使用大數值類型

發行項
11/10/2015

在 SQL Server 2005 之前，使用大數值資料類型時需要進行特殊處理。大數值資料類型是指那些最大資料列大小超過 8 KB 的資料類型。 SQL Server 2005 推出了 max 規範，這個規範可用於 varchar、nvarchar 和 varbinary 資料類型，以允許儲存最大可達 2^31 -1 個位元組的值。資料表資料行和 Transact-SQL 變數可以指定 varchar(max)、nvarchar(max) 或 varbinary(max) 資料類型。

[!附註]

大數值資料類型的最大大小可介於 1 和 8 KB 之間，或者也可以指定為無限制。

先前，只有 text、ntext 和 image 之類的 SQL Server 資料類型可以達到此種長度。 varchar、nvarchar 和 varbinary 的 max 規範使這些資料類型變得多餘。不過，因為 long 資料類型仍可使用，所以大部分 OLE DB 和 ODBC 資料存取元件的介面都仍保持原樣。為了與先前的版本保持回溯相容性，SQL Server Native Client OLE DB 提供者中的 DBCOLUMNFLAGS_ISLONG 旗標以及 SQL Server Native Client ODBC 驅動程式中的 SQL_LONGVARCHAR 仍會繼續使用。當新類型設定為無限制的最大長度時，針對 SQL Server 2005 和更新版本所撰寫的提供者和驅動程式仍會為其繼續使用這些詞彙。

[!附註]

您也可以將 varchar(max)、nvarchar(max) 和 varbinary(max) 資料類型指定為預存程序的輸入和輸出參數類型、函數傳回類型，或者指定於 CAST 和 CONVERT 函數。

[!附註]

如果複寫資料，您可能需要將 max text repl size 伺服器組態選項設定為 -1。

SQL Server Native Client OLE DB 提供者

SQL Server Native Client OLE DB 提供者會將 varchar(max)、varbinary(max) 和 nvarchar(max) 類型分別公開為 DBTYPE_STR、DBTYPE_BYTES 和 DBTYPE_WSTR。

在 max 大小設定為無限制的資料行中，varchar(max)、varbinary(max) 和 nvarchar(max) 資料類型會在傳回資料行資料類型的核心 OLE DB 結構描述資料列集和介面中表示為 ISLONG。

命令物件的 IAccessor 實作已變更為允許以 DBTYPE_IUNKNOWN 的形式進行繫結。如果取用者指定 DBTYPE_IUNKNOWN 並將 pObject 設定為 null，則提供者會將 ISequentialStream 介面傳回給取用者，讓取用者可以將 varchar(max)、nvarchar(max) 或 varbinary(max) 用資料流的形式傳出輸出變數。

以資料流傳輸的輸出參數值會在任何結果資料列之後傳回。如果應用程式藉由呼叫 IMultipleResults::GetResult (而不取用所有的傳回輸出參數值) 嘗試繼續前往下一個結果集，就會傳回 DB_E_OBJECTOPEN。

為了支援資料流作業，SQL Server Native Client OLE DB 提供者需要循序存取可變長度的參數。這表示每當 varchar(max)、nvarchchar(max) 或 varbinary(max) 資料行或輸出參數繫結至 DBTYPE_IUNKNOWN 時，DBPROP_ACCESSORDER 就必須設定為 DBPROPVAL_AO_SEQUENTIALSTORAGEOBJECTS 或 DBPROPVAL_AO_SEQUENTIAL。如果未遵守這項存取順序的限制，則對 IRowset::GetData 的呼叫會失敗，且傳回 DBSTATUS_E_UNAVAILABLE。當沒有任何使用 DBTYPE_IUNKNOWN 的輸出繫結時，這項限制就不適用。

SQL Server Native Client OLE DB 提供者也支援將輸出參數針對大數值資料類型而繫結為 DBTYPE_IUNKNOWN，這在預存程序要將大數值類型以傳回值 (對用戶端公開為 DBTYPE_IUNKNOWN) 傳回時很有用。

