SQL group with Recursive CTE - sql

Question

I'm working on SQL Server 2008. I believe the answer to my Q lies in a recursive CTE but any solution would be greatly appreciated.
In the sam_DB.dbo.example table below where the PID is not null it links back to an ID
ID | PID
------ | ------
1 | NULL
2 | 1
3 | 2
4 | 3
5 | NULL
6 | 5
7 | 6
8 | NULL
9 | NULL
10 | 9
I want my output to have a new field (CID) that identifies each record in a chain of linkages from PID to ID