procedure TForm2.Button1Click(Sender: TObject);
begin
SimpleDataSet1.DataSet.CommandText:='select * from userlist where firstName=:Edit1.Text and lastName=:Edit2.Text';
if SimpleDataSet1.DataSet.RecordCount=0 then
begin
ShowMessage('Такого Логина нету');
Edit1.Clear;
Edit2.Clear;
end;
end;