Question

I have a user registration process which stores user info to my database using MYDAC components. Currently it allow duplicate users, which is not my intention. My code is below, but I don’t know where the problem is.

procedure TForm1.Button1Click(Sender: TObject);
begin    
  if (edit1.Text <> '') and (edit2.Text <> '') and (edit3.Text <> '') and 
    (edit4.Text <> '') then
  begin
    MyQuery1.Close;
    MyQuery1.SQL.Text := 'select * from uyeler '+
                         'where nick=:0 and mail=:0 and site=:0';

    MyQuery1.Params[0].AsString:=edit1.text;
    MyQuery1.Params[0].AsString:=edit2.text;
    MyQuery1.Params[0].AsString:=edit3.text;

    MyQuery1.open;

    if MyQuery1.RecordCount = 0 then
      MessageDlg('The same information! Try again.', mtError, [mbOK], 0)
    else
      MyQuery1.Close;

    MyQuery1.SQL.Text := 'INSERT INTO uyeler (nick, mail, site, sifre) VALUES '+
                                            '(:nick, :mail, :site, :sifre)';

    MyQuery1.ParamByName('nick').AsString := Edit1.text;
    MyQuery1.ParamByName('mail').AsString := Edit2.text;
    MyQuery1.ParamByName('site').AsString := Edit3.text;
    MyQuery1.ParamByName('sifre').AsString := Edit4.text;
    MyQuery1.Execute;

    Button1.Enabled := False;
    MessageDlg('Mission complate!', mtInformation, [mbOK], 0);

    Edit1.Clear;
    Edit2.Clear;
    Edit3.clear;
    Edit4.Clear;

    PageControl2.Visible := False;
    PageControl1.Visible := True;
  end
  else
  begin
    MessageDlg('Information is missing! Try again.', mtWarning,[mbOK],0);    
  end;
end;

How can I prevent signing up with the same? What should I do in this case?

Answer 1

IMO the answer posted by @Ken White should work fine, but since you are finding troubles. I suggest you to try with this code. Its just the difference in the executing the queries.

I am considering the field datatypes to be Char or VarChar, hence ” ” while entering the data values

procedure TForm1.Button1Click(Sender: TObject);
begin
  Button1.Enabled:=false;

  if (edit1.Text <> '') and (edit2.Text <> '') and 
     (edit3.Text <> '') and (edit4.Text <> '') then
  begin
    MyQuery1.SQL.Clear;
    MyQuery1.SQL.Add(' select* from uyeler where nick="'+edit1.text+'"' +
                      'and mail="'+edit2.text+'" and site="'+edit3.text+'"');
    MyQuery1.Execute;

    if not MyQuery1.IsEmpty then   //--- can also use MyQuery1.RecordCount >0 
      MessageDlg('The same information! Try again.', mtError,[mbOK],0)
    else
    begin            //--- no duplicates present 
      MyQuery1.SQL.Clear;
      MyQuery1.SQL.Add(' INSERT INTO uyeler (nick, mail, site, sifre) VALUES '+
                            '("'+edit1.text+'", "'+edit2.text+'","'+edit3.text+'", "'+edit4.text+'")');

     try
        MyQuery1.Execute;
      finally
        MyQuery1.SQL.Clear;
      end;

     MessageDlg('Mission complate!', mtInformation,[mbOK],0);

     Edit1.Clear;
     Edit2.Clear;
     Edit3.clear;
     Edit4.Clear;

     PageControl2.Visible:=false;
     PageControl1.Visible:=true;
    end;                        
  end;                 
  else
   MessageDlg('Information is missing! Try again.', mtWarning,[mbOK],0);
end;