How to write this recursive CTE for SQL Server? - sql

Question

For simplicity my schema:
Folders table (FolderId, ParentFolderId)
DeviceFolderProperties table (FolderId, LogDataRetentionDaysEvent)
Not every folder has a retention day. However this is an inherited value. How can you write something in SQL to return every folder and its retention day and if that is null its inherited value.
There are multiple levels to inheritance, so it will need to walk the tree.
This is what I have tried:
;
WITH [cte]
AS
(
SELECT f.FolderId, f.ParentFolderId, dfp.LogDataRetentionDaysEvent
FROM [Folders] f
LEFT JOIN DeviceFolderProperties dfp
ON f.FolderId = dfp.FolderId
),
[cte_collapse]
AS --recurse where r days is null
(
SELECT c.FolderId, c.ParentFolderId, c.LogDataRetentionDaysEvent
FROM [cte] c
WHERE c.LogDataRetentionDaysEvent IS NULL
UNION ALL
SELECT c.FolderId, c.ParentFolderId, ISNULL(c.LogDataRetentionDaysEvent, cc.LogDataRetentionDaysEvent)
FROM [cte] c
JOIN [cte_collapse] cc ON cc.FolderId = c.ParentFolderId
)
SELECT
*
FROM
[cte_collapse]

Answer 1

You could write this as:
with
data as (
select f.FolderID, f.ParentFolderId, dfp.LogDataRetentionDaysEvent
from Folders f
left join DeviceFolderProperties dfp on dfp.FolderID = f.FolderID
),
cte as (
select d.*, FolderID OriginalFolderId
from data d
union all
select d.*, c.OriginalFolderId
from cte c
inner join data d on d.FolderID = c.ParentFolderId
where c.LogDataRetentionDaysEvent is null
)
select OriginalFolderId, max(LogDataRetentionDaysEvent) LogDataRetentionDaysEvent
from cte
group by OriginalFolderId
We start by generating a derived table that contains information from both tables. Then, for each record, the recursive query climbs up the hierarchy, searching for a non-null the retention at each level. The trick is to stop as soon as a match is met.