http://habrahabr.ru/post/43955/

gistfile1.txt

SQL> select * from nc_objects2;

 OBJECT_ID  PARENT_ID
---------- ----------
         0
         2          1
         1
         3          2

SQL> select
  2    level,
  3    obj.*
  4  from nc_objects2 obj
  5  start with object_id = 1
  6  connect by rownum between 1 and 10
  7  /

     LEVEL  OBJECT_ID  PARENT_ID
---------- ---------- ----------
         1          1
         2          0
         3          0
         4          0
         5          0
         6          0
         7          0
         8          0
         9          0
        10          0

10 rows selected.

SQL> select
  2    level,
  3    obj.*
  4  from nc_objects2 obj
  5  start with object_id = 0
  6  connect by rownum between 1 and 10
  7  /

     LEVEL  OBJECT_ID  PARENT_ID
---------- ---------- ----------
         1          0
         2          0
         3          0
         4          0
         5          0
         6          0
         7          0
         8          0
         9          0
        10          0

10 rows selected.