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This article is the first of a series OLE DB articles that cover database
access using the OLE DB Templates. This is aimed primarily at beginners while
later articles will cover the intermeadiate and advanced aspects of using
the OLE DB Consumers. The code for these articles is developed on Win 2k under
Developer Studio 6 and Developer Studio 7. It will be stated and in some cases
obvious which version of developer studio the code was written in. The code for
this article was written in Developer Studio 6.
First of all an ODBC Driver needs to be specified on the system. To do this
open the Control Panel and double click on the ODBC Data Sources. This is
located in the Control Panel on Windows 98 and in Control Panel\Administrative
Tools in Windows 2000. Click on the System DSN tab and click on the add button.
If you are following this using the Microsoft NWind database supplied select a
Microsoft Access Driver ( *.mdb ) and call the Driver "AccessNWind" in the
DataSource Name box and click O.K. What this does is create an ODBC DataSource
that uses the access driver to access the database. All this will work
transparently behind the scenes once we create the OLE DB Consumer class.
To Insert an OLE DB Consumer class in the project first of all create a
standard MFC project. I've created a simple dialog project but in a real
working environment you'd probably want a better display system, that is, if
you wanted a display system at all. Once you have your project go to the Insert
menu and clin the "New ATL Object" menu. You will be asked if you want to add
ATL support to your MFC project. Answer yes to this and the main ATL dialog box
Select the Consumer and click next.
The image above shows the completed dialog for now yours wont look too much
like this so Click on the Select Datasource button and we'll set it up.
Although the picture above shows the Microsoft OLE DB Provider for ODBC Drivers
it's better to select one of the Microsoft Jet drivers as the
Microsoft OLE for ODBC drivers is no longer supported in the beta release
( 2 ) of Developer Studio 7. I don't know if this will be the case in the
final release but presume it will.. Click on the Next button,
Select the ODBC Driver for the AccessNWind Database that was set up earlier in
the use datasource name option. The next two pages of the dialog are simple so
I'll go through them quickly here. Click the Test Connection button just to
make sure that the connection to the Database is working correctly. As long as
you've set up the ODBC driver properly there shouldn't be any problem. If you
click on the Advanced tab the page presents you with the options that you
require for accessing the database. Note that these options are to tell the
driver the options that you require and at this point they are not going to be
reflected in your codes access to the the database. For this example it will be
fine if you select read/write access to the database. The final page in the
dialog displays the results of what you have selected.
Once you click on the O.K. button you will be given the dialog below
This dialog gives you the tables that are available in this database so that
the required code can be generated to access the database from your code. You
dont have to choose the Suppliers Database here but it might help if you want
to follow the examples coming up later.
Once you have selected the database table you will be taken back to the
starting dialog. which will be filled in and almost ready to finish and get on
with the code.
On the dialog you should select the table type. The two types will in
fact make little difference within this example but I tend to stick to the
guideline that tables are for when you want to go through the entire set of data
within the table on a row by row basis and the Command type is for when you
want to customize the data that you get back from the table. There will be a
lot more on commands in later examples as these will will be primarily what we
The final part of this dialog is the Support that we are going to add to the
database. In this example I've clicked them all but you should always remember
at this point that there's no point specifying a delete option here if you
didn't tell the driver to have write access to the database. At this point if
you've forgotten something you can change it easily.
Once you're happy with the dialog and the settings click on O.K.
The Generated Code
You should now have a Suppliers.h file added to your project and this is the
code that will do all your database access for you. If you look at the file you
will see there are two classes generated the first is the Accessor class and
the second is the
class. If you knew to OLE DB the accessor class
comes as something of a surprise because the first you know about it is when
you open the generated header file.
