Need help with self join query

Question

Hello,
I have table A with the following data
oid parent_oid
10 4
4 2
2 2
12 6
6 6
parent_oid is the parent of oid. I'd like a query that shows the final parent of the oid. The result should show the following
oid final parent
10 2
4 2
2 2
12 6
6 6
I'm using Oracle 10g. I'm familiar with self joins, but that alone will not do the job. Thanks!

Answer 1

Hi,
arizona9952 wrote:
... I'm familiar with self joins, but that alone will not do the job.You're absolutely right!
A 2-way self join would work for rows have no parent, or rows that are directly connected to their final ancestor (such as oid=4), but not for anything farther away.
A 3-way self-join would work for one more level away from the final row, but no more. That would be enough for the small set of sample data that you posted, but it would not work if you added a new row with parent_id=10.
An N-way self-join would work for up to N+1 levels, but no more.
You need something that can go any number of levels, such as CONNECT BY:
SELECT     CONNECT_BY_ROOT oid     AS oid
,     parent_oid          AS final_parent
FROM     a
WHERE     CONNECT_BY_ISLEAF     = 1
CONNECT BY     oid     = PRIOR parent_oid
     AND     oid     != parent_oid
;Edited by: Frank Kulash on Feb 22, 2010 7:09 PM
Upon sober reflection, I think that a Top-Down query, like the one below, would be more efficient than a Bottom-Up query, like the one above:
SELECT     oid
,     CONNECT_BY_ROOT     parent_oid     AS final_parent
FROM     a
START WITH     parent_oid     = oid
CONNECT BY     parent_oid     = PRIOR oid
     AND     oid          != PRIOR oid
;