procedure TForm2.Button2Click(Sender: TObject);
begin
SimpleDataSet1.DataSet.CommandText:='select * from userlist';// where firstName=:Edit3.Text and lastName=:Edit6.Text';
SimpleDataSet1.DataSet.Active:=true;
SimpleDataSet1.Active:=true;

if DataSource1.DataSet.RecordCount = 1  then
begin
      ShowMessage('Такого Логина нету');
      Edit1.Clear;
      Edit2.Clear;
end; 
end;