项目
04/01/2012

CLR 表值函数

表值函数是返回表的用户定义函数。

从 SQL Server 2005 开始，SQL Server 通过允许您在任何托管语言中定义表值函数以扩展表值函数的功能。数据通过 IEnumerable 或 IEnumerator 对象从表值函数中返回。

注意
对于表值函数，返回表类型的列不能包含时间戳列或非 Unicode 字符串数据类型列（例如，char、varchar 和 text）。不支持 NOT NULL 约束。

Transact-SQL 与 CLR 表值函数之间的差异

Transact-SQL 表值函数将调用此函数的结果具体化到某个中间表中。由于它们使用中间表，因此它们可以对于结果支持约束和唯一索引。当返回大型结果时，这些功能极其有用。

相比较而言，CLR 表值函数表示一种流替代方法。此时，不要求在单个表中具体化整个结果集。调用表值函数的查询的执行计划直接调用由托管函数返回的 IEnumerable 对象，并且结果将以递增方式使用。这种流模型可确保在第一行可用之后立即使用结果，而不是等待填充整个表。如果返回的行非常多，则这还是一个更好的替代方法，因为它们不必在内存中作为一个整体进行具体化。例如，可以使用托管表值函数分析文本文件并将其中的每行作为一行返回。

实现表值函数

在 Microsoft .NET Framework 程序集中将表值函数作为类的方法实现。表值函数代码必须实现 IEnumerable 接口。IEnumerable 接口在 .NET Framework 中定义。在 .NET Framework 中表示数组和集合的类型已经实现 IEnumerable 接口。这样，就可以轻松地编写将集合或数组转换为结果集的表值函数。

表值参数

表值参数即传递到某一过程或函数的用户定义表类型，它提供了一种将多行数据传递到服务器的高效方法。表值参数提供与参数数组类似的功能，但灵活性更高并且与 Transact-SQL 的集成更紧密。它们还提供提升性能的潜力。表值参数还有助于减少到服务器的往返次数。可以将数据作为表值参数发送到服务器，而不是向服务器发送多个请求（例如，对于标量参数列表）。用户定义表类型不能作为表值参数传递到在 SQL Server 进程中执行的托管存储过程或函数，也不能从这些存储过程或函数中返回。有关表值参数的详细信息，请参阅表值参数（数据库引擎）。

输出参数和表值函数

通过使用输出参数，可以从表值函数返回信息。在实现代码表值函数中的相应参数应将按引用传递参数用作参数。请注意，Visual Basic 不支持采用与 Visual C# 的相同方法输出参数。您必须按引用指定参数并且应用 <Out()> 属性以表示输出参数，如下所示：

Imports System.Runtime.InteropServices
…
Public Shared Sub FillRow ( <Out()> ByRef value As SqlInt32)

在 Transact-SQL 中定义表值函数

定义 CLR 表值函数的语法与定义 Transact-SQL 表值函数的语法类似，但增加了 EXTERNAL NAME 子句。例如：

CREATE FUNCTION GetEmpFirstLastNames()
RETURNS TABLE (FirstName NVARCHAR(4000), LastName NVARCHAR(4000))
EXTERNAL NAME MyDotNETAssembly.[MyNamespace.MyClassname]. GetEmpFirstLastNames;

表值函数用于以相关格式表示数据，以便在查询中进一步处理，例如：

select * from function();
select * from tbl join function() f on tbl.col = f.col;
select * from table t cross apply function(t.column);

在以下情况下，表值函数可以返回表：

当从标量输入参数中创建表值函数时。例如，使用以逗号分隔的数字字符串并将它们透视到某个表中的表值函数。
当从外部数据生成表值函数时。例如，读取事件日志并将其显示为表的表值函数。

注意表值函数只能通过 InitMethod 方法（而非 FillRow 方法）中的 Transact-SQL 查询执行数据访问。如果执行 Transact-SQL 查询，则应使用 SqlFunction.DataAccess.Read 属性标记 InitMethod。