為了使用這些類型，應用程式具有下列選項：

繫結為具有資料行基底類型所支援繫結的類型 (例如，對於 nvarchar(max)，繫結為可繫結到 nvarchar 的類型)。如果緩衝區不夠大，就會進行截斷，這與基底類型完全相同，只是現在使用的是較大數值。
繫結為具有資料行基底類型所支援轉換的類型，並同時指定 DBTYPE_BYREF。
繫結為 DBTYPE_IUNKNOWN 並使用資料流。

在報告資料行的最大大小時，SQL Server Native Client OLE DB 提供者會報告：

定義的最大大小，例如 varchar(2000) 資料行的最大大小為 2000，或者
"unlimited" 這個值，如果是 varchar(max) 資料行則等於 0。這個值會針對 DBCOLUMN_COLUMNSIZE 中繼資料屬性設定。

標準轉換規則適用於 varchar(max) 資料行，代表任何對 varchar(2000) 資料行有效的轉換也對 varchar(max) 資料行有效。相同的規則也適用於 nvarchar(max) 和 varbinary(max) 資料行。

在擷取大數值類型時，最有效的方法是繫結為 DBTYPE_IUNKNOWN，並將資料列集屬性 DBPROP_ACCESSORDER 設定為 DBPROPVAL_AO_SEQUENTIALSTORAGEOBJECTS。這樣會導致數值直接從網路以資料流的方式傳出，而不必經過中間的緩衝作業，如下列範例所示：

#define UNICODE
#define _UNICODE
#define DBINITCONSTANTS
#define INITGUID
#define OLEDBVER 0x0250  // To include the correct interfaces.

#include <stdio.h>
#include <tchar.h>
#include <stddef.h>
#include <iostream>

using std::cout;
using std::endl;

#include <windows.h>

#include <oledb.h>
#include "sqlncli.h"
#include <oledberr.h>

#define CHKHR_GOTO(hr, errMsg, Label) \
   if (FAILED(hr)) \
   { \
      cout << errMsg << endl; \
      goto Label; \
   }

#define MAX_COL_SIZE 8000

// ROUNDUP on all platforms pointers must be aligned properly.
#define ROUNDUP_AMOUNT 8
#define ROUNDUP_(size,amount) (((ULONG)(size)+((amount)-1))&~((amount)-1))
#define ROUNDUP(size) ROUNDUP_(size, ROUNDUP_AMOUNT)

HRESULT InitializeAndEstablishConnection(IDBInitialize** ppIDBInitialize);
void UnInitializeConnection(IDBInitialize* pIDBInitialize);
HRESULT CreateAndSetCommand(IDBInitialize* pIDBInitialize, ICommandText** ppICommandText);
HRESULT ProcessResultSet(IRowset* pIRowset);

void DisplayTime()
{
   SYSTEMTIME st;
   GetSystemTime(&st);
   cout<< st.wHour << ":" << st.wMinute << ":" << st.wSecond << "." << st.wMilliseconds << endl;
}

void main()
{
   HRESULT hr;
   IDBInitialize* pIDBInitialize = NULL;
   ICommandText* pICommandText = NULL;
   IMultipleResults* pIMultipleResults = NULL;
   IRowset* pIRowset = NULL;

   hr = InitializeAndEstablishConnection(&pIDBInitialize);
   CHKHR_GOTO(hr, L"Failed to establish connection.", _ExitMain);

   hr = CreateAndSetCommand(pIDBInitialize, &pICommandText);
   CHKHR_GOTO(hr, L"Failed to set up command object.", _ExitMain);

   DisplayTime();

   hr = pICommandText->Execute(NULL, 
      IID_IMultipleResults, 
      NULL, 
      NULL, 
     (IUnknown **) &pIMultipleResults);

   CHKHR_GOTO(hr, L"Failed to execute command.", _ExitMain);

   while (1)
   {
      hr = pIMultipleResults->GetResult(
         NULL, 
         DBRESULTFLAG_DEFAULT, 
         IID_IRowset, 
         NULL, 
         (IUnknown**)&pIRowset);

   CHKHR_GOTO(hr, L"Failed to obtain a results from MR object.", _ExitMain);

   if (hr == DB_S_NORESULT)
      break;

      if (pIRowset)
      {
         hr = ProcessResultSet(pIRowset); 
         CHKHR_GOTO(hr, L"Failed to process the current Rowset.", _ExitMain);

         pIRowset->Release();
         pIRowset = NULL;
      }
   }

   DisplayTime();

