Reference CTE inside CTE? - sql

Question

I found this stored proc in our codebase:
ALTER PROCEDURE [dbo].[MoveNodes]
(
#id bigint,
#left bigint,
#right bigint,
#parentid bigint,
#offset bigint,
#caseid bigint,
#userid bigint
)
AS
BEGIN
WITH q AS
(
SELECT id, parent, lft, rgt, title, type, caseid, userid, 0 AS level,
CAST(LEFT(CAST(id AS VARCHAR) + REPLICATE('0', 10), 10) AS VARCHAR) AS bc
FROM [dbo].DM_FolderTree hc
WHERE id = #id and caseid = #caseid
UNION ALL
SELECT hc.id, hc.parent, hc.lft, hc.rgt, hc.title, hc.type, hc.caseid, hc.userid, level + 1,
CAST(bc + '.' + LEFT(CAST(hc.id AS VARCHAR) + REPLICATE('0', 10), 10) AS VARCHAR)
FROM q
JOIN [dbo].DM_FolderTree hc
ON hc.parent = q.id
)
UPDATE [dbo].DM_FolderTree
SET lft = ((-lft) + #offset), rgt = ((-rgt) + #offset), userid = #userid
WHERE id in (select id from q) AND lft <= (-(#left)) AND rgt >= (-(#right)) AND caseid = #caseid;
UPDATE [dbo].DM_FolderTree SET parent = #parentid, userid = #userid WHERE id = #id AND caseid = #caseid;
END
where you'll notice that the CTE q is being used called on the UNION. What exactly are we calling here? Everything before the UNION, the whole CTE? What exactly is happening here.
I'm assuming that this code is legal, since its been in production for quite some time (FLW, I know). But still, I have no idea what's happening here.

Answer 1

This is a recursive query. It calls the CTE again and again until all ID's and CaseIDs have walked the tree.
Think about nesting of folders in a directory. This query simply walks all the directors to get final the "file path" for all files in all folders.
Notice how Level starts at 0 and then gets added to. The second time though level is now 1 and becomes 2 and then 3 and so on.
To better understand:
Grab the select cte portion (with q as...) and replace the update with Select * from q and run it. Just so you can see what it does. Bit rough learning to start with but walking though an example by doing the above will help.
Specific answers to questions:
What exactly are we calling here?
Your building a baseline which denotes the all roots you wish to start with and then traversing all the levels under that root/folder. So in essense you're crawling the entire structure for hc.parent = q.id
Everything before the UNION, the whole CTE?
The whole cte. Recursion powerfully cool stuff!