Recuperare righe utilizzando i segnalibri (OLE DB)
Si applica a: 
SQL Server Database SQL di Azure Istanza gestita di SQL di Azure Azure Synapse Analytics Piattaforma di strumenti analitici (PDW)
Il consumer imposta il valore del campo dwFlag della struttura di associazione su DBCOLUMNSINFO_ISBOOKMARK per indicare che la colonna viene utilizzata come segnalibro. Imposta inoltre la proprietà del set di righe DBPROP_BOOKMARKS su VARIANT_TRUE, consentendo alla colonna 0 di essere presente nel set di righe. Viene quindi utilizzato IRowsetLocate::GetRowsAt per recuperare le righe, a partire da quella specificata in corrispondenza di un offset da un segnalibro.
Importante
Se possibile, usare l'autenticazione di Windows. Se non è disponibile, agli utenti verrà richiesto di immettere le credenziali in fase di esecuzione. Evitare di archiviare le credenziali in un file. Se è necessario rendere persistenti le credenziali, è consigliabile crittografarle usando l'API di crittografia Win32.
Per recuperare righe utilizzando i segnalibri
Stabilire una connessione all'origine dati.
Impostare la proprietà DBPROP_IRowsetLocate del set di righe su VARIANT_TRUE.
Eseguire il comando .
Impostare il campo dwFlag della struttura di associazione sul flag DBCOLUMNSINFO_ISBOOKMARK della colonna che verrà utilizzata come segnalibro.
Usare IRowsetLocate::GetRowsAt per recuperare le righe, a partire da quella specificata da un offset del segnalibro.
Esempio
In questo esempio viene illustrato come recuperare righe mediante un segnalibro. Questo esempio non è supportato in IA64.
In questo esempio la quinta riga viene recuperata dal set di risultati prodotto in seguito all'esecuzione di un'istruzione SELECT.
Gli esempi di codice Transact-SQL in questo articolo utilizzano il database campione AdventureWorks2022 o AdventureWorksDW2022, che è possibile scaricare dalla home page di Esempi di Microsoft SQL Server e progetti collettivi.
Compilare il listato di codice C++ seguente con ole32.lib oleaut32.lib ed eseguirlo. In questa applicazione viene eseguita la connessione all'istanza predefinita di SQL Server nel computer in uso. In alcuni sistemi operativi Windows sarà necessario modificare (local) o (localhost) impostando il valore sul nome dell'istanza di SQL Server. Per connettersi a un'istanza denominata, modificare il stringa di connessione da L"(local)" a L"(local)\name", dove name è l'istanza denominata. Per impostazione predefinita, SQL Server Express viene installato in un'istanza denominata. Verificare che nella variabile di ambiente INCLUDE sia presente la directory che contiene msoledbsql.h.
// compile with: ole32.lib oleaut32.lib
int InitializeAndEstablishConnection();
int ProcessResultSet();
#define UNICODE
#define _UNICODE
#define DBINITCONSTANTS
#define INITGUID
#define OLEDBVER 0x0250 // to include correct interfaces
#include <stdio.h>
#include <tchar.h>
#include <stddef.h>
#include <windows.h>
#include <iostream>
#include <oledb.h>
#include <msoledbsql.h>
using namespace std;
IDBInitialize* pIDBInitialize = NULL;
IDBProperties* pIDBProperties = NULL;
IDBCreateSession* pIDBCreateSession = NULL;
IDBCreateCommand* pIDBCreateCommand = NULL;
ICommandProperties* pICommandProperties = NULL;
ICommandText* pICommandText = NULL;
IRowset* pIRowset = NULL;
IColumnsInfo* pIColumnsInfo = NULL;
DBCOLUMNINFO* pDBColumnInfo = NULL;
IAccessor* pIAccessor = NULL;
IRowsetLocate* pIRowsetLocate = NULL;
DBPROP InitProperties[4];
DBPROPSET rgInitPropSet[1];
DBPROPSET rgPropSets[1];
DBPROP rgProperties[1];
ULONG i, j;
HRESULT hresult;
DBROWCOUNT cNumRows = 0;
DBORDINAL lNumCols;
WCHAR* pStringsBuffer;
DBBINDING* pBindings;
DBLENGTH ConsumerBufferColOffset = 0;
HACCESSOR hAccessor;
DBCOUNTITEM lNumRowsRetrieved;
HROW hRows[5];
HROW* pRows = &hRows[0];
char* pBuffer;
int main() {
// The command to execute.