The Accessor Class
An accessor class is well, basically an accessor to the database. It defines
the structure of the database for your database class, which in this case is
memset(this, 0, sizeof(*this));
The class above is map that the code will use for a generated table. As you can
see it is nothing more than a copy of the columns in the database and once it
is initialised you will be able to move through the database using
CRowset::MoveNext();. The only thing that stands out here is the macros
BEGIN_COLUMN_MAP( classname )
END_COLUMN_MAP(). These macros are used
for the positioning of the member variables in a Command Accessor. If when we
were at the dialogs we had clicked on the Command radio button our class would
have looked like this,
COLUMN_ENTRY( 1, m_SupplierID )
COLUMN_ENTRY( 2, m_CompanyName )
COLUMN_ENTRY( 3, m_ContactName )
COLUMN_ENTRY( 4, m_Address )
COLUMN_ENTRY( 5, m_City )
COLUMN_ENTRY( 6, m_Region )
COLUMN_ENTRY( 7, m_PostalCode )
COLUMN_ENTRY( 8, m_Country )
COLUMN_ENTRY( 9, m_Phone )
COLUMN_ENTRY( 10, m_Fax )
COLUMN_ENTRY( 11, m_HomePage )
DEFINE_COMMAND( CSuppliersAccessor, _T( " \
FROM Suppliers" ) )
memset(this, 0, sizeof(*this));
You can see hear that the Accessor has filled in the
macros with the
COLUMN_ENTRY( column, variable )
bind the data in the rows when it is retrieved using the SQL statement which is
a new part of the class that is part of the macro,
At the moment as I said earlier it doesn't matter if you use the
Table or the Command Accessor at this point. I'll be highlighting the power of
the Command Accessor in the next essay.
The CSuppliers Class
class is the class that you will actually use in the code. The
supplier class inherits the accessor class that is defined above.
class CSuppliers : public CTable< CAccessor< CSuppliersAccessor > >
hr = OpenDataSource();
hr = db.Open(_T("MSDASQL"), &dbinit);
DBPROPVAL_UP_CHANGE | DBPROPVAL_UP_INSERT |
return CTable< CAccessor< CSupplierAccessor >
<CAccessor<CSuppliersAccessor> >::Open(m_session, _
As can be seen from the class declaration
class CSuppliers :
public CTable< CAccessor< CSuppliersAccessor > > <CAccessor<CSuppliersAccessor>
is wrapped by the
class which in turn is wrapped
by the CTable class. and the whole lot is then inherited by the
class which has very little functionality of its own. The
its functionality through the
classes that it
inherits from. What the
class does do is open the connection to the
database through the
function. This function calls the
remaining two functions declared in the class.
function sets up the properties for opening the
database by declaring and then Adding the properties to a
object. It also creates a
object and opens the database using the
method which takes two parameters the first being the name of
the provider of the data which in this case is the MSDASQL provider and then as
a second parameter it takes the
object that has been set up.
function then sets up the properties
for opening the
rowset by declaring another
object and initialising it with the
value. The properties are then added according to the access
requirements that were ticked in the check boxes earlier and then it calls
The Sample Code
The sample application uses the created files to load the data from the
database and to display it in a simple listbox format. which uses a header
control so that the view of each column can be spaced in order to be able to
read it properly.
To get the data from the database simply click on the Get Data button in the
top left hand corner of the dialog box.
class is used by including it in the Suppliers.h header file in
stdafx.h and then declaring an instance of the
CSupplier class as a member
class. The class is used in the
function and one the files have been generated it's use in a project like
this couldn't be easier.
HRESULT hResult = m_Suppliers.Open();
if( FAILED( hResult ) )
AfxMessageBox( _T( "Error opening the database" ) );
First of all the open function is called followed by the move first function.
That's all there is too it. All we have to do from here on in is load the data
into the list box like so.
int nRow = 0;
char szBuffer[ 25 ];
m_ListBox.InsertItem( LVIF_TEXT | LVIF_STATE, nRow,
ltoa( m_Suppliers.m_SupplierID, szBuffer, 10 ), 0,
LVIS_SELECTED, 0, NULL );
m_ListBox.SetItemText( nRow, 1, m_Suppliers.m_CompanyName );
m_ListBox.SetItemText( nRow, 2, m_Suppliers.m_ContactName );
m_ListBox.SetItemText( nRow, 3, m_Suppliers.m_ContactTitle );
m_ListBox.SetItemText( nRow, 4, m_Suppliers.m_Address );
m_ListBox.SetItemText( nRow, 5, m_Suppliers.m_City );
m_ListBox.SetItemText( nRow, 6, m_Suppliers.m_Region );
m_ListBox.SetItemText( nRow, 7, m_Suppliers.m_PostalCode );
m_ListBox.SetItemText( nRow, 8, m_Suppliers.m_Country );
m_ListBox.SetItemText( nRow, 9, m_Suppliers.m_Phone );
m_ListBox.SetItemText( nRow, 10, m_Suppliers.m_Fax );
m_ListBox.SetItemText( nRow, 11, m_Suppliers.m_HomePage );
while( m_Suppliers.MoveNext() == S_OK );
And that's how you set up and access an OLE DB database using OLE DB Templates.
In the next article we'll look at saving and modifying data and how to
customize queries using the Command class.