record count, using COUNT(*)= " + dr.GetInt32(0) + Environment.NewLine; // Move to the data result set. dr.NextResult( ); int count = 0; // Iterate over the records in the DataReader. while(dr.Read( )) { count++; // . . . Do something interesting with the data here. } // Close the DataReader and the connection. dr.Close( ); resultTextBox.Text += "Orders table record count, " + "iterating over result set = " + count + Environment.NewLine; // Create the stored procedure to use in the DataReader. cmd = new SqlCommand("SP0207_GetOrders", conn); cmd.CommandType = CommandType.StoredProcedure; // Create the output paramter to return @@ROWCOUNT. cmd.Parameters.Add("@RowCount", SqlDbType.Int).Direction = ParameterDirection.Output; // Create a DataReader for the result set returned by // the stored procedure. dr = cmd.ExecuteReader( ); // . . . Process the data in the DataReader. // Close the DataReader. dr.Close( ); // The output parameter containing the row count is now available. resultTextBox.Text += "Orders table record count, " + "returning @@ROWCOUNT from stored procedure = " + cmd.Parameters["@RowCount"].Value; conn.Close( ); Discussion The DataReader provides forward-only, read-only access to a stream of rows from a data source. It is optimized for performance by reading data directly from a connection to a data source. As a result, there is no way to determine the number of records in the result set for the DataReader without iterating through all of the records. Additionally, because the DataReader is forward-only, you cannot move backwards in DataReader so when iterating, you must process data at the same time. This technique provides the only accurate count of records in the DataReader. A second technique demonstrated in the solution counts the records using the COUNT aggregate function for the command text used to build the DataReader. This technique can have discrepancies with the number of records actually in the DataReader because of the timing lag between issuing the COUNT function and creating the DataReader. The solution also demonstrates using the SQL Server @@ROWCOUNT variable, which returns the number of rows affected by the previous statement, to return the number of records in the result set used to create the DataReader. The count is returned as an output parameter and is therefore not available until the DataReader is closed. Although this does not improve the availability of the record count, centralizing the count in the stored procedure is less prone to coding errors than the counting approach. For more information about output parameters, see Recipe 2.9 . The HasRows( ) method of the DataReader was introduced in Version 1.1 of the .NET Framework. It returns a Boolean value indicating whether the DataReader contains at least one row. There is also no way to determine the number of result sets in a DataReader built using a batch query without iterating over the result sets using the NextResult( ) method. [ Team LiB ] . returns a Boolean value indicating whether the DataReader contains at least one row. There is also no way to determine the number of result sets in a DataReader. [ Team LiB ] Recipe 2.7 Counting Records in a DataReader Problem You want to determine how many records there are in a DataReader. Solution