with t as
(
select 1 group_id, '1001' code, ''     up_code from dual union all
select 1 group_id, '3001' code, '1001' up_code from dual union all
select 1 group_id, '3002' code, '1001' up_code from dual union all
select 1 group_id, '5001' code, '3001' up_code from dual union all
select 1 group_id, '5002' code, '3002' up_code from dual union all
select 2 group_id, '1001' code, ''     up_code from dual union all
select 2 group_id, '3001' code, '1001' up_code from dual union all
select 2 group_id, '3002' code, '1001' up_code from dual union all
select 2 group_id, '5001' code, '3001' up_code from dual union all
select 2 group_id, '5001' code, '3002' up_code from dual
)
select *
  from t
 where 1 = 1
   and group_id = 1
 start with t.up_code is null
connect by prior t.code = t.up_code
;

위의 Data는 1 Group 과 2Group 이 같은Data를 가지고 있는데 
그룹별로 계층구조를 나열하고 싶습니다.

1    1001    
1    3001    1001
1    5001    3001
1    3002    1001
1    5002    3002
2    1001    
2    3001    1001
2    5001    3001
2    3002    1001
2    5001    3002

가능한가요?