[[AgustinOrtu]]

Estaba convencido de que se podia

Código Delphi [-]
procedure TForm1.Button1Click(Sender: TObject);
begin
  ADOQuery1.Close;
  if Edit1.Text <> '' then
    ADOQuery1.Parameters.ParamByName('ParamId').Value := Edit1.Text
  else
    ADOQuery1.Parameters.ParamByName('ParamId').Value := NULL;
  ADOQuery1.Open;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  ADOQuery1.SQL.Text := ' SELECT * FROM Productos WHERE Id = IsNull(:ParamId, Id) ';
  ADOQuery1.Prepared := True;
end;

Saludos