示例表值函数

下面的表值函数返回系统事件日志中的信息。此函数采用单个字符串参数，其中包含要读取的事件日志的名称。

示例代码

using System;
using System.Data.Sql;
using Microsoft.SqlServer.Server;
using System.Collections;
using System.Data.SqlTypes;
using System.Diagnostics;

public class TabularEventLog
{
    [SqlFunction(FillRowMethodName = "FillRow")]
    public static IEnumerable InitMethod(String logname)
    {
        return new EventLog(logname).Entries;    }

    public static void FillRow(Object obj, out SqlDateTime timeWritten, out SqlChars message, out SqlChars category, out long instanceId)
    {
        EventLogEntry eventLogEntry = (EventLogEntry)obj;
        timeWritten = new SqlDateTime(eventLogEntry.TimeWritten);
        message = new SqlChars(eventLogEntry.Message);
        category = new SqlChars(eventLogEntry.Category);
        instanceId = eventLogEntry.InstanceId;
    }
}

Imports System
Imports System.Data.Sql
Imports Microsoft.SqlServer.Server
Imports System.Collections
Imports System.Data.SqlTypes
Imports System.Diagnostics
Imports System.Runtime.InteropServices

Public Class TabularEventLog
    <SqlFunction(FillRowMethodName:="FillRow")> _
    Public Shared Function InitMethod(ByVal logname As String) As IEnumerable
        Return New EventLog(logname).Entries
    End Function

    Public Shared Sub FillRow(ByVal obj As Object, <Out()> ByRef timeWritten As SqlDateTime, <Out()> ByRef message As SqlChars, <Out()> ByRef category As SqlChars, <Out()> ByRef instanceId As Long)
        Dim eventLogEnTry As EventLogEntry = CType(obj, EventLogEntry)
        timeWritten = New SqlDateTime(eventLogEnTry.TimeWritten)
        message = New SqlChars(eventLogEnTry.Message)
        category = New SqlChars(eventLogEnTry.Category)
        instanceId = eventLogEnTry.InstanceId
    End Sub
End Class

声明和使用示例表值函数

在编译了示例表值函数后，就可以在 Transact-SQL 中声明它，如下所示：

use master
-- Replace SQL_Server_logon with your SQL Server user credentials.
GRANT EXTERNAL ACCESS ASSEMBLY TO [SQL_Server_logon] 
-- Modify the following line to specify a different database.
ALTER DATABASE master SET TRUSTWORTHY ON

-- Modify the next line to use the appropriate database.
CREATE ASSEMBLY tvfEventLog 
FROM 'D:\assemblies\tvfEventLog\tvfeventlog.dll' 
WITH PERMISSION_SET = EXTERNAL_ACCESS
GO
CREATE FUNCTION ReadEventLog(@logname nvarchar(100))
RETURNS TABLE 
(logTime datetime,Message nvarchar(4000),Category nvarchar(4000),InstanceId bigint)
AS 
EXTERNAL NAME tvfEventLog.TabularEventLog.InitMethod
GO

请注意，从 SQL Server 2005 开始，在兼容级别为“80”的 SQL Server 数据库中，不能创建托管的用户定义类型、存储过程、函数、聚合或触发器。若要利用 SQL Server 的这些 CLR 集成功能，必须使用 sp_dbcmptlevel 存储过程将数据库兼容级别设置为“90”。

SQL Server 2005 中不再支持执行使用 /clr:pure 编译的 Visual C++ 数据库对象。例如，此类数据库对象包含表值函数。

若要测试此示例，请尝试以下 Transact-SQL 代码：

-- Select the top 100 events,
SELECT TOP 100 *
FROM dbo.ReadEventLog(N'Security') as T
go

-- Select the last 10 login events.
SELECT TOP 10 T.logTime, T.Message, T.InstanceId 
FROM dbo.ReadEventLog(N'Security') as T
WHERE T.Category = N'Logon/Logoff'
go

示例：返回 SQL Server 查询的结果

以下示例演示查询 SQL Server 数据库的表值函数。本示例使用 AdventureWorks2008R2 轻型数据库。有关下载 AdventureWorks2008R2 的详细信息，请参阅 https://www.codeplex.com/sqlserversamples。

