* A DAG
         0
      /  |  \
     7  8    9
      \  /\  /\
       \/  \/  \
        6  5    \
         \/ \    \
          4  3  10
           \/ \/
            1  2

-- Each row in the table represents an edge in the graph
insert into hq_tab(child_id, parent_id) values(1,3);
insert into hq_tab(child_id, parent_id) values(1,4);
insert into hq_tab(child_id, parent_id) values(2,3);
insert into hq_tab(child_id, parent_id) values(2,10);
insert into hq_tab(child_id, parent_id) values(3,5);
insert into hq_tab(child_id, parent_id) values(4,5);
insert into hq_tab(child_id, parent_id) values(4,6);
insert into hq_tab(child_id, parent_id) values(10,9);
insert into hq_tab(child_id, parent_id) values(6,7);
insert into hq_tab(child_id, parent_id) values(6,8);
insert into hq_tab(child_id, parent_id) values(5,8);
insert into hq_tab(child_id, parent_id) values(5,9);
insert into hq_tab(child_id, parent_id) values(7,0);
insert into hq_tab(child_id, parent_id) values(8,0);
insert into hq_tab(child_id, parent_id) values(9,0);
insert into hq_tab(child_id, parent_id) values(0,null);

** Going down the graph
-- Give me the id structure in a way that the "child_id" of the previous row
-- is the parent_id of current row, starting from the row whose child_id=8
-- What's important is the connect by clause. It defines the direction with
-- which you walk the graph. Start with just gives you a strating point.
-- You can use child_id or parent_id with "start with", as long as it is
-- the node that you want.
SQL> select * from hq_tab
  2  start with child_id=8
  3  connect by prior child_id=parent_id;

 PARENT_ID   CHILD_ID
---------- ----------
         0          8
         8          6
         6          4
         4          1
         8          5
         5          3
         3          1
         3          2
         5          4
         4          1

10 rows selected.

-- Note that "edge" (4,1) was listed twice, since it is accessible to 2 paths.
-- Note that child_id 1 was listed 3 times, since there are 3 possible paths
-- of reaching that "node".
SQL> 
SQL> select * from hq_tab
  2  start with parent_id=5
  3  connect by prior child_id=parent_id;

 PARENT_ID   CHILD_ID
---------- ----------
         5          3
         3          1
         3          2
         5          4
         4          1

SQL> 

** Going up the graph
SQL> select * from hq_tab
  2  start with child_id=1
  3  connect by prior parent_id=child_id;

 PARENT_ID   CHILD_ID
---------- ----------
         3          1
         5          3
         8          5
         0          8
                    0
         9          5
         0          9
                    0
         4          1
         5          4
         8          5
         0          8
                    0
         9          5
         0          9
                    0
         6          4
         7          6
         0          7
                    0
         8          6
         0          8
                    0

23 rows selected.

SQL> 

-- Note the repeated edges and repeated parent nodes.

* A tree

          1
         /\
        2  3
       /\   \
      4  5   6
     /\
    7  8

insert into hq_tab(child_id, parent_id) values(7,4);
insert into hq_tab(child_id, parent_id) values(8,4);
insert into hq_tab(child_id, parent_id) values(4,2);
insert into hq_tab(child_id, parent_id) values(5,2);
insert into hq_tab(child_id, parent_id) values(2,1);
insert into hq_tab(child_id, parent_id) values(6,3);
insert into hq_tab(child_id, parent_id) values(3,1);
insert into hq_tab(child_id, parent_id) values(1,null);

-- Walking down the tree
SQL> select * from hq_tab
  2  start with child_id=1
  3  connect by prior child_id=parent_id;

 PARENT_ID   CHILD_ID
---------- ----------
                    1
         1          2
         2          4
         4          7
         4          8
         2          5
         1          3
         3          6

8 rows selected.

SQL>           
-- Entire graph walked, no edge repeated, no child node repeated.

-- Walking up the tree
SQL> select * from hq_tab
  2  start with child_id=7
  3  connect by prior parent_id=child_id;

 PARENT_ID   CHILD_ID
---------- ----------
         4          7
         2          4
         1          2
                    1

SQL> 
-- Entire graph walked, no edge repeated, no parent node repeated.