How to get a root node and all its child nodes for multiple hierarchy in a table

Question

I have the following heirarchies in the DB

A
|_A1
|  |_A11
|
|_A2

and

B
|_B1
|
|_B2
   |_B21
   |_B22

I want a query which displays the result as

Parent    Child
A          A1
A          A11
A          A2
B          B1
B          B2
B          B21
B          B22

Answer 1

with w(child, parent) as
(
  select 'A', null from dual
  union all
  select 'A1', 'A' from dual
  union all
  select 'A11', 'A1' from dual
  union all
  select 'A2', 'A' from dual
  union all
  select 'B', null from dual
  union all
  select 'B1', 'B' from dual
  union all
  select 'B2', 'B' from dual
  union all
  select 'B21', 'B2' from dual
  union all
  select 'B22', 'B2' from dual
)
select connect_by_root child parent, child
from w
connect by w.parent = prior w.child
start with w.parent is null
;

This returns:

1   A   A
2   A   A1
3   A   A11
4   A   A2
5   B   B
6   B   B1
7   B   B2
8   B   B21
9   B   B22

You can filter lines with parent == child by adding where connect_by_root child != child between the from and the connect.