IndexNulls-Eigenschaft – Beispiel (VC++)

In diesem Beispiel wird die IndexNulls-Eigenschaft eines Index veranschaulicht. Der Code erstellt einen neuen Index und legt den Wert von IndexNulls basierend auf der Benutzereingabe fest. Anschließend wird der Index an die Tabelle "Mitarbeiter" im Northwind-Katalog angefügt. Der neue Index wird auf ein Recordset basierend auf der Tabelle Employees (Mitarbeiter) angewendet, und das Recordset wird geöffnet. Ein neuer Datensatz wird der Tabelle Employees (Mitarbeiter) mit einem Null-Wert im indizierten Feld hinzugefügt. Ob dieser neue Datensatz angezeigt wird, hängt von der Einstellung der IndexNulls-Eigenschaft ab.

// BeginIndexNullCpp.cpp  
// compile with: /EHsc  
#import "msado15.dll" rename("EOF", "EndOfFile")  
#import "msadox.dll" no_namespace  
  
#include "iostream"  
#include "icrsint.h"  
using namespace std;  
  
// This class extracts LastName,Country,FirstName from Employees table  
class CEmployeeRs : public CADORecordBinding {  
   BEGIN_ADO_BINDING(CEmployeeRs)  
      // Column LastName is the 2nd field in the table  
      ADO_VARIABLE_LENGTH_ENTRY2(2, adVarChar, m_szemp_LastName, sizeof(m_szemp_LastName), lemp_LastNameStatus, TRUE)  
  
      // Column Country is the 11th field in the table  
      ADO_VARIABLE_LENGTH_ENTRY2(11, adVarChar, m_szemp_Country, sizeof(m_szemp_Country), lemp_CountryStatus, TRUE)  
  
      // Column Country is the 17th field in the table  
      ADO_VARIABLE_LENGTH_ENTRY2(17, adVarChar, m_szemp_FirstName, sizeof(m_szemp_FirstName), lemp_FirstNameStatus, TRUE)  
      END_ADO_BINDING()  
  
public:  
   CHAR m_szemp_LastName[21];  
   ULONG lemp_LastNameStatus;  
   CHAR m_szemp_Country[16];  
   ULONG lemp_CountryStatus;  
   CHAR m_szemp_FirstName[11];  
   ULONG lemp_FirstNameStatus;  
};  
  
// Function declarations  
inline void TESTHR(HRESULT x) {if FAILED(x) _com_issue_error(x);};  
void IndexNullsX(_bstr_t);  
  
int main() {  
   if ( FAILED(::CoInitialize(NULL)) )  
      return -1;   
  
   printf("\nShow records having indexed field value = NULL? (Y/N):");  
   // char input = getche();  
   // Let's pretend you said Yes  
   char input = 'Y';  
  
   if ( toupper(input) == 'Y' )  
      IndexNullsX("Allow");  
   else if ( toupper(input) == 'N' )  
      IndexNullsX("Ignore");  
   else  
      exit(0);  
  
   ::CoUninitialize();  
}  
  
void IndexNullsX(_bstr_t strSel) {  
   HRESULT hr = S_OK;  
  
   // Define and initialize ADOX object pointers. These are in the ADODB namespace.  
   _CatalogPtr m_pCatalog = NULL;  
   _IndexPtr m_pIndexNew = NULL;  
  
   // Define ADODB object pointers  
   ADODB::_ConnectionPtr m_pCnn = NULL;  
   ADODB::_RecordsetPtr m_pRstEmployees = NULL;  
  
   // Define other variables  
   IADORecordBinding *picRs = NULL;   // Interface Pointer Declared  
   CEmployeeRs emprs;   // C++ Class Object  
  
   // Define string variable.  
   _bstr_t strCnn("Provider='Microsoft.JET.OLEDB.4.0';data source='c:\\Northwind.mdb';");  
  
   try {  
      TESTHR(hr = m_pCnn.CreateInstance(__uuidof(ADODB::Connection)));  
      TESTHR(hr = m_pCatalog.CreateInstance(__uuidof (Catalog)));  
      TESTHR(hr = m_pIndexNew.CreateInstance(__uuidof(Index)));  
      TESTHR(hr = m_pRstEmployees.CreateInstance(__uuidof(ADODB::Recordset)));  
  
