Question

Say I have table A:

id child_id
1  2
2  3
3  NULL
4  NULL
5  6
6  7
7  8
8  NULL

I need a query to get root parent id of each row, this output for example:

id root_parent_id
1  1
2  1
3  1
4  4
5  5
6  5
7  5
8  5

I've tried both CONNECT BY and CTE by examples, but seems all I found based on rows with parent_id and not child_id and doesn't work.

CONNECT BY query I tried:

SELECT id, child_id, LEVEL, connect_by_root id
FROM a
CONNECT BY  id = PRIOR child_id

CTE query I tried:

WITH recursion_view (base,
                     id,
                     child_id)
     AS (
         SELECT id base, id, child_id FROM a
         UNION ALL
         SELECT 
                previous_level.base,
                current_level.id,
                current_level.child_id
           FROM recursion_view previous_level, a current_level
          WHERE current_level.id = previous_level.child_id)
  SELECT base,
         id,
         child_id
    FROM recursion_view
ORDER BY base, id, child_id

Answer 1

You need to traverse the hierarchy in reverse order - view the root parents as leaves. Something like this:

with table_a ( id, child_id ) as (
       select 1, 2    from dual union all
       select 2, 3    from dual union all
       select 3, NULL from dual union all
       select 4, NULL from dual union all
       select 5, 6    from dual union all
       select 6, 7    from dual union all
       select 7, 8    from dual union all
       select 8, NULL from dual
     )
select     connect_by_root id as id, id as root_parent_id
from       table_a
where      connect_by_isleaf = 1
connect by child_id = prior id
order by   id
;

ID ROOT_PARENT_ID
-- --------------
 1              1
 2              1
 3              1
 4              4
 5              5
 6              5
 7              5
 8              5

Oracle SQL: Find root parent for hierarchical data with child_id (No parent_id)

1 个答案: