Tài liệu Executing Queries That Use COMPUTE BY pptx

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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 ]