i was 
create user nur identified by nur;
then i 
create role admin;
grant dba to admin;
grant admin to nur;

and i also 
create user aiznu identified by aiznu;
create role r_staff;
grant r_staff to aiznu;

in my form:

DECLARE
   v_alert NUMBER;
BEGIN
   if :USERNAME is null then
      message('Username must be entered');
      go_item('LOGON.USER');
      return;
   end if;

   if :PASSWORD is null then
      MESSAGE('Password must be entered');
      go_item('LOGON.PASSWORD');
      return;
   end if;

    IF (:counter < 3 ) THEN
       :counter := :counter + 1;

       LOGON(:USERNAME, :PASSWORD || '@'||'DBA', TRUE);

       :GLOBAL.USER := :USERNAME;
       :GLOBAL.PASSWORD := :PASSWORD;

       BEGIN
          SELECT ROLE
          INTO :GLOBAL.v_role
          FROM SESSION_ROLES
          WHERE ROLE IN ('ADMIN','R_STAFF','R_MEMBER');

          IF (:GLOBAL.v_role='R_STAFF' ) THEN
             NEW_FORM(:global.Form_Path ||'main.fmx');

          ELSIF (:GLOBAL.v_role='ADMIN') THEN
             NEW_FORM(:global.Form_Path ||'admin.fmx');

          ELSIF (:GLOBAL.v_role='R_MEMBER' ) THEN
             NEW_FORM(:global.Form_Path ||'membermain.fmx');
          END IF;

       EXCEPTION
          WHEN OTHERS THEN
             v_alert:=Show_Alert('LOGON_FAILURE');
             :USERNAME :='';
             :PASSWORD :='';
       END;
    ELSE
       LOGON(:USERNAME, :PASSWORD || '@'||'DBA', FALSE);

       v_alert:=Show_Alert('ATTEMPT_FAILURE');
       :USERNAME :='';
       :PASSWORD :='';

       IF v_alert=ALERT_BUTTON1 THEN
          --Exit_Form;
       NEW_FORM(:global.Form_Path ||'logon.fmx');

       END IF;
   END IF;
END;

EXCEPTION
          WHEN OTHERS THEN
             v_alert:=Show_Alert('LOGON_FAILURE');
             :USERNAME :='';
             :PASSWORD :='';
       END;

SELECT ROLE
      INTO :GLOBAL.v_role
      FROM SESSION_ROLES
      WHERE ROLE IN ('ADMIN','R_STAFF','R_MEMBER');

SQL> select role
  2  from session_roles
  3  where role in ('ADMIN', 'R_STAFF', 'R_MEMBER');

no rows selected

SQL>

SQL> conn nurul/nurul@dba
Connected.
SQL> select role
  2  from session_roles
  3  where role in ('ADMIN', 'R_STAFF', 'R_MEMBER');

ROLE
------------------------------
R_MEMBER
R_STAFF
ADMIN

 declare
   l_role session_roles.role%type;
 begin
   select role
     into l_role
     from session_roles
     where role in ('ADMIN', 'R_STAFAF', 'R_MEMBER');
 end;
 /

SQL> declare
  2     l_role session_roles.role%type;
  3   begin
  4     select role
  5       into l_role
  6       from session_roles
  7       where role in ('ADMIN', 'R_STAFAF', 'R_MEMBER');
  8   end;
  9   /
declare
*
ERROR at line 1:
ORA-01422: exact fetch returns more than requested number of rows
ORA-06512: at line 4

SQL> CREATE PROCEDURE ROLE_PROC
  2  declare
  3     l_role session_roles.role%type;
  4   begin
  5     select role
  6       into l_role
  7       from session_roles
  8       where role in ('ADMIN', 'R_STAFAF', 'R_MEMBER');
  9   end;
 10   /

Warning: Procedure created with compilation errors.

ERROR at line 1:
ORA-01422: exact fetch returns more than requested number of rows