SQL> select * from test_Table;

         A          B
---------- ----------
         1         -1
         2          1
         3          1
         4          2
         5          2
         6          4
         7          4
         8          5
         9          5
        10          3
        11          3

11 rows selected.

SQL> SELECT a
  2    FROM test_table
  3    CONNECT BY PRIOR b=a
  4  /

SQL> select a from test_Table connect by prior b=a;

         A
----------
         1
         2
         1
         3
         1
         4
         2
         1
         5
         2
         1

         A
----------
         6
         4
         2
         1
         7
         4
         2
         1
         8
         5
         2

         A
----------
         1
         9
         5
         2
         1
        10
         3
         1
        11
         3
         1