Find top parent of child, multiple levels - sql

Question

ENTRY TABLE
__________________
| ID | PARENT_ID |
| 1 | null |
| 2 | 1 |
| 3 | 2 |
| 4 | null |
| 5 | 4 |
| 6 | 5 |
...
I make copies of the entries in some cases and they are conneted by parent ID.
Each entry can have one copy:
THIS WONT HAPPEN
__________________
| ID | PARENT_ID |
| 1 | null |
| 2 | 1 |
| 3 | 1 |
...
Sometimes I need to take a copy and query for it's top level parent. I need to find the top parent entries for all the entries I search for.
For example, if I query for the parents of ID 6 and 3, I would get ID 4 and 1.
If I query for the parents of ID 5 and 2, I would get ID 4 and 1.
But also If I query for ID 5 and 1, it should return ID 4 and 1 because the entry ID 1 is already the top parent itself.
I don't know where to begin since I don't know how to recursively query in such case.
Can anyone point me in the right direction?
I know that the query below will just return the child elemements (ID 6 and 3), but I don't know where to go from here honestly.
I am using OracleSQL by the way.
SELECT * FROM entry WHERE id IN (6, 3);

Answer 1

You can use a hierarchical query and CONNECT_BY_ROOT.
Either starting at the root of the hierarchy and working down:
SELECT id,
CONNECT_BY_ROOT(id) AS root_id
FROM entry
WHERE id IN (6, 3)
START WITH parent_id IS NULL
CONNECT BY PRIOR id = parent_id;
Or, from the entry back up to the root:
SELECT CONNECT_BY_ROOT(id) AS id,
id AS root_id
FROM entry
WHERE parent_id IS NULL
START WITH id IN (6, 3)
CONNECT BY PRIOR parent_id = id;
Which, for the sample data:
CREATE TABLE entry( id, parent_id ) AS
SELECT 1, NULL FROM DUAL UNION ALL
SELECT 2, 1 FROM DUAL UNION ALL
SELECT 3, 2 FROM DUAL UNION ALL
SELECT 4, NULL FROM DUAL UNION ALL
SELECT 5, 4 FROM DUAL UNION ALL
SELECT 6, 5 FROM DUAL UNION ALL
SELECT 7, 6 FROM DUAL
Both output:
ID
ROOT_ID
3
1
6
4
db<>fiddle here

Answer 2

You can use recursive CTE to walk the graph and find the initial parent. For example:
with
n (starting_id, current_id, parent_id, v) as (
select id, id, parent_id, 0 from entry where id in (6, 3)
union all
select n.starting_id, e.id, e.parent_id, n.v - 1
from n
join entry e on e.id = n.parent_id
)
select starting_id, current_id as initial_id
from (
select n.*, row_number() over(partition by starting_id order by v) as rn
from n
) x
where rn = 1
Result:
STARTING_ID INITIAL_ID
------------ ----------
3 1
6 4
See running example at db<>fiddle.