Execute, Requery, and Clear Methods Example (VC++)
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Execute, Requery, and Clear Methods Example (VC++)

 

This example demonstrates the Execute method when run from both a Command object and a Connection object. It also uses the Requery method to retrieve current data in a recordset, and the Clear method to clear the contents of the Errors collection. The ExecuteCommand and PrintOutput functions are required for this example to run.

// Execute_Requery_Clear_Method_Sample.cpp
// compile with: /EHsc
#include <ole2.h>
#include <stdio.h>

#import "msado15.dll" no_namespace rename("EOF", "EndOfFile")

// Function declarations
inline void TESTHR(HRESULT x) { if FAILED(x) _com_issue_error(x); };
void ExecuteX();
void ExecuteCommand(_CommandPtr pCmdTemp, _RecordsetPtr pRstTemp);
void PrintOutput(_RecordsetPtr pRstTemp);
void PrintProviderError(_ConnectionPtr pConnection);
void PrintComError(_com_error &e);

int main() {
   if ( FAILED(::CoInitialize(NULL)) )
      return -1;

   ExecuteX();
   ::CoUninitialize();
}

void ExecuteX() {
   // Define string variables.
   _bstr_t strSQLChange("UPDATE Titles SET Type = 'self_help' WHERE Type = 'psychology'");
   _bstr_t strSQLRestore("UPDATE Titles SET Type = 'psychology' WHERE Type = 'self_help'");
   _bstr_t strCnn("Provider='sqloledb'; Data Source='My_Data_Source'; Initial Catalog='pubs'; Integrated Security='SSPI';");

   // Define ADO object pointers.  Initialize pointers on define.
   // These are in the ADODB::  namespace.
   _ConnectionPtr pConnection = NULL;
   _CommandPtr pCmdChange = NULL;
   _RecordsetPtr pRstTitles = NULL;

   try {
      // Open connection.
      TESTHR(pConnection.CreateInstance(__uuidof(Connection)));
      pConnection->Open (strCnn, "", "", adConnectUnspecified);

      // Create command object.
      TESTHR(pCmdChange.CreateInstance(__uuidof(Command)));
      pCmdChange->ActiveConnection = pConnection;
      pCmdChange->CommandText = strSQLChange;

      // Open titles table, casting Connection pointer to an 
      // IDispatch type so converted to correct type of variant.
      TESTHR(pRstTitles.CreateInstance(__uuidof(Recordset)));
      pRstTitles->Open ("Titles", _variant_t((IDispatch *) pConnection, true), 
         adOpenStatic, adLockOptimistic, adCmdTable);

      // Print report of original data.
      printf("\n\nData in Titles table before executing the query: \n");

      // Call function to print loop recordset contents.
      PrintOutput(pRstTitles);

      // Clear extraneous errors from the Errors collection.
      pConnection->Errors->Clear();

      // Call ExecuteCommand subroutine to execute pCmdChange command.
      ExecuteCommand(pCmdChange, pRstTitles);

      // Print report of new data.
      printf("\n\n\tData in Titles table after executing the query: \n");
      PrintOutput(pRstTitles);

      // Use Connection object's Execute method to execute SQL statement to restore data.
      pConnection->Execute(strSQLRestore, NULL, adExecuteNoRecords);

      // Retrieve the current data by requerying the recordset.
      pRstTitles->Requery(adCmdUnknown);

      // Print report of restored data.
      printf("\n\n\tData after exec. query to restore original info: \n");
      PrintOutput(pRstTitles);
   }
   catch (_com_error &e) {
      PrintProviderError(pConnection);
      PrintComError(e);
   }

   // Clean up objects before exit.
   if (pRstTitles)
      if (pRstTitles->State == adStateOpen)
         pRstTitles->Close();
   if (pConnection)
      if (pConnection->State == adStateOpen)
         pConnection->Close();
}

void ExecuteCommand(_CommandPtr pCmdTemp, _RecordsetPtr pRstTemp) {
   try {
      // CommandText property already set before function was called.
      pCmdTemp->Execute(NULL, NULL, adCmdText);

      // Retrieve the current data by requerying the recordset.
      pRstTemp->Requery(adCmdUnknown);
   }

   catch(_com_error &e) {
      // Notify user of any errors that result from executing the query.
      // Pass a connection pointer accessed from the Recordset.
      PrintProviderError(pRstTemp->GetActiveConnection());
      PrintComError(e);
   }
}

void PrintOutput(_RecordsetPtr pRstTemp) {
   // Ensure at top of recordset.
   pRstTemp->MoveFirst();

   // If EOF is true, then no data and skip print loop.
   if ( pRstTemp->EndOfFile )
      printf("\tRecordset empty\n");
   else {
      // Define strings for output conversions.  Initialize to first record's values.
      _bstr_t bstrTitle;
      _bstr_t bstrType;

      // Enumerate Recordset and print from each.
      while ( !(pRstTemp->EndOfFile) ) {
         // Convert variant string to convertable string type.
         bstrTitle = pRstTemp->Fields->GetItem("Title")->Value;
         bstrType  = pRstTemp->Fields->GetItem("Type")->Value;
         printf("\t%s, %s \n", (LPCSTR) bstrTitle, (LPCSTR) bstrType);

         pRstTemp->MoveNext();
      }
   }
}

void PrintProviderError(_ConnectionPtr pConnection) {
   // Print Provider Errors from Connection object.
   // pErr is a record object in the Connection's Error collection.
   ErrorPtr pErr = NULL;

   if ( (pConnection->Errors->Count) > 0 ) {
      long nCount = pConnection->Errors->Count;
      // Collection ranges from 0 to nCount -1.
      for ( long i = 0 ; i < nCount ; i++ ) {
         pErr = pConnection->Errors->GetItem(i);
         printf("\t Error number: %x\t%s", pErr->Number, pErr->Description);
      }
   }
}

void PrintComError(_com_error &e) {
   _bstr_t bstrSource(e.Source());
   _bstr_t bstrDescription(e.Description());

   // Print Com errors.
   printf("Error\n");
   printf("\tCode = %08lx\n", e.Error());
   printf("\tCode meaning = %s\n", e.ErrorMessage());
   printf("\tSource = %s\n", (LPCSTR) bstrSource);
   printf("\tDescription = %s\n", (LPCSTR) bstrDescription);
}
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