将源代码文件命名为 FindInvalidEmails.cs 或 FindInvalidEmails.vb。

using System;
using System.Collections;
using System.Data;
using System.Data.SqlClient;
using System.Data.SqlTypes;

using Microsoft.SqlServer.Server;

public partial class UserDefinedFunctions {
   private class EmailResult {
      public SqlInt32 CustomerId;
      public SqlString EmailAdress;

      public EmailResult(SqlInt32 customerId, SqlString emailAdress) {
         CustomerId = customerId;
         EmailAdress = emailAdress;
      }
   }

   public static bool ValidateEmail(SqlString emailAddress) {
      if (emailAddress.IsNull)
         return false;

      if (!emailAddress.Value.EndsWith("@adventure-works.com"))
         return false;

      // Validate the address. Put any more rules here.
      return true;
   }

   [SqlFunction(
       DataAccess = DataAccessKind.Read,
       FillRowMethodName = "FindInvalidEmails_FillRow",
       TableDefinition="CustomerId int, EmailAddress nvarchar(4000)")]
   public static IEnumerable FindInvalidEmails(SqlDateTime modifiedSince) {
      ArrayList resultCollection = new ArrayList();

      using (SqlConnection connection = new SqlConnection("context connection=true")) {
         connection.Open();

         using (SqlCommand selectEmails = new SqlCommand(
             "SELECT " +
             "[CustomerID], [EmailAddress] " +
             "FROM [AdventureWorksLT2008].[SalesLT].[Customer] " +
             "WHERE [ModifiedDate] >= @modifiedSince",
             connection)) {
            SqlParameter modifiedSinceParam = selectEmails.Parameters.Add(
                "@modifiedSince",
                SqlDbType.DateTime);
            modifiedSinceParam.Value = modifiedSince;

            using (SqlDataReader emailsReader = selectEmails.ExecuteReader()) {
               while (emailsReader.Read()) {
                  SqlString emailAddress = emailsReader.GetSqlString(1);
                  if (ValidateEmail(emailAddress)) {
                     resultCollection.Add(new EmailResult(
                         emailsReader.GetSqlInt32(0),
                         emailAddress));
                  }
               }
            }
         }
      }

      return resultCollection;
   }

   public static void FindInvalidEmails_FillRow(
       object emailResultObj,
       out SqlInt32 customerId,
       out SqlString emailAdress) {
      EmailResult emailResult = (EmailResult)emailResultObj;

      customerId = emailResult.CustomerId;
      emailAdress = emailResult.EmailAdress;
   }
};

Imports System.Collections
Imports System.Data
Imports System.Data.SqlClient
Imports System.Data.SqlTypes
Imports Microsoft.SqlServer.Server

Public Partial Class UserDefinedFunctions
   Private Class EmailResult
      Public CustomerId As SqlInt32
      Public EmailAdress As SqlString

      Public Sub New(customerId__1 As SqlInt32, emailAdress__2 As SqlString)
         CustomerId = customerId__1
         EmailAdress = emailAdress__2
      End Sub
   End Class

   Public Shared Function ValidateEmail(emailAddress As SqlString) As Boolean
      If emailAddress.IsNull Then
         Return False
      End If

      If Not emailAddress.Value.EndsWith("@adventure-works.com") Then
         Return False
      End If

      ' Validate the address. Put any more rules here.
      Return True
   End Function

   <SqlFunction(DataAccess := DataAccessKind.Read, FillRowMethodName := "FindInvalidEmails_FillRow", TableDefinition := "CustomerId int, EmailAddress nvarchar(4000)")> _
   Public Shared Function FindInvalidEmails(modifiedSince As SqlDateTime) As IEnumerable
      Dim resultCollection As New ArrayList()

      Using connection As New SqlConnection("context connection=true")
         connection.Open()

         Using selectEmails As New SqlCommand("SELECT " & "[CustomerID], [EmailAddress] " & "FROM [AdventureWorksLT2008].[SalesLT].[Customer] " & "WHERE [ModifiedDate] >= @modifiedSince", connection)
            Dim modifiedSinceParam As SqlParameter = selectEmails.Parameters.Add("@modifiedSince", SqlDbType.DateTime)
            modifiedSinceParam.Value = modifiedSince

