create table test (
    test_id number( 9 )
  , constraint pk_test primary key (test_id)
)
/
insert into test (test_id)
  select rownum from all_objects where rownum <=  40 
/
delete test
where test_id between  4  and  7 
   or test_id between  10  and  15 
   or test_id between  20  and  25 
/
commit;

select t.*
     , case 
         when t.lead_test_id > t.test_id +  1  
          and t.lag_test_id = t.test_id -  1  
         then test_id +  1  
       end begin_hole
     , case 
         when t.lag_test_id < t.test_id -  1  
          and t.lead_test_id = t.test_id +  1  
         then test_id -  1  
       end end_hole
from (
  select t.test_id
       , lag(t.test_id) over (order by test_id) lag_test_id
       , lead(t.test_id) over (order by test_id) lead_test_id
  from test t
) t
/