_ExitMain:

   if (pIRowset)
   {
      pIRowset->Release();
      pIRowset = NULL;
   }

   if (pIMultipleResults)
   {
      pIMultipleResults->Release();
      pIMultipleResults = NULL;
   }

   if (pICommandText)
   {
      pICommandText->Release();
      pICommandText = NULL;
   }

   UnInitializeConnection(pIDBInitialize);
   return;
};

HRESULT InitializeAndEstablishConnection(IDBInitialize** ppIDBInitialize)
{
   HRESULT hr;
   IDBInitialize* pIDBInitialize = NULL;
   IDBProperties* pIDBProperties = NULL;

   const int NUM_DBINIT_PROPS = 3;
   const wchar_t* const g_wszServer = L".";
   const wchar_t* const g_wszCatalog = L"AdventureWorks";
   const wchar_t* const g_wszSecurity = L"SSPI";

   DBPROPSET rgdbPropSetInit[1];
   DBPROP rgdbPropInit [NUM_DBINIT_PROPS];

   *ppIDBInitialize = NULL;
   hr = CoInitialize(NULL);
   CHKHR_GOTO(hr, L"Failed to initialize COM.", _ExitInitialize);

   hr = CoCreateInstance(CLSID_SQLNCLI11, 
      NULL, 
      CLSCTX_INPROC_SERVER,
      IID_IDBInitialize, 
      (void**)&pIDBInitialize);

   CHKHR_GOTO(hr, L"Failed to create SQLNCLI11 DataSource object.", _ExitInitialize);

   for(int idxProp = 0; idxProp < NUM_DBINIT_PROPS; idxProp++) 
   {
      VariantInit(&rgdbPropInit[idxProp].vValue);
   }

   rgdbPropInit[0].dwPropertyID = DBPROP_INIT_DATASOURCE;
   rgdbPropInit[0].vValue.vt = VT_BSTR;
   rgdbPropInit[0].vValue.bstrVal= SysAllocString(g_wszServer);
   rgdbPropInit[0].dwOptions = DBPROPOPTIONS_REQUIRED;
   rgdbPropInit[0].colid = DB_NULLID;

   if (rgdbPropInit[0].vValue.bstrVal == NULL)
   {
      hr = E_OUTOFMEMORY;
      goto _ExitInitialize;
   }

   rgdbPropInit[1].dwPropertyID = DBPROP_INIT_CATALOG;
   rgdbPropInit[1].vValue.vt = VT_BSTR;
   rgdbPropInit[1].vValue.bstrVal= SysAllocString(g_wszCatalog);
   rgdbPropInit[1].dwOptions = DBPROPOPTIONS_REQUIRED;
   rgdbPropInit[1].colid = DB_NULLID;

   if (rgdbPropInit[1].vValue.bstrVal == NULL)
   {
      hr = E_OUTOFMEMORY;
      goto _ExitInitialize;
   }

   rgdbPropInit[2].dwPropertyID = DBPROP_AUTH_INTEGRATED;
   rgdbPropInit[2].vValue.vt = VT_BSTR;
   rgdbPropInit[2].vValue.bstrVal= SysAllocString(g_wszSecurity);
   rgdbPropInit[2].dwOptions = DBPROPOPTIONS_REQUIRED;
   rgdbPropInit[2].colid = DB_NULLID;

   if (rgdbPropInit[2].vValue.bstrVal == NULL)
   {
      hr = E_OUTOFMEMORY;
      goto _ExitInitialize;
   }

   rgdbPropSetInit[0].guidPropertySet = DBPROPSET_DBINIT;
   rgdbPropSetInit[0].cProperties = NUM_DBINIT_PROPS;
   rgdbPropSetInit[0].rgProperties = rgdbPropInit;

   hr = pIDBInitialize->QueryInterface(IID_IDBProperties, (void **)&pIDBProperties);
   CHKHR_GOTO(hr, L"Failed to QI DataSource object for IDBProperties.", _ExitInitialize);

   hr = pIDBProperties->SetProperties(1, rgdbPropSetInit); 
   CHKHR_GOTO(hr, L"Failed to set DataSource object Properties.", _ExitInitialize);

   pIDBProperties->Release();
   pIDBProperties = NULL;

   hr = pIDBInitialize->Initialize();
   CHKHR_GOTO(hr, L"Failed to establish connection with the server.", _ExitInitialize);