// WCHAR* wCmdString = OLESTR(" SELECT title_id, title FROM titles ");
WCHAR* wCmdString = OLESTR(" SELECT Name FROM Production.Product");
// Initialize and establish a connection to the data source.
if (InitializeAndEstablishConnection() == -1) {
// Handle error.
cout << "Failed to initialize and connect to the server.\n";
return -1;
}
// Create a session object.
if (FAILED(pIDBInitialize->QueryInterface(IID_IDBCreateSession, (void**) &pIDBCreateSession))) {
cout << "Failed to obtain IDBCreateSession interface.\n";
// Handle error.
return -1;
}
if (FAILED(pIDBCreateSession->CreateSession(NULL, IID_IDBCreateCommand, (IUnknown**) &pIDBCreateCommand))) {
cout << "pIDBCreateSession->CreateSession failed.\n";
// Handle error.
return -1;
}
// Access the ICommandText interface.
if (FAILED(pIDBCreateCommand->CreateCommand( NULL, IID_ICommandText, (IUnknown**) &pICommandText))) {
cout << "Failed to access ICommand interface.\n";
// Handle error.
return -1;
}
// Set DBPROP_IRowsetLocate
if (FAILED(pICommandText->QueryInterface( IID_ICommandProperties, (void **) &pICommandProperties ))) {
cout << "Failed to obtain ICommandProperties interface.\n";
// Handle error.
return -1;
}
// Set DBPROP_IRowsetLocate to VARIANT_TRUE to get the IRowsetLocate interface.
VariantInit(&rgProperties[0].vValue);
rgPropSets[0].guidPropertySet = DBPROPSET_ROWSET;
rgPropSets[0].cProperties = 1;
rgPropSets[0].rgProperties = rgProperties;
// Set properties in the property group (DBPROPSET_ROWSET)
rgPropSets[0].rgProperties[0].dwPropertyID = DBPROP_IRowsetLocate;
rgPropSets[0].rgProperties[0].dwOptions = DBPROPOPTIONS_REQUIRED;
rgPropSets[0].rgProperties[0].colid = DB_NULLID;
rgPropSets[0].rgProperties[0].vValue.vt = VT_BOOL;
rgPropSets[0].rgProperties[0].vValue.boolVal= VARIANT_TRUE;
// Set the rowset properties.
hresult = pICommandText->QueryInterface( IID_ICommandProperties,(void **)&pICommandProperties);
if (FAILED(hresult)) {
printf("Failed to get ICommandProperties to set rowset properties.\n");
// Release any references and return.
return -1;
}
hresult = pICommandProperties->SetProperties(1, rgPropSets);
if (FAILED(hresult)) {
printf("Execute failed to set rowset properties.\n");
// Release any references and return.
return -1;
}
pICommandProperties->Release();
// Specify the command text.
if (FAILED(pICommandText->SetCommandText(DBGUID_DBSQL, wCmdString))) {
cout << "Failed to set command text.\n";
// Handle error.
return -1;
}
// Execute the command.
if (FAILED(hresult =
pICommandText->Execute( NULL, IID_IRowset, NULL, &cNumRows, (IUnknown **) &pIRowset))) {
cout << "Failed to execute command.\n";
// Handle error.
return -1;
}
ProcessResultSet();
pIRowset->Release();
// Free up memory.
pICommandText->Release();
pIDBCreateCommand->Release();
pIDBCreateSession->Release();
pIDBInitialize->Uninitialize();
pIDBInitialize->Release();
// Release COM library.
CoUninitialize();
return -1;
}
int InitializeAndEstablishConnection() {
// Initialize the COM library.
CoInitialize(NULL);
// Obtain access to the OLE DB Driver for SQL Server.
CoCreateInstance( CLSID_MSOLEDBSQL, NULL, CLSCTX_INPROC_SERVER, IID_IDBInitialize, (void **) &pIDBInitialize);
// Initialize the property values that are the same for each property.
for ( i = 0 ; i < 4 ; i++ ) {
VariantInit(&InitProperties[i].vValue);
InitProperties[i].dwOptions = DBPROPOPTIONS_REQUIRED;
InitProperties[i].colid = DB_NULLID;
}
// Server name.
InitProperties[0].dwPropertyID = DBPROP_INIT_DATASOURCE;
InitProperties[0].vValue.vt = VT_BSTR;
InitProperties[0].vValue.bstrVal = SysAllocString(L"(local)");
// Database.
InitProperties[1].dwPropertyID = DBPROP_INIT_CATALOG;
InitProperties[1].vValue.vt = VT_BSTR;
InitProperties[1].vValue.bstrVal = SysAllocString(L"AdventureWorks");
InitProperties[2].dwPropertyID = DBPROP_AUTH_INTEGRATED;
InitProperties[2].vValue.vt = VT_BSTR;
InitProperties[2].vValue.bstrVal = SysAllocString(L"SSPI");
// Construct the PropertySet array.
rgInitPropSet[0].guidPropertySet = DBPROPSET_DBINIT;
rgInitPropSet[0].cProperties = 4;
rgInitPropSet[0].rgProperties = InitProperties;
// Set initialization properties.
