create table test_connect_by (
parent number,
child number,
constraint uq_tcb unique (child)
);

insert into test_connect_by values ( 5, 2);
insert into test_connect_by values ( 5, 3);
insert into test_connect_by values (18,11);
insert into test_connect_by values (18, 7);
insert into test_connect_by values (17, 9);
insert into test_connect_by values (17, 8);
insert into test_connect_by values (26,13);
insert into test_connect_by values (26, 1);
insert into test_connect_by values (26,12);
insert into test_connect_by values (15,10);
insert into test_connect_by values (15, 5);
insert into test_connect_by values (38,15);
insert into test_connect_by values (38,17);
insert into test_connect_by values (38, 6);
insert into test_connect_by values (null, 38);
insert into test_connect_by values (null, 26);
insert into test_connect_by values (null, 18);

COMMIT;

In Oracle 10g:
SQL> select level, lpad(' ',2*(level-1)) || to_char(child) s
2 from test_connect_by
3 start with level = 1
4 connect by prior child = parent;

no rows selected


In Oracle 9i:
SQL> select level, lpad(' ',2*(level-1)) || to_char(child) s
2 from test_connect_by
3 start with level = 1
4 connect by prior child = parent;

LEVEL S



--------------------------------------------------------------------------------
---------------
1 2
1 3
1 10
1 5
2 2
2 3
1 9
1 8
1 11
1 7
1 13
1 1
1 12
1 15
2 10
2 5
3 2
3 3
1 17
2 9
2 8
1 6
1 38
2 15
3 10
3 5
4 2
4 3
2 17
3 9
3 8
2 6
1 26
2 13
2 1
2 12
1 18
2 11
2 7

39 rows selected

/*+ no_connect_by_filtering */ 

SQL>ed
Wrote file afiedt.buf

  1  select lpad(' ',2*(level-1)) || to_char(child) s
  2  from test_connect_by
  3  start with level = 1
  4* connect by prior child = parent
SQL>/
from test_connect_by
     *
ERROR at line 2:
ORA-01788: CONNECT BY clause required in this query block


SQL>select * from v$version;

BANNER
================================================================
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Prod
PL/SQL Release 10.2.0.1.0 - Production
CORE    10.2.0.1.0      Production
TNS for 32-bit Windows: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production

5 rows selected.

_allow_level_without_connect_by=true

SCOTT@orcl_11g> select banner from v$version
  2  /

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.1.0.6.0 - Production
PL/SQL Release 11.1.0.6.0 - Production
CORE	11.1.0.6.0	Production
TNS for 32-bit Windows: Version 11.1.0.6.0 - Production
NLSRTL Version 11.1.0.6.0 - Production

SCOTT@orcl_11g> column level format 99999
SCOTT@orcl_11g> column s     format a15
SCOTT@orcl_11g> select level, lpad(' ',2*(level-1)) || to_char(child) s
  2  from test_connect_by
  3  start with level = 1
  4  connect by prior child = parent
  5  /

no rows selected

SCOTT@orcl_11g> select level, lpad(' ',2*(level-1)) || to_char(child) s
  2  from test_connect_by
  3  --start with level = 1
  4  connect by prior child = parent
  5  /

 LEVEL S
------ ---------------
     1 2
     1 3
     1 5
     2   2
     2   3
     1 10
     1 8
     1 9
     1 7
     1 11
     1 1
     1 12
     1 13
     1 6
     1 15
     2   5
     3     2
     3     3
     2   10
     1 17
     2   8
     2   9
     1 18
     2   7
     2   11
     1 26
     2   1
     2   12
     2   13
     1 38
     2   6
     2   15
     3     5
     4       2
     4       3
     3     10
     2   17
     3     8
     3     9

39 rows selected.

SCOTT@orcl_11g>