LOGIN with my_own_users table

Question

Hi,
I need help!!
I have my_own_users table that contains username and password.
is there a way (possible) for the form to detect that?
I have tried following: (but it gives me frm-40735 error)
on the on-logon trigger,
declare
     un varchar2(80);
     pw varchar2(80);
     cn varchar2(80);
     cnt number;
     al_id alert;
begin
     Logon_Screen;
     un := GET_APPLICATION_PROPERTY(username);
     pw := GET_APPLICATION_PROPERTY(password);
     select count(*) into cnt
     from my_own_users
     where username = un
     and password = pw;
if cnt < 1 then
al_id := FIND_ALERT('invalid');
SET_ALERT_PROPERTY(al_id, alert_message_text, 'user'|| un);
end if;
end;

Answer 1

ok, I changed my mind, this is what i am doing but i am still having the same problem.
problem: when the user enter wrong username and password, it falls into WHEN_NO_DATA_FOUND, which is fine (the way I want it), but I want it to keep repating the loop until the user type inthe correct information. for some reason, it won't go back and won't give me the logon_screen.
Please tell me what I did wrong.
thanks
================================================
Here is my code:
declare
un varchar2(30);
rn varchar2(30);
pw varchar2(30);
al_btn number;
al_id alert;
begin     
LOOP
logon_screen;
un:=get_application_property(username);
pw:=get_application_property(password);
begin
     SELECT role_name
     INTO rn
FROM my_group_role gr, my_users r
WHERE gr.group_id = r.group_id
AND gr.role_name in ('be_browse', 'be_edit', 'be_admin')
AND r.username = un
AND r.password = pw;
exception
     when no_data_found then
al_id := FIND_ALERT('invalid');
SET_ALERT_PROPERTY(al_id,alert_message_text , 'Invalid user/password for '|| un );
al_btn:= show_alert(al_id);
raise form_trigger_failure;
when others then
al_id := FIND_ALERT('invalid');
-- SET_ALERT_PROPERTY(al_id,alert_message_text , 'INVALID user/password for '|| un);
SET_ALERT_PROPERTY(al_id, alert_message_text,'others'|| SQLCODE ||' * ' || SQLERRM);
al_btn:= show_alert(al_id);
raise form_trigger_failure;
end;
EXIT WHEN FORM_SUCCESS;
END LOOP;