-- Ваши тестовые данные
with
  orders (id, order_time, is_back,link_id) as 
    (
      select * 
	    from (
		       values 
                 (1, '07:00', null,null),
                 (2, '08:00', null,4),
                 (3, '09:00', null,7),
                 (4, '10:00', 'true',2),
                 (5, '11:00', null,null),
                 (6, '12:00', null,null),
                 (7, '13:00', 'true',3),
                 (8, '14:00', null,null)
             ) v(a,b,c,d)
    )
-- Возможный вариант решения:
select o_b.*
  from orders o_b
  left join orders o_l
    on o_b.id = o_l.link_id
   and o_l.is_back is null
 order by coalesce(o_l.order_time, o_b.order_time), o_b.order_time