_ExitInitialize:

   if (pIDBProperties)
   {
      pIDBProperties->Release();
      pIDBProperties = NULL;
   }

   if (FAILED(hr))
   {
      if (pIDBInitialize)
      {
         pIDBInitialize->Release();
         pIDBInitialize = NULL;
      }
   }

   *ppIDBInitialize = pIDBInitialize;
   return hr;
}

void UnInitializeConnection(IDBInitialize* pIDBInitialize)
{
   if (pIDBInitialize)
   {
      pIDBInitialize->Uninitialize();
      pIDBInitialize->Release();
      pIDBInitialize = NULL;
   }
   CoUninitialize();
}

HRESULT CreateAndSetCommand(IDBInitialize* pIDBInitialize, ICommandText** ppICommandText)
{
   HRESULT hr;
   IDBCreateSession* pIDBCreateSession = NULL;
   IDBCreateCommand* pIDBCreateCommand = NULL;
   ICommandText* pICommandText = NULL;
   ICommandProperties* pICommandProperties = NULL;
   DBPROPSET rgCmdPropSet[1];
   DBPROP rgCmdProperties[1];

const wchar_t* const g_wCmdString = L"declare @x xml, @y nvarchar(max); select @x = (SELECT * FROM Sales.SalesOrderHeader FOR XML AUTO); select @x;";

   *ppICommandText = NULL;

   if (!pIDBInitialize)
   {
      hr = E_FAIL;
      goto _ExitCreateAndSetCommand;
   }

   hr = pIDBInitialize->QueryInterface(IID_IDBCreateSession, (void**) &pIDBCreateSession);
   CHKHR_GOTO(hr, L"Failed to obtain IDBCreateSession interface from DSO.", _ExitCreateAndSetCommand);

   hr = pIDBCreateSession->CreateSession(
      NULL, 
      IID_IDBCreateCommand, 
      (IUnknown**) &pIDBCreateCommand);

   CHKHR_GOTO(hr, L"Failed to Create a Session for command execution.", _ExitCreateAndSetCommand);

   hr = pIDBCreateCommand->CreateCommand(
      NULL, 
      IID_ICommandText, 
      (IUnknown**)&pICommandText);

   CHKHR_GOTO(hr, L"Failed to Create a Command object.", _ExitCreateAndSetCommand);

   hr = pICommandText->SetCommandText(DBGUID_DBSQL, g_wCmdString);
   CHKHR_GOTO(hr, L"Failed to Set Command Text.", _ExitCreateAndSetCommand);

   hr = pICommandText->QueryInterface(IID_ICommandProperties, (void**) &pICommandProperties);
   CHKHR_GOTO(hr, L"Failed to obtain ICommandProperties interface from the command object.", _ExitCreateAndSetCommand);

   rgCmdProperties[0].dwPropertyID = DBPROP_ACCESSORDER;
   rgCmdProperties[0].vValue.vt = VT_I4;
   rgCmdProperties[0].vValue.lVal = DBPROPVAL_AO_SEQUENTIAL;
   rgCmdProperties[0].dwOptions = DBPROPOPTIONS_REQUIRED;
   rgCmdProperties[0].colid = DB_NULLID;

   rgCmdPropSet[0].guidPropertySet = DBPROPSET_ROWSET;
   rgCmdPropSet[0].cProperties = 1;
   rgCmdPropSet[0].rgProperties = rgCmdProperties;

   hr = pICommandProperties->SetProperties(1, rgCmdPropSet); 
   CHKHR_GOTO(hr, L"Failed to Set Command object Properties.", _ExitCreateAndSetCommand);

_ExitCreateAndSetCommand:

   if (pICommandProperties)
   {
      pICommandProperties->Release();
      pICommandProperties = NULL;
   }

   if (pIDBCreateCommand)
   {
      pIDBCreateCommand->Release();
      pIDBCreateCommand = NULL;
   }

   if (pIDBCreateSession)
   {
      pIDBCreateSession->Release();
      pIDBCreateSession = NULL;
   }

   if (FAILED(hr))
   {
      if (pICommandText)
      {
         pICommandText->Release();
         pICommandText = NULL;
      }
   }

