Find all ancestors without direct id-parentid in same table - sql

Question

I have a parent-child structure across two tables. The first table has BOM_ID for bills and ITEM_ID for associated children items. The other has BOM_ID and ITEM_ID for the parent item.
I am able to find the first level of parents' ITEM_ID with the following query
SELECT item_id
FROM bomversion
WHERE bom_id IN (SELECT bom_id FROM bom WHERE item_id = 1)
So, in order to find all ancestors, I would have to repeat this step. I've tried to look at CTE and recursion techniques but all examples have parentid and childid in the same table. I cannot figure this one out.
If 1 is child of 2, 2 is child of 3, 3 is child of 4, and 2 is also child of 5, I am looking for the following result:
ChildID
ParentID
1
2
2
3
2
5
3
4
The starting point will be a specific ChildID.
S O L U T I O N
Based on Adnan Sharif's proposal, I found the solution for my problem:
WITH items_CTE AS (
-- create the mapping of items to consider
SELECT
B.ITEMID AS Child_id,
BV.ITEMID AS Parent_Id
FROM BOM AS B
INNER JOIN BOMVERSION AS BV
ON B.BOMID = BV.BOMID
), parent_child_cte AS (
-- select those items as initial query
SELECT
Child_id,
Parent_id
FROM items_CTE
WHERE Child_id = '111599' -- this is the starting point
UNION ALL
-- recursive approach to find all the ancestors
SELECT
c.Child_Id,
c.Parent_Id
FROM items_CTE c
JOIN parent_child_cte pc
ON pc.Parent_Id = c.Child_id
)
SELECT * FROM parent_child_cte

Answer 1

From the dbfiddle that you shared in the comment, if I understand you correctly, you want to have the rows showing all the parents of a child.
For example, lets consider the hierarchy, 1 is a child of 2, 2 is a child of 3, and 3 is a child of 4. You want the final result as,
child_id
parent_id
1
2
1
3
1
4
2
3
2
4
3
4
If that's the case, we can use recursive CTE to build that table. First of all, we need to be build a relation between those two tables based on bom_id and then we need to find out parent-child relation. After that we will add rows based on the initial query. Please see the below code.
WITH RECURSIVE items_CTE AS (
-- create the mapping of items to consider
SELECT
B.Item_id AS Child_id,
BV.Item_id AS Parent_Id
FROM BOM AS B
INNER JOIN BOMVERSION AS BV
ON B.bom_id = BV.bom_id
), parent_child_cte AS (
-- select those items as initial query
SELECT
Child_id,
Parent_id
FROM items_CTE
UNION
-- recursive approach to find all the ancestors
SELECT
parent_child_cte.Child_Id,
items_CTE.Parent_Id
FROM items_CTE
INNER JOIN parent_child_cte
ON parent_child_cte.Parent_Id = items_CTE.Child_id
)
SELECT * FROM parent_child_cte