操作数据

在引入多重活动结果集 (MARS) 之前，开发人员必须使用多个连接或服务器端游标来解决某些情况。 此外，如果在事务情境下使用多个连接，则需要绑定的连接（使用 sp_getbindtoken 和 sp_bindsession）。 以下场景说明了如何使用启用了 MARS 的连接，而不是使用多个连接。

对 MARS 使用多个命令

下面的控制台应用程序演示如何使用两个包含两个 SqlCommand 对象的 SqlDataReader 对象和一个启用了 MARS 的 SqlConnection 对象。

示例

该示例将打开与 AdventureWorks 数据库的单个连接。 使用 SqlCommand 对象时，将创建一个 SqlDataReader。 使用阅读器时，打开第二个 SqlDataReader，使用第一个 SqlDataReader 的数据作为第二个阅读器的 WHERE 子句的输入。

备注

下面的示例使用随 SQL Server 提供的 AdventureWorks 示例数据库。 示例代码中提供的连接字符串假定数据库已安装并且在本地计算机上可用。 根据环境需要修改连接字符串。

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";  
  }  
}  

使用 MARS 读取和更新数据

MARS 允许将连接用于读取操作和数据操作语言 (DML) 操作，其中有多个待处理操作。 此功能使应用程序无需处理连接繁忙错误。 此外，MARS 可以替换服务器端游标用户，这通常会消耗更多资源。 最后，因为可以在单个连接上执行多个操作，所以，这些操作可以共享相同的事务上下文，不需要使用 sp_getbindtoken 和 sp_bindsession 系统存储过程。

示例

下面的控制台应用程序演示如何使用两个包含三个 SqlCommand 对象的 SqlDataReader 对象和一个启用了 MARS 的 SqlConnection 对象。 第一个命令对象检索信用评级为 5 的供应商列表。 第二个命令对象使用 SqlDataReader 提供的供应商 ID 为第二个 SqlDataReader 加载特定供应商的所有产品。 每个产品记录由第二个 SqlDataReader 访问。 通过执行计算来确定新的 OnOrderQty。 然后，通过第三个命令对象来使用新值更新 ProductVendor 表。 整个过程发生在单个事务中，该事务在结束时回滚。

备注

下面的示例使用随 SQL Server 提供的 AdventureWorks 示例数据库。 示例代码中提供的连接字符串假定数据库已安装并且在本地计算机上可用。 根据环境需要修改连接字符串。

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";  
  }  
}  

请参阅

• 多重活动结果集 (MARS)
• ADO.NET 概述