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.

// BeginExecuteCpp
#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);
void ExecuteCommand(_CommandPtr pCmdTemp, _RecordsetPtr pRstTemp);
void PrintOutput(_RecordsetPtr pRstTemp);
void PrintProviderError(_ConnectionPtr pConnection);
void PrintComError(_com_error &e);

////////////////////////////////
//      Main Function         //
////////////////////////////////

void main()
{
    if(FAILED(::CoInitialize(NULL)))
        return;

    ExecuteX();

    ::CoUninitialize();
}

///////////////////////////////////
//      ExecuteX Function        //
///////////////////////////////////

void ExecuteX(void) 
{
   HRESULT    hr = S_OK;

    // 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='MySqlServer';"
            "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 the 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();
}

//////////////////////////////////////////
//      ExecuteCommand Function         //
//////////////////////////////////////////

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);
    }
}

/////////////////////////////////////
//      PrintOutput Function       //
/////////////////////////////////////

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 temporary 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();
        }
    }
}

///////////////////////////////////////////////
//      PrintProviderError Function          //
///////////////////////////////////////////////

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);
        }
    }
}

//////////////////////////////////////
//      PrintComError Function      //
//////////////////////////////////////

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);
}
// EndExecuteCpp

See Also

Clear Method | Command Object | Connection Object | Errors Collection | Execute Method (ADO Command) | Execute Method (ADO Connection) | Requery Method