Find oldest ancestor for each entry in a SQL table - sql

Question

I have a SQL table where the rows in the table have a parent-child relationship with each other. I would like to write a recursive SQL query to find the oldest ancestor for each of these rows (or the row itself if it has no parent). So in other words, if my table looks like this:
Child
Parent
1
null
2
1
3
2
4
null
5
4
6
null
7
null
8
3
9
5
Then my final output should be:
Child
OldestAncestor
1
1
2
1
3
1
4
4
5
4
6
6
7
7
8
1
9
4
Can this be done? I know how to use recursive SQL to find an individual parent-child relationship, I'm just not sure if that can be taken a step further to find the parent's parent, etc.

Answer 1

Update - I was able to figure out a solution based on this article:https://www.webcodeexpert.com/2020/04/sql-server-cte-recursive-query-to-get.html
WITH parentChild(Child, Parent) AS
(
SELECT 1, null UNION ALL
SELECT 2, 1 UNION ALL
SELECT 3, 2 UNION ALL
SELECT 4, null UNION ALL
SELECT 5, 4 UNION ALL
SELECT 6, null UNION ALL
SELECT 7, null UNION ALL
SELECT 8, 3 UNION ALL
SELECT 9, 5
),
MyCTE AS
(
SELECT pc.Child, pc.Child AS 'OldestAncestor'
FROM parentChild pc
WHERE pc.Parent IS NULL
UNION ALL
SELECT pc2.Child, m.OldestAncestor AS 'OldestAncestor'
FROM parentChild pc2
INNER JOIN MyCTE m ON pc2.Parent = m.Child
)
SELECT * FROM MyCTE ORDER BY Child

Answer 2

In Oracle, you can use:
SELECT child,
CONNECT_BY_ROOT child AS oldest_ancestor
FROM table_name
START WITH parent IS NULL
CONNECT BY parent = PRIOR child
ORDER BY child;
or a recursive sub-query factoring clause (WITH clause):
WITH rsqfc (child, oldest_ancestor) AS (
SELECT child, child
FROM table_name
WHERE parent IS NULL
UNION ALL
SELECT t.child, r.oldest_ancestor
FROM rsqfc r
INNER JOIN table_name t
ON (t.parent = r.child)
)
SELECT *
FROM rsqfc
ORDER BY child;
Which, for the sample data:
CREATE TABLE table_name (Child, Parent) 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, null FROM DUAL UNION ALL
SELECT 7, null FROM DUAL UNION ALL
SELECT 8, 3 FROM DUAL UNION ALL
SELECT 9, 5 FROM DUAL;
Both output:
CHILD
OLDEST_ANCESTOR
1
1
2
1
3
1
4
4
5
4
6
6
7
7
8
1
9
4
db<>fiddle here

Answer 3

You can use a recursive CTE to find the paths from the parents to the leaf nodes, and then join the CTE back onto the original table:
with recursive cte(n1, n2) as (
select child, child from tbl where parent is null
union all
select c.n1, t.child from cte c join tbl t on c.n2 = t.parent
)
select t1.child, c.n1 from tbl t1 join cte c on t1.child = c.n2;