操作数据 (MARS)

2011-05-20

在引入多个活动结果集 (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方案。此外，如果在事务情况下使用多个连接，需要绑定的连接（使用 sp_getbindtoken 和 sp_bindsession）。以下方案显示如何使用启用 MARS 的连接来取代多个连接。

对 MARS 使用多个命令

以下控制台应用程序演示如何对两个 SqlCommand 对象和单个启用了 MARS 的 SqlConnection 对象使用两个 SqlDataReader 对象。

示例

该示例打开单个与 AdventureWorks 数据库的连接。使用 SqlCommand 对象创建一个 SqlDataReader 对象。在使用该读取器时，打开第二个 SqlDataReader，使用来自第一个 SqlDataReader 的数据作为第二个读取器的 WHERE 子句的输入。

注意
下面的示例使用随 SQL Server 2005 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在本地计算机上已安装并且可用。根据环境的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 such as SQL Server 2005.
    ' 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 retrive 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 such as SQL Server 2005.
  // 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 retrive 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 允许连接供读取操作以及数据操作语言 (DML) 操作使用，包含多个挂起操作。通过此功能，应用程序不需要处理连接忙的错误。此外，MARS 可以取代服务器端游标的用户，后者通常会占用更多资源。最后，因为可以在单个连接上执行多个操作，所以，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

以下控制台应用程序演示如何对三个 SqlCommand 对象和单个启用了 MARS 的 SqlConnection 对象使用两个 SqlDataReader 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。每个产品记录通过第二个 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。整个过程在单个事务中进行，在结束时回滚。

注意
下面的示例使用随 SQL Server 2005 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在本地计算机上已安装并且可用。根据环境的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 such as SQL Server 2005.
    ' 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 retrive 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 such as SQL Server 2005.
  // 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 retrive it from a configuration file.
  return "Data Source=(local);Integrated Security=SSPI;" + 
    "Initial Catalog=AdventureWorks;" + 
    "MultipleActiveResultSets=True";
  }
}

请参见

其他资源

多活动结果集 (MARS)