select e.emp_id
from employees e
minus
select h.emp_id
from hand_punch h
where h.date_column between to_date('20.10.2012 07:00', 'dd.mm.yyyy')
                        and to_date('20.10.2012 07:30', 'dd.mm.yyyy')

select e.emp_id
  from employees e
 where not exists (select 1 
                     from hand_punch h
                         where h.emp_id = e.emp_id 
                           and h.date_column between to_date('20.10.2012 07:00', 'dd.mm.yyyy')
                                                 and to_date('20.10.2012 07:30', 'dd.mm.yyyy')
                           )