Tài liệu Executing Queries That Use COMPUTE BY pptx

[ Team LiB ] Recipe 3.13 Executing Queries That Use COMPUTE BY Problem The SQL Server .NET data provider does not support the COMPUTE BY clause but you want to execute a COMPUTE BY statement using ADO.NET. Solution Use the COMPUTE BY statement from the Command object of the OLE DB .NET data provider. The sample code defines a COMPUTE BY statement and executes it using the ExecuteReader( ) method of the OleDbCommand object. Multiple result sets are returned by the DataReader and then these are displayed. The C# code is shown in Example 3-13 . Example 3-13. File: ComputeForm.cs // Namespaces, variables, and constants using System; using System.Text; using System.Data; using System.Data.OleDb; // . . . StringBuilder result = new StringBuilder( ); String sqlSelect = "select OrderID, ProductID, Quantity " + "FROM [Order Details] " + "ORDER BY ProductID " + "COMPUTE SUM(quantity) by ProductID"; OleDbConnection conn = new OleDbConnection( ConfigurationSettings.AppSettings["OleDb_Shape_ConnectString"]); OleDbCommand cmd = new OleDbCommand(sqlSelect, conn); conn.Open( ); OleDbDataReader dr = cmd.ExecuteReader( ); do { result.Append("Order\tProduct\tQuantity" + Environment.NewLine); while(dr.Read( )) { result.Append(dr.GetInt32(0) + "\t" + dr.GetInt32(1) + "\t" + dr.GetInt16(2) + Environment.NewLine); } // Get the sum. dr.NextResult( ); dr.Read( ); result.Append("SUM\t\t" + dr.GetInt32(0) + Environment.NewLine); result.Append(Environment.NewLine); } while(dr.NextResult( )); dr.Close( ); conn.Close( ); resultTextBox.Text = result.ToString( ); Discussion The SQL Server .NET data provider does not support the COMPUTE BY clause, but the OLE DB .NET data provider does. The results are returned as multiple pairs of result sets, the first of which contains the selected details and the second containing the results of the aggregate functions specified (the sum of the quantity ordered for the product in this example) in the COMPUTE BY clause. This pattern is repeated for the remaining pairs of result sets. Microsoft states that the COMPUTE and COMPUTE BY clauses are provided in SQL Server 7.0 and later versions for backward compatibility. The ROLLUP operator provides similar functionality and is recommended instead. The main difference is that ROLLUP returns a single result set instead of multiple result sets. For more information about the ROLLUP operator, see Microsoft SQL Server Books Online. [ Team LiB ] . 3.13 Executing Queries That Use COMPUTE BY Problem The SQL Server .NET data provider does not support the COMPUTE BY clause but you want to execute a COMPUTE. in the COMPUTE BY clause. This pattern is repeated for the remaining pairs of result sets. Microsoft states that the COMPUTE and COMPUTE BY clauses are