pIDBInitialize->QueryInterface(IID_IDBProperties, (void **)&pIDBProperties);
hresult = pIDBProperties->SetProperties(1, rgInitPropSet);
if (FAILED(hresult)) {
cout << "Failed to set initialization properties.\n";
// Handle error.
return -1;
}
pIDBProperties->Release();
// Call the initialization method to establish the connection.
if (FAILED(pIDBInitialize->Initialize())) {
cout << "Problem initializing and connecting to the data source.\n";
// Handle error.
return -1;
}
return 0;
}
#ifdef _WIN64
#define BUFFER_ALIGNMENT 8
#else
#define BUFFER_ALIGNMENT 4
#endif
#define ROUND_UP(value) (value + (BUFFER_ALIGNMENT - 1) & ~(BUFFER_ALIGNMENT - 1))
int ProcessResultSet() {
HRESULT hr;
// Retrieve 5th row from the rowset (for example).
DBBKMARK iBookmark = 5;
pIRowset->QueryInterface(IID_IColumnsInfo, (void **)&pIColumnsInfo);
pIColumnsInfo->GetColumnInfo( &lNumCols, &pDBColumnInfo, &pStringsBuffer );
// Create a DBBINDING array.
pBindings = new DBBINDING[lNumCols];
if (!(pBindings /* = new DBBINDING[lNumCols] */ )) {
// Handle error.
return -1;
}
// Using the ColumnInfo strucuture, fill out the pBindings array.
for ( j = 0 ; j < lNumCols ; j++ ) {
pBindings[j].iOrdinal = j;
pBindings[j].obValue = ConsumerBufferColOffset;
pBindings[j].pTypeInfo = NULL;
pBindings[j].pObject = NULL;
pBindings[j].pBindExt = NULL;
pBindings[j].dwPart = DBPART_VALUE;
pBindings[j].dwMemOwner = DBMEMOWNER_CLIENTOWNED;
pBindings[j].eParamIO = DBPARAMIO_NOTPARAM;
pBindings[j].cbMaxLen = pDBColumnInfo[j].ulColumnSize + 1; // + 1 for null terminator
pBindings[j].dwFlags = 0;
pBindings[j].wType = pDBColumnInfo[j].wType;
pBindings[j].bPrecision = pDBColumnInfo[j].bPrecision;
pBindings[j].bScale = pDBColumnInfo[j].bScale;
// Recalculate the next buffer offset.
ConsumerBufferColOffset = ConsumerBufferColOffset + pDBColumnInfo[j].ulColumnSize;
ConsumerBufferColOffset = ROUND_UP(ConsumerBufferColOffset);
};
// Indicate that the first field is used as a bookmark by setting
// dwFlags to DBCOLUMNFLAGS_ISBOOKMARK.
pBindings[0].dwFlags = DBCOLUMNFLAGS_ISBOOKMARK;
// Get IAccessor interface.
hr = pIRowset->QueryInterface( IID_IAccessor, (void **)&pIAccessor);
if (FAILED(hr)) {
printf("Failed to get IAccessor interface.\n");
// Handle error.
return -1;
}
// Create accessor.
hr = pIAccessor->CreateAccessor( DBACCESSOR_ROWDATA,
lNumCols,
pBindings,
0,
&hAccessor,
NULL);
if (FAILED(hr)) {
printf("Failed to create an accessor.\n");
// Handle error.
return -1;
}
hr = pIRowset->QueryInterface( IID_IRowsetLocate, (void **) &pIRowsetLocate);
if (FAILED(hr)) {
printf("Failed to get IRowsetLocate interface.\n");
// Handle error.
return -1;
}
hr = pIRowsetLocate->GetRowsAt( 0,
NULL,
sizeof(DBBKMARK),
(BYTE *) &iBookmark,
0,
1,
&lNumRowsRetrieved,
&pRows);
if (FAILED(hr)) {
printf("Calling the GetRowsAt method failed.\n");
// Handle error.
return -1;
}
// Create buffer and retrieve data.
pBuffer = new char[ConsumerBufferColOffset];
if (!(pBuffer /* = new char[ConsumerBufferColOffset] */ )) {
// Handle error.
return -1;
}
memset(pBuffer, 0, ConsumerBufferColOffset);
hr = pIRowset->GetData(hRows[0], hAccessor, pBuffer);
if (FAILED(hr)) {
printf("Failed GetDataCall.\n");
// Handle error.
return -1;
}
char szTitle[7] = {0};
strncpy_s(szTitle, &pBuffer[pBindings[1].obValue], 6);
printf("%S\n", &pBuffer[pBindings[1].obValue]);
pIRowset->ReleaseRows(lNumRowsRetrieved, hRows, NULL, NULL, NULL);
// Release allocated memory.
delete [] pBuffer;
pIAccessor->ReleaseAccessor(hAccessor, NULL);
pIAccessor->Release();
delete [] pBindings;
return 0;
}