Manipulating Data

Before the introduction of Multiple Active Result Sets (MARS), developers had to use either multiple connections or server-side cursors to solve certain scenarios. In addition, when multiple connections were used in a transactional situation, bound connections (with sp_getbindtoken and sp_bindsession) were required. The following scenarios show how to use a MARS-enabled connection instead of multiple connections.

Using Multiple Commands with MARS

The following Console application demonstrates how to use two SqlDataReader objects with two SqlCommand objects and a single SqlConnection object with MARS enabled.

Example

The example opens a single connection to the AdventureWorks database. Using a SqlCommand object, a SqlDataReader is created. As the reader is used, a second SqlDataReader is opened, using data from the first SqlDataReader as input to the WHERE clause for the second reader.

Option Strict On  
Option Explicit On  
  
Imports System  
Imports System.Data  
Imports System.Data.SqlClient  
Module Module1  
  Sub Main()  
    ' By default, MARS is disabled when connecting  
    ' to a MARS-enabled host.  
    ' It must be enabled in the connection string.  
    Dim connectionString As String = GetConnectionString()  
  
    Dim vendorID As Integer  
  
    Dim vendorCmd As SqlCommand  
    Dim productCmd As SqlCommand  
    Dim productReader As SqlDataReader  
  
    Dim vendorSQL As String = & _
      "SELECT VendorId, Name FROM Purchasing.Vendor"  
    Dim productSQL As String = _  
        "SELECT Production.Product.Name FROM Production.Product " & _  
        "INNER JOIN Purchasing.ProductVendor " & _  
        "ON Production.Product.ProductID = " & _  
        "Purchasing.ProductVendor.ProductID " & _  
        "WHERE Purchasing.ProductVendor.VendorID = @VendorId"  
  
    Using awConnection As New SqlConnection(connectionString)  
      vendorCmd = New SqlCommand(vendorSQL, awConnection)  
      productCmd = New SqlCommand(productSQL, awConnection)  
      productCmd.Parameters.Add("@VendorId", SqlDbType.Int)  
  
      awConnection.Open()  
      Using vendorReader As SqlDataReader = vendorCmd.ExecuteReader()  
        While vendorReader.Read()  
          Console.WriteLine(vendorReader("Name"))  
  
          vendorID = CInt(vendorReader("VendorId"))  
  
          productCmd.Parameters("@VendorId").Value = vendorID  
  
          ' The following line of code requires  
          ' a MARS-enabled connection.  
          productReader = productCmd.ExecuteReader()  
          Using productReader  
            While productReader.Read()  
              Console.WriteLine("  " & CStr(productReader("Name")))  
            End While  
          End Using  
        End While  
      End Using  
    End Using  
  
    Console.WriteLine("Press any key to continue")  
    Console.ReadLine()  
  End Sub  
  
  Function GetConnectionString() As String  
    ' To avoid storing the connection string in your code,  
    ' you can retrieve it from a configuration file.  
    Return "Data Source=(local);Integrated Security=SSPI;" & _  
      "Initial Catalog=AdventureWorks; MultipleActiveResultSets=True"  
  End Function  
End Module  

using System;  
using System.Data;  
using System.Data.SqlClient;  
  
class Class1  
{  
static void Main()  
{  
  // By default, MARS is disabled when connecting  
  // to a MARS-enabled host.  
  // It must be enabled in the connection string.  
  string connectionString = GetConnectionString();  
  
  int vendorID;  
  SqlDataReader productReader = null;  
  string vendorSQL =
    "SELECT VendorId, Name FROM Purchasing.Vendor";  
  string productSQL =
    "SELECT Production.Product.Name FROM Production.Product " +  
    "INNER JOIN Purchasing.ProductVendor " +  
    "ON Production.Product.ProductID = " +
    "Purchasing.ProductVendor.ProductID " +  
    "WHERE Purchasing.ProductVendor.VendorID = @VendorId";  
  
  using (SqlConnection awConnection =
    new SqlConnection(connectionString))  
  {  
    SqlCommand vendorCmd = new SqlCommand(vendorSQL, awConnection);  
    SqlCommand productCmd =
      new SqlCommand(productSQL, awConnection);  
  
    productCmd.Parameters.Add("@VendorId", SqlDbType.Int);  
  
    awConnection.Open();  
    using (SqlDataReader vendorReader = vendorCmd.ExecuteReader())  
    {  
      while (vendorReader.Read())  
      {  
        Console.WriteLine(vendorReader["Name"]);  
  
        vendorID = (int)vendorReader["VendorId"];  
  
        productCmd.Parameters["@VendorId"].Value = vendorID;  
        // The following line of code requires  
        // a MARS-enabled connection.  
        productReader = productCmd.ExecuteReader();  
        using (productReader)  
        {  
          while (productReader.Read())  
          {  
            Console.WriteLine("  " +  
              productReader["Name"].ToString());  
          }  
        }  
      }  
  }  
      Console.WriteLine("Press any key to continue");  
      Console.ReadLine();  
    }  
  }  
  private static string GetConnectionString()  
  {  
    // To avoid storing the connection string in your code,  
    // you can retrieve it from a configuration file.  
    return "Data Source=(local);Integrated Security=SSPI;" +
      "Initial Catalog=AdventureWorks;MultipleActiveResultSets=True";  
  }  
}  