            Using emailsReader As SqlDataReader = selectEmails.ExecuteReader()
               While emailsReader.Read()
                  Dim emailAddress As SqlString = emailsReader.GetSqlString(1)
                  If ValidateEmail(emailAddress) Then
                     resultCollection.Add(New EmailResult(emailsReader.GetSqlInt32(0), emailAddress))
                  End If
               End While
            End Using
         End Using
      End Using

      Return resultCollection
   End Function

   Public Shared Sub FindInvalidEmails_FillRow(emailResultObj As Object, ByRef customerId As SqlInt32, ByRef emailAdress As SqlString)
      Dim emailResult As EmailResult = DirectCast(emailResultObj, EmailResult)

      customerId = emailResult.CustomerId
      emailAdress = emailResult.EmailAdress
   End Sub
End ClassImports System.Collections
Imports System.Data
Imports System.Data.SqlClient
Imports System.Data.SqlTypes
Imports Microsoft.SqlServer.Server

Public Partial Class UserDefinedFunctions
   Private Class EmailResult
      Public CustomerId As SqlInt32
      Public EmailAdress As SqlString

      Public Sub New(customerId__1 As SqlInt32, emailAdress__2 As SqlString)
         CustomerId = customerId__1
         EmailAdress = emailAdress__2
      End Sub
   End Class

   Public Shared Function ValidateEmail(emailAddress As SqlString) As Boolean
      If emailAddress.IsNull Then
         Return False
      End If

      If Not emailAddress.Value.EndsWith("@adventure-works.com") Then
         Return False
      End If

      ' Validate the address. Put any more rules here.
      Return True
   End Function

   <SqlFunction(DataAccess := DataAccessKind.Read, FillRowMethodName := "FindInvalidEmails_FillRow", TableDefinition := "CustomerId int, EmailAddress nvarchar(4000)")> _
   Public Shared Function FindInvalidEmails(modifiedSince As SqlDateTime) As IEnumerable
      Dim resultCollection As New ArrayList()

      Using connection As New SqlConnection("context connection=true")
         connection.Open()

         Using selectEmails As New SqlCommand("SELECT " & "[CustomerID], [EmailAddress] " & "FROM [AdventureWorksLT2008].[SalesLT].[Customer] " & "WHERE [ModifiedDate] >= @modifiedSince", connection)
            Dim modifiedSinceParam As SqlParameter = selectEmails.Parameters.Add("@modifiedSince", SqlDbType.DateTime)
            modifiedSinceParam.Value = modifiedSince

            Using emailsReader As SqlDataReader = selectEmails.ExecuteReader()
               While emailsReader.Read()
                  Dim emailAddress As SqlString = emailsReader.GetSqlString(1)
                  If ValidateEmail(emailAddress) Then
                     resultCollection.Add(New EmailResult(emailsReader.GetSqlInt32(0), emailAddress))
                  End If
               End While
            End Using
         End Using
      End Using

      Return resultCollection
   End Function

   Public Shared Sub FindInvalidEmails_FillRow(emailResultObj As Object, customerId As SqlInt32, emailAdress As SqlString)
      Dim emailResult As EmailResult = DirectCast(emailResultObj, EmailResult)

      customerId = emailResult.CustomerId
      emailAdress = emailResult.EmailAdress
   End Sub
End Class

将源代码编译为 DLL 并将此 DLL 复制到 C 驱动器的根目录下。然后，执行以下 Transact-SQL 查询。

use AdventureWorksLT2008
go

IF EXISTS (SELECT name FROM sysobjects WHERE name = 'FindInvalidEmails')
   DROP FUNCTION FindInvalidEmails
go

IF EXISTS (SELECT name FROM sys.assemblies WHERE name = 'MyClrCode')
   DROP ASSEMBLY MyClrCode
go

CREATE ASSEMBLY MyClrCode FROM 'C:\FindInvalidEmails.dll'
WITH PERMISSION_SET = SAFE -- EXTERNAL_ACCESS
GO

CREATE FUNCTION FindInvalidEmails(@ModifiedSince datetime) 
RETURNS TABLE (
   CustomerId int,
   EmailAddress nvarchar(4000)
)
AS EXTERNAL NAME MyClrCode.UserDefinedFunctions.[FindInvalidEmails]
go

SELECT * FROM FindInvalidEmails('2000-01-01')
go

请参阅

概念

数据库兼容级别选项

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