Retrieving root id in hierarchial data structure.

Question

HI,
I have data arranged in tree having parent child relationship as below .But i need to retrieve the root in the tree.Please let me know how can i achieve this.
parent child
A B
B C
C D
D E
M N
N O
In the above case i need to get root element of tree through query i.e in my case it will be A and N.How can we achieve this

Answer 1

And why not 'M'?
I'm not sure about the exact requirement, but you could try to fiddle a bit with connect_by_root etc.:
SQL>  -- generating sample data:
SQL>  with t as (
  2   select 'A' p, 'B' c from dual union
  3   select 'B', 'C' from dual union
  4   select 'C', 'D' from dual union
  5   select 'D', 'E' from dual union
  6   select 'M', 'N' from dual union
  7   select 'N', 'O' from dual
  8   )
  9   --
10   -- actual query:
11   --
12   select connect_by_root(p) root_p
13   ,      ltrim(sys_connect_by_path(p, '/'), '/') scbp
14   ,      connect_by_isleaf
15   ,      level lvl
16   from   t
17   where  connect_by_isleaf = 1
18   connect by p = prior c
19   order siblings by p;
R SCBP       CONNECT_BY_ISLEAF        LVL
A A/B/C/D                    1          4
B B/C/D                      1          3
C C/D                        1          2
D D                          1          1
M M/N                        1          2
N N                          1          1