Reading and Updating Data with MARS

MARS allows a connection to be used for both read operations and data manipulation language (DML) operations with more than one pending operation. This feature eliminates the need for an application to deal with connection-busy errors. In addition, MARS can replace the use of server-side cursors, which generally consume more resources. Finally, because multiple operations can operate on a single connection, they can share the same transaction context, eliminating the need to use sp_getbindtoken and sp_bindsession system stored procedures.

Example

The following Console application demonstrates how to use two SqlDataReader objects with three SqlCommand objects and a single SqlConnection object with MARS enabled. The first command object retrieves a list of vendors whose credit rating is 5. The second command object uses the vendor ID provided from a SqlDataReader to load the second SqlDataReader with all of the products for the particular vendor. Each product record is visited by the second SqlDataReader. A calculation is performed to determine what the new OnOrderQty should be. The third command object is then used to update the ProductVendor table with the new value. This entire process takes place within a single transaction, which is rolled back at the end.

Option Strict On  
Option Explicit On  
  
Imports System  
Imports System.Data  
Imports System.Data.SqlClient  
  
Module Module1  
  
  Sub Main()  
    ' By default, MARS is disabled when connecting  
    ' to a MARS-enabled host.  
    ' It must be enabled in the connection string.  
    Dim connectionString As String = GetConnectionString()  
  
    Dim updateTx As SqlTransaction  
    Dim vendorCmd As SqlCommand  
    Dim prodVendCmd As SqlCommand  
    Dim updateCmd As SqlCommand  
  
    Dim prodVendReader As SqlDataReader  
  
    Dim vendorID As Integer  
    Dim productID As Integer  
    Dim minOrderQty As Integer  
    Dim maxOrderQty As Integer  
    Dim onOrderQty As Integer  
    Dim recordsUpdated As Integer  
    Dim totalRecordsUpdated As Integer  
  
    Dim vendorSQL As String = _  
        "SELECT VendorID, Name FROM Purchasing.Vendor " & _  
        "WHERE CreditRating = 5"  
    Dim prodVendSQL As String = _  
        "SELECT ProductID, MaxOrderQty, MinOrderQty, OnOrderQty " & _  
        "FROM Purchasing.ProductVendor " & _  
        "WHERE VendorID = @VendorID"  
    Dim updateSQL As String = _  
        "UPDATE Purchasing.ProductVendor " & _
        "SET OnOrderQty = @OrderQty " & _  
        "WHERE ProductID = @ProductID AND VendorID = @VendorID"  
  
    Using awConnection As New SqlConnection(connectionString)  
      awConnection.Open()  
      updateTx = awConnection.BeginTransaction()  
  
      vendorCmd = New SqlCommand(vendorSQL, awConnection)  
      vendorCmd.Transaction = updateTx  
  
      prodVendCmd = New SqlCommand(prodVendSQL, awConnection)  
      prodVendCmd.Transaction = updateTx  
      prodVendCmd.Parameters.Add("@VendorId", SqlDbType.Int)  
  
      updateCmd = New SqlCommand(updateSQL, awConnection)  
      updateCmd.Transaction = updateTx  
      updateCmd.Parameters.Add("@OrderQty", SqlDbType.Int)  
      updateCmd.Parameters.Add("@ProductID", SqlDbType.Int)  
      updateCmd.Parameters.Add("@VendorID", SqlDbType.Int)  
  
      Using vendorReader As SqlDataReader = vendorCmd.ExecuteReader()  
        While vendorReader.Read()  
          Console.WriteLine(vendorReader("Name"))  
  
          vendorID = CInt(vendorReader("VendorID"))  
          prodVendCmd.Parameters("@VendorID").Value = vendorID  
          prodVendReader = prodVendCmd.ExecuteReader()  
  
          Using prodVendReader  
            While (prodVendReader.Read)  
              productID = CInt(prodVendReader("ProductID"))  
  
              If IsDBNull(prodVendReader("OnOrderQty")) Then  
                minOrderQty = CInt(prodVendReader("MinOrderQty"))  
                onOrderQty = minOrderQty  
              Else  
                maxOrderQty = CInt(prodVendReader("MaxOrderQty"))  
                onOrderQty = CInt(maxOrderQty / 2)  
              End If  
  
              updateCmd.Parameters("@OrderQty").Value = onOrderQty  
              updateCmd.Parameters("@ProductID").Value = productID  
              updateCmd.Parameters("@VendorID").Value = vendorID  
  
              recordsUpdated = updateCmd.ExecuteNonQuery()  
              totalRecordsUpdated += recordsUpdated  
            End While  
          End Using  
        End While  
      End Using  
  
