create or replace trigger LOGOFF
before logoff on database
declare
   v_numses number(2);
begin
   select count(userid) into v_numses
   from bob.logtable
   where userid=user and
         machine=userenv('TERMINAL') and
         sesid=userenv('SESSIONID')
         and logouttime IS NULL;
   if v_numses=1 then
         update bob.logtable
         set logouttime=sysdate,
         loggedhours=(sysdate-logintime)*24
         where userid=user and
            machine=userenv('TERMINAL') and
            sesid=userenv('SESSIONID');
   End if;
end;
/