How to display and modify NULL-Values in a DBGrid

You have to write two event handlers to visualize and edit NULL values
in a DBGrid.

If the user want to reset a field to NULL, he must enter the field
and type Ctrl-0.

procedure TForm1.DBGrid1DrawColumnCell(Sender: TObject;
  const Rect: TRect; DataCol: Integer; Column: TColumn;
  State: TGridDrawState);
begin
   if not (gdFocused in State) then
   begin
      if Column.Field.IsNull then
      begin
         with TDBGrid(Sender).Canvas do
         begin
            Brush.Color := clDkGray;  // change color to your personal
                                      // prefs
            Pen.Color := clWhite;
            Rectangle(Rect);
         end;
      end;
   end;
end;

procedure TForm1.DBGrid1KeyDown(Sender: TObject; var Key: Word;
  Shift: TShiftState);
var
   TheGrid : TDBGrid;
begin
   // check for Ctrl-0  (zero, not uppercase Letter O)
   if (char(Key)='0') and (shift = [ssCtrl]) then
   begin
      TheGrid := (Sender as TDBGrid);
      if TheGrid.DataSource.AutoEdit then
         TheGrid.DataSource.Edit;
      TheGrid.SelectedField.Clear;
   end;
end;