      // Connect the catalog.  
      m_pCnn->Open(strCnn, "", "", NULL);  
      m_pCatalog->PutActiveConnection(_variant_t((IDispatch *)m_pCnn));  
  
      // Append Country column to new index.  
      m_pIndexNew->Columns->Append("Country", adVarWChar, 0);  
      m_pIndexNew->Name = "NewIndex";  
  
      // Set IndexNulls based on user input  
      if (strcmp((LPSTR)strSel, "Allow") == 0 )  
         m_pIndexNew->IndexNulls = adIndexNullsAllow;  
      else if (strcmp((LPSTR)strSel, "Ignore") == 0 )  
         m_pIndexNew->IndexNulls = adIndexNullsIgnore;  
  
      // Append new index to Employees table  
      m_pCatalog->Tables->GetItem("Employees")->Indexes->Append(_variant_t((IDispatch *)m_pIndexNew));  
      m_pRstEmployees->Index = m_pIndexNew->Name;  
      m_pRstEmployees->Open("Employees",   
         _variant_t((IDispatch *)m_pCnn),   
         ADODB::adOpenKeyset,   
         ADODB::adLockOptimistic,  
         ADODB::adCmdTableDirect);  
  
      // Add a new record to the Employees table.  
      m_pRstEmployees->AddNew();  
      m_pRstEmployees->Fields->GetItem("FirstName")->Value = (_bstr_t) "Gary";  
      m_pRstEmployees->Fields->GetItem("LastName")->Value = (_bstr_t) "Haarsager";  
      m_pRstEmployees->Update();  
  
      // Bookmark the newly added record.  
      _variant_t varBookmark = m_pRstEmployees->Bookmark;  
  
      // Use the new index to set the order of the records.  
      m_pRstEmployees->MoveFirst();  
      printf("\n\nIndex = %s,", (LPSTR) m_pRstEmployees->Index);  
      printf("IndexNulls = %d\n\n", m_pIndexNew->IndexNulls);  
      cout<<"Country            -        Name" << endl;  
  
      // Open an IADORecordBinding interface pointer for binding Recordset to a class  
      TESTHR(hr = m_pRstEmployees->QueryInterface(__uuidof(IADORecordBinding), (LPVOID*)&picRs));  
  
      // Bind the Recordset to a C++ class here   
      TESTHR(hr = picRs->BindToRecordset(&emprs));  
  
      // Enumerate the Recordset.The value of the IndexNulls property will determine if the newly  
      // added record appears in the output.  
      while ( !(m_pRstEmployees->EndOfFile) ) {  
         printf("%s    -    %s %s\n",  
            emprs.lemp_CountryStatus == adFldOK ? emprs.m_szemp_Country :"[Null]",  
            emprs.lemp_FirstNameStatus == adFldOK ? emprs.m_szemp_FirstName :"<NULL>",  
            emprs.lemp_LastNameStatus == adFldOK ? emprs.m_szemp_LastName :"<NULL>");  
  
         m_pRstEmployees->MoveNext();  
      }  
  
      // Delete new record because this is a demonstration.  
      m_pRstEmployees->Bookmark = varBookmark;  
      m_pRstEmployees->Delete(ADODB::adAffectCurrent);  
   }  
   catch(_com_error &e) {  
      // Notify the user of errors if any.  
      _bstr_t bstrSource(e.Source());  
      _bstr_t bstrDescription(e.Description());  
      printf("\n\tSource :  %s \n\tdescription : %s \n ", (LPCSTR)bstrSource, (LPCSTR)bstrDescription);  
   }  
   catch(...) {  
      cout << "Error occurred in include files...." << endl;  
   }  
  
   if (m_pRstEmployees)  
      if (m_pRstEmployees->State == 1) {  
         m_pRstEmployees->Close();  
         m_pRstEmployees = NULL;  
      }  
  
      // Delete new Index because this is a demonstration.  
      if (m_pCatalog)  
         m_pCatalog->Tables->GetItem("Employees")->Indexes->Delete(m_pIndexNew->Name);  
  
      if (m_pCnn)  
         if (m_pCnn->State == 1) {  
            m_pCnn->Close();  
            m_pCnn = NULL;  
         }  
  
         m_pCatalog = NULL;  
}