   *ppICommandText = pICommandText;
   return hr;
}

HRESULT ProcessResultSet(IRowset* pIRowset)
{
   HRESULT hr;

   IColumnsInfo* pIColumnsInfo = NULL;
   DBCOLUMNINFO* pDBColumnInfo = NULL;
   ULONG lNumCols = 0;
   wchar_t* pStringsBuffer = NULL;

   DBBINDING* pBindings = NULL;
   DBOBJECT dbobj;
   ULONG idxBinding;
   IAccessor* pIAccessor = NULL;
   HACCESSOR hAccessor = DB_NULL_HACCESSOR;
   HROW hRows[1] = {DB_NULL_HROW};
   HROW* pRow = &hRows[0];
   BYTE* pBuffer = NULL;

   ULONG lNumRowsRetrieved;
   DBLENGTH dwOffset = 0;

   hr = pIRowset->QueryInterface(IID_IColumnsInfo, (void **)&pIColumnsInfo);
   CHKHR_GOTO(hr, L"Failed to QI Rowset for IColumnsInfo.", _ExitProcessResultSet);

   hr = pIColumnsInfo->GetColumnInfo(&lNumCols, &pDBColumnInfo, &pStringsBuffer);
   CHKHR_GOTO(hr, L"Failed to obtain Column Information.", _ExitProcessResultSet);

   pBindings = new DBBINDING[lNumCols];

   if (!pBindings)
   {
      hr = E_OUTOFMEMORY;
      goto _ExitProcessResultSet;
   }

   memset(pBindings, 0, sizeof(DBBINDING) * lNumCols);

   dbobj.dwFlags = STGM_READ;
   dbobj.iid = IID_ISequentialStream;

   for (idxBinding = 0; idxBinding < lNumCols; idxBinding++) 
   {
      pBindings[idxBinding].iOrdinal = idxBinding + 1;
      pBindings[idxBinding].obStatus = dwOffset;
      pBindings[idxBinding].obLength = dwOffset + sizeof(DBSTATUS);
      pBindings[idxBinding].obValue = dwOffset + sizeof(DBSTATUS) + sizeof(DBLENGTH);

      pBindings[idxBinding].pTypeInfo = NULL;
      pBindings[idxBinding].pBindExt = NULL;
      pBindings[idxBinding].dwPart = DBPART_VALUE | DBPART_LENGTH | DBPART_STATUS;
      pBindings[idxBinding].dwMemOwner = DBMEMOWNER_CLIENTOWNED;
      pBindings[idxBinding].eParamIO = DBPARAMIO_NOTPARAM;
      pBindings[idxBinding].bPrecision = pDBColumnInfo[idxBinding].bPrecision;
      pBindings[idxBinding].bScale = pDBColumnInfo[idxBinding].bScale;

      pBindings[idxBinding].cbMaxLen = 0;
      pBindings[idxBinding].wType = DBTYPE_WSTR;

   // Determine the maximum number of bytes required in our buffer to
   // contain the Unicode string representation of the provider's native
   // data type, including room for the NULL-termination character
   switch( pDBColumnInfo[idxBinding].wType )
   {
      case DBTYPE_NULL:
      case DBTYPE_EMPTY:
      case DBTYPE_I1:
      case DBTYPE_I2:
      case DBTYPE_I4:
      case DBTYPE_UI1:
      case DBTYPE_UI2:
      case DBTYPE_UI4:
      case DBTYPE_R4:
      case DBTYPE_BOOL:
      case DBTYPE_I8:
      case DBTYPE_UI8:
      case DBTYPE_R8:
      case DBTYPE_CY:
      case DBTYPE_ERROR:
      // When the above types are converted to a string, they
      // will all fit into 25 characters, so use that plus space
      // for the NULL-terminator.

      pBindings[idxBinding].cbMaxLen = (25 + 1) * sizeof(WCHAR);
      break;

      case DBTYPE_DECIMAL:
      case DBTYPE_NUMERIC:
      case DBTYPE_DATE:
      case DBTYPE_DBDATE:
      case DBTYPE_DBTIMESTAMP:
      case DBTYPE_GUID:
      // Converted to a string, the above types will all fit into
      // 50 characters, so use that plus space for the terminator.

