Retrieving data

The following code example demonstrates how to retrieve data.

1. Create a Command object:

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

2. Set the SQL statement:

   hr = pICommandText->SetCommandText(
        DBGUID_DBSQL,
        L"SELECT * FROM testReadStringData");
   

3. Create and describe the rowset data structure. This structure contains fields for each column you want accessed, as shown:

   IAccessor* pIAccessor;
   hr = pICommandText->QueryInterface(IID_IAccessor, (void**)&pIAccessor);
   
   static DBBINDING ExactBindings [1] = {
   {
        1, // iOrdinal
        offsetof (ExactlyTheSame,s), // obValue
        0, // No length binding
        0, // No Status binding
        NULL, // No TypeInfo
        NULL, // No Object
        NULL, // No Extensions
        DBPART_VALUE,
        DBMEMOWNER_CLIENTOWNED, // Ignored
        DBPARAMIO_NOTPARAM,
        sizeof(mystr), // number of bytes
        0,
        DBTYPE_WSTR | DBTYPE_BYREF,
        0, // No Precision
        0 // No Scale
   }
   };
   
   DBBINDSTATUS status[1];
   HACCESSOR hAccessor;
   HRESULT hr = pIAccessor->CreateAccessor(
        DBACCESSOR_ROWDATA, 1, ExactBindings,
        sizeof(ExactlyTheSame), &hAccessor, status);
   pIAccessor->Release();
   

4. Execute the rowset:

   DBROWCOUNT cRowsAffected;
   IRowset* pIRowset;
   hr = pICommandText->Execute(
   NULL, IID_IRowset, params,
   &cRowsAffected, (IUnknown**)&pIRowset);
   

5. Use the following code to get the rows one row at a time:

   DBCOUNTITEM cRowsReturned;
   HROW hRow[1];
   HROW* pRow = hRow;
   hr = pIRowset->GetNextRows(NULL, 0, 1, &cRowsReturned, &pRow);
   

6. Use IMalloc to free the memory allocated by GetData:

   CComPtr<IMalloc> pIMalloc = NULL;
   hr = CoGetMalloc( MEMCTX_TASK, &pIMalloc );
   
   while (hr == S_OK)
   {
   

7. Retrieve the data for the specified row, for example:

        ExactlyTheSame pData[1] = { {NULL} };
        hr = pIRowset->GetData(hRow[0], hAccessor, pData);
        wchar_t* value = pData[0].s;
   

8. Free the allocated memory:

        // client owned memory must be freed by the client
        pIMalloc->Free(pData[0].s);
        pData[0].s = NULL;
   

9. Release the rows:

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

10. Get the next row:

         hr = pIRowset->GetNextRows(NULL, 0, 1,
              &cRowsReturned, &pRow);
    }
    
    pIRowset->Release();
    pICommandText->Release();
    

To retrieve rows from a database, execute a SELECT statement using ICommandText::Execute. This opens a cursor on the statement.Then, use IRowset::GetNextRows to fetch rows through the cursor. When an application frees the statement by releasing the rowset, it closes the cursor.