Recursive CTE and SELECT - sql

Question

I have written a fairly simple recursive CTE statement. The purpose is that it looks up a structure and returns the top level item
Here is the code
WITH cte_BOM(parent_serial_id, serial_id, serial_batch_no, sort)
AS (SELECT BOM.parent_serial_id, BOM.serial_id, p.serial_batch_no, 1
FROM serial_status AS BOM
INNER JOIN item_serial_nos p ON BOM.parent_serial_id = p.serial_id
WHERE BOM.serial_id = '16320' AND BOM.is_current = 'Y'
UNION ALL
SELECT
BOM1.parent_serial_id, bom1.serial_id, p1.serial_batch_no, cte_BOM.sort + 1
FROM cte_BOM
INNER JOIN serial_status AS BOM1 ON cte_BOM.parent_serial_id =
BOM1.serial_id
INNER JOIN item_serial_nos p1 ON BOM1.parent_serial_id = p1.serial_id
WHERE BOM1.is_current = 'Y'
)
SELECT TOP 1
cte_BOM.*
FROM
cte_BOM
ORDER BY sort desc
As you can see I just hard code the serial_id at the moment. What I now need to accomplish is to run this cte against a subset of data. I’m now stuck on how I can do this.
So I would produce a list of serial_ids by means of another select statement, and then for each row use this serial_id in place of the ones that is currently hard coded and return the 1st record. Importantly if a serial_id has no parent that should still return a row
The second SELECT would be this:
SELECT serial_id
FROM
item_serial_nos
WHERE
item_serial_nos.item_id = '15683'
Any suggestions appreciated. (using SQL 2008 R2)

Answer 1

If I understand your goal correctly, you can just remove the hardcoded serial_id from the cte and add the start serial_id:
WITH cte_BOM(parent_serial_id, serial_id, serial_batch_no, sort, Original_Serial_id)
AS (
SELECT BOM.parent_serial_id
, BOM.serial_id
, p.serial_batch_no
, 1
, BOM.serial_id
FROM serial_status AS BOM
INNER JOIN item_serial_nos p
ON BOM.parent_serial_id = p.serial_id
WHERE BOM.is_current = 'Y'
UNION ALL
SELECT BOM1.parent_serial_id
, bom1.serial_id
, p1.serial_batch_no
, cte_BOM.sort + 1
, cte_BOM.Original_Serial_id
FROM cte_BOM
INNER JOIN serial_status AS BOM1
ON cte_BOM.parent_serial_id = BOM1.serial_id
INNER JOIN item_serial_nos p1
ON BOM1.parent_serial_id = p1.serial_id
WHERE BOM1.is_current = 'Y'
)
SELECT cte_BOM.*
FROM cte_BOM
INNER JOIN (
SELECT cte_BOM.Original_Serial_id
, MAX(sort) sort_max
FROM cte_BOM
WHERE cte_BOM.Original_Serial_id IN (
SELECT serial_id
FROM item_serial_nos
WHERE item_serial_nos.item_id = '15683'
)
GROUP BY cte_BOM.Original_Serial_id
) max_cte
ON max_cte.Original_Serial_id = cte_BOM.Original_Serial_id
AND max_cte.sort_max = cte_BOM.sort
The recursive CTE is only executed when it's called from the last select, so it is only executed for those records that are selected in the IN query. So you shouldn't suffer a performance hit because of this.