      pBindings[idxBinding].cbMaxLen = (50 + 1) * sizeof(WCHAR);
      break;

      case DBTYPE_BYTES:
      // In converting DBTYPE_BYTES to a string, each byte
      // becomes two characters (e.g. 0xFF -> "FF"), so we
      // will use double the maximum size of the column plus
      // include space for the NULL-terminator.

      pBindings[idxBinding].cbMaxLen = (pDBColumnInfo[idxBinding].ulColumnSize * 2 + 1) * sizeof(WCHAR);
      break;

      case DBTYPE_STR:
      case DBTYPE_WSTR:
      case DBTYPE_BSTR:
      // Going from a string to our string representation,
      // we can just take the maximum size of the column,
      // a count of characters, and include space for the
      // terminator, which is not included in the column size.

      pBindings[idxBinding].cbMaxLen = (pDBColumnInfo[idxBinding].ulColumnSize + 1) * sizeof(WCHAR);
      break;

      default:
      // For any other type, we will simply use our maximum
      // column buffer size, since the display size of these
      // columns may be variable (e.g. DBTYPE_VARIANT) or
      // unknown (e.g. provider-specific types).
      pBindings[idxBinding].cbMaxLen = MAX_COL_SIZE;
      break;
   }

   // If the provider's native data type for this column is
   // DBTYPE_IUNKNOWN or this is a BLOB column and the user
   // has requested that we bind BLOB columns as ISequentialStream
   // objects, bind this column as an ISequentialStream object if
   // the provider supports our creating another ISequentialStream
   // binding.
   if(pDBColumnInfo[idxBinding].dwFlags & DBCOLUMNFLAGS_ISLONG)
   {
      pBindings[idxBinding].wType = DBTYPE_IUNKNOWN;

      pBindings[idxBinding].cbMaxLen = sizeof(ISequentialStream*);

      pBindings[idxBinding].pObject = (DBOBJECT *)CoTaskMemAlloc(sizeof(DBOBJECT));

      if (!pBindings[idxBinding].pObject)
      {
         hr = E_OUTOFMEMORY;
         goto _ExitProcessResultSet;
      }

      // Direct the provider to create an ISequentialStream
      // object over the data for this column.
      pBindings[idxBinding].pObject->iid = IID_ISequentialStream;

      // We want read access on the ISequentialStream
      // object that the provider will create for us
      pBindings[idxBinding].pObject->dwFlags = STGM_READ;
      }

      // Ensure that the bound maximum length is no more than the
      // maximum column size in bytes that we've defined.
      pBindings[idxBinding].cbMaxLen = min(pBindings[idxBinding].cbMaxLen, MAX_COL_SIZE);

      // Update the offset past the end of this column's data, so
      // that the next column will begin in the correct place in
      // the buffer.
      dwOffset = pBindings[idxBinding].cbMaxLen + pBindings[idxBinding].obValue;

      // Ensure that the data for the next column will be correctly
      // aligned for all platforms, or, if we're done with columns,
      // that if we allocate space for multiple rows that the data
      // for every row is correctly aligned.
      dwOffset = ROUNDUP(dwOffset);
   }

   hr = pIRowset->QueryInterface(IID_IAccessor, (void **) &pIAccessor);
   CHKHR_GOTO(hr, L"Failed to obtain Accessor interface", _ExitProcessResultSet);

   hr = pIAccessor->CreateAccessor(DBACCESSOR_ROWDATA,
      lNumCols,
      pBindings,
      0,
      &hAccessor,
      NULL);

   CHKHR_GOTO(hr, L"Failed to create Accessor", _ExitProcessResultSet);
   for (idxBinding = 0; idxBinding < lNumCols; idxBinding++) 
   {
      cout << pDBColumnInfo[idxBinding].pwszName << endl;
   }

   lNumRowsRetrieved = 0;

   hr = pIRowset->GetNextRows(
      NULL,
      0,
      1,
      &lNumRowsRetrieved,
      &pRow);

