procedure TForm2.Button2Click(Sender: TObject);
var s:string;
begin
s:='select * from userlist where firstName="'+Edit3.Text+'" and lastName="'+Edit6.Text+'"';
ShowMessage(s);
SimpleDataSet1.DataSet.CommandText:=s;
SimpleDataSet1.DataSet.Active:=false;
SimpleDataSet1.Active:=false;
SimpleDataSet1.DataSet.Active:=true;
SimpleDataSet1.Active:=true;

if DataSource1.DataSet.RecordCount = 0 then
  begin
    ShowMessage('Такого Логина нету');
  end
else   ShowMessage('OK');
end;