Loop within Recursive Query

create table t

( id        number

, parent_id number

);

insert into t values (1, 2);

insert into t values (2, 1);

commit;

with tr (id,parent_id) as

( select id

       , parent_id

    from t

   where id = 1

   union all

  select t.id

       , t.parent_id

    from t

         join tr on t.parent_id = tr.id

) cycle id set is_cycle to '1' default '0'

select id

     , parent_id

     , is_cycle

  from tr;