   CHKHR_GOTO(hr, L"Failed to fetch a row from the rowset", _ExitProcessResultSet);

   pBuffer = new BYTE[sizeof(DBSTATUS) + sizeof(DBLENGTH) + sizeof(IUnknown*)];

   if (!pBuffer)
   {
      hr = E_OUTOFMEMORY;
      goto _ExitProcessResultSet;
   }

   while(lNumRowsRetrieved && hr != DB_S_ENDOFROWSET) 
   {
      memset(pBuffer, 0, sizeof(DBSTATUS) + sizeof(DBLENGTH) + sizeof(IUnknown*));

      hr = pIRowset->GetData(hRows[0], hAccessor, pBuffer);
      CHKHR_GOTO(hr, L"Failed to obtain row data", _ExitProcessResultSet);

      for (idxBinding = 0; idxBinding < lNumCols; idxBinding++)
      {
         if (pBindings[idxBinding].wType == DBTYPE_IUNKNOWN)
         {
            BYTE pbBuff[3000];
            ULONG cbNeeded = sizeof(pbBuff)/sizeof(BYTE);
            ULONG cbRead;
            ULONG cbReadTotal = 0;
            ISequentialStream* pISequentialStream = NULL;

            IUnknown* pIUnknown = *((IUnknown**)(pBuffer + pBindings[idxBinding].obValue));
            pIUnknown->QueryInterface(IID_ISequentialStream, (void**)&pISequentialStream);

            do
            {
               hr = pISequentialStream->Read(pbBuff, cbNeeded, &cbRead);
               cbReadTotal += cbRead;
            }
            while (SUCCEEDED(hr) && hr != S_FALSE && cbRead == cbNeeded);

               cout << "Total Bytes Read: " << cbReadTotal << endl;

               pISequentialStream->Release();
               pISequentialStream = NULL;
               pIUnknown->Release();
               pIUnknown = NULL;
            }
         }

         pIRowset->ReleaseRows(1, pRow, NULL, NULL, NULL);

         hr = pIRowset->GetNextRows(NULL,
            0,
            1,
            &lNumRowsRetrieved,
            &pRow);

         CHKHR_GOTO(hr, L"Failed to fetch a row from the rowset.", _ExitProcessResultSet);
   }

_ExitProcessResultSet:

   pIRowset->ReleaseRows(1, pRow, NULL, NULL, NULL);
   delete [] pBuffer;

   if (pIAccessor)
   {
      if (hAccessor != DB_NULL_HACCESSOR)
      {
         pIAccessor->ReleaseAccessor(hAccessor, NULL);
      }

      pIAccessor->Release();
      pIAccessor = NULL;
   }

   if (pBindings)
   {
      for (idxBinding = 0; idxBinding < lNumCols; idxBinding++)
      {
         if (pBindings[idxBinding].pObject)
         CoTaskMemFree(pBindings[idxBinding].pObject);
      }
   }

   delete [] pBindings;

   CoTaskMemFree(pDBColumnInfo);
   CoTaskMemFree(pStringsBuffer);

   if (pIColumnsInfo)
   {
      pIColumnsInfo->Release();
      pIColumnsInfo = NULL;
   }

   return hr;
}

如需有關 SQL Server Native Client OLE DB 提供者如何公開大數值資料類型的詳細資訊，請參閱＜BLOB 與 OLE 物件＞。

SQL Server Native Client ODBC 驅動程式

SQL Server Native Client ODBC 驅動程式會在接受或傳回 ODBC SQL 資料類型的 ODBC API 函數中，將 varchar(max)、varbinary(max) 和 nvarchar(max) 類型公開為 SQL_VARCHAR、SQL_VARBINARY 和 SQL_WVARCHAR。

在報告資料行的最大大小時，驅動程式會報告下列其中一項：

定義的最大大小，例如 varchar(2000) 資料行的最大大小為 2000，或者
"unlimited" 這個值，如果是 varchar(max) 資料行則等於 0。

下列是 ODBC API 函數的清單，這些函數經過改良，可用於大數值資料類型：

請參閱

其他資源

SQL Server Native Client 功能

共用方式為

使用大數值類型

SQL Server Native Client OLE DB 提供者

SQL Server Native Client ODBC 驅動程式

請參閱

其他資源

其他資源