      Console.WriteLine("Total Records Updated: " & _
        CStr(totalRecordsUpdated))  
      updateTx.Rollback()  
      Console.WriteLine("Transaction Rolled Back")  
    End Using  
  
    Console.WriteLine("Press any key to continue")  
    Console.ReadLine()  
  
  End Sub  
  
  Function GetConnectionString() As String  
    ' To avoid storing the connection string in your code,  
    ' you can retrieve it from a configuration file.  
    Return "Data Source=(local);Integrated Security=SSPI;" & _  
      "Initial Catalog=AdventureWorks;MultipleActiveResultSets=True"  
  End Function  
End Module  

using System;  
using System.Collections.Generic;  
using System.Text;  
using System.Data;  
using System.Data.SqlClient;  
  
class Program  
{  
static void Main()  
{  
  // By default, MARS is disabled when connecting  
  // to a MARS-enabled host.  
  // It must be enabled in the connection string.  
  string connectionString = GetConnectionString();  
  
  SqlTransaction updateTx = null;  
  SqlCommand vendorCmd = null;  
  SqlCommand prodVendCmd = null;  
  SqlCommand updateCmd = null;  
  
  SqlDataReader prodVendReader = null;  
  
  int vendorID = 0;  
  int productID = 0;  
  int minOrderQty = 0;  
  int maxOrderQty = 0;  
  int onOrderQty = 0;  
  int recordsUpdated = 0;  
  int totalRecordsUpdated = 0;  
  
  string vendorSQL =  
      "SELECT VendorID, Name FROM Purchasing.Vendor " +
      "WHERE CreditRating = 5";  
  string prodVendSQL =  
      "SELECT ProductID, MaxOrderQty, MinOrderQty, OnOrderQty " +  
      "FROM Purchasing.ProductVendor " +
      "WHERE VendorID = @VendorID";  
  string updateSQL =  
      "UPDATE Purchasing.ProductVendor " +
      "SET OnOrderQty = @OrderQty " +  
      "WHERE ProductID = @ProductID AND VendorID = @VendorID";  
  
  using (SqlConnection awConnection =
    new SqlConnection(connectionString))  
  {  
    awConnection.Open();  
    updateTx = awConnection.BeginTransaction();  
  
    vendorCmd = new SqlCommand(vendorSQL, awConnection);  
    vendorCmd.Transaction = updateTx;  
  
    prodVendCmd = new SqlCommand(prodVendSQL, awConnection);  
    prodVendCmd.Transaction = updateTx;  
    prodVendCmd.Parameters.Add("@VendorId", SqlDbType.Int);  
  
    updateCmd = new SqlCommand(updateSQL, awConnection);  
    updateCmd.Transaction = updateTx;  
    updateCmd.Parameters.Add("@OrderQty", SqlDbType.Int);  
    updateCmd.Parameters.Add("@ProductID", SqlDbType.Int);  
    updateCmd.Parameters.Add("@VendorID", SqlDbType.Int);  
  
    using (SqlDataReader vendorReader = vendorCmd.ExecuteReader())  
    {  
      while (vendorReader.Read())  
      {  
        Console.WriteLine(vendorReader["Name"]);  
  
        vendorID = (int) vendorReader["VendorID"];  
        prodVendCmd.Parameters["@VendorID"].Value = vendorID;  
        prodVendReader = prodVendCmd.ExecuteReader();  
  
        using (prodVendReader)  
        {  
          while (prodVendReader.Read())  
          {  
            productID = (int) prodVendReader["ProductID"];  
  
            if (prodVendReader["OnOrderQty"] == DBNull.Value)  
            {  
              minOrderQty = (int) prodVendReader["MinOrderQty"];  
              onOrderQty = minOrderQty;  
            }  
            else  
            {  
              maxOrderQty = (int) prodVendReader["MaxOrderQty"];  
              onOrderQty = (int)(maxOrderQty / 2);  
            }  
  
            updateCmd.Parameters["@OrderQty"].Value = onOrderQty;  
            updateCmd.Parameters["@ProductID"].Value = productID;  
            updateCmd.Parameters["@VendorID"].Value = vendorID;  
  
            recordsUpdated = updateCmd.ExecuteNonQuery();  
            totalRecordsUpdated += recordsUpdated;  
          }  
        }  
      }  
    }  
    Console.WriteLine("Total Records Updated: " +
      totalRecordsUpdated.ToString());  
    updateTx.Rollback();  
    Console.WriteLine("Transaction Rolled Back");  
  }  
  
  Console.WriteLine("Press any key to continue");  
  Console.ReadLine();  
}  
private static string GetConnectionString()  
{  
  // To avoid storing the connection string in your code,  
  // you can retrieve it from a configuration file.  
  return "Data Source=(local);Integrated Security=SSPI;" +
    "Initial Catalog=AdventureWorks;" +
    "MultipleActiveResultSets=True";  
  }  
}  

See also

• Multiple Active Result Sets (MARS)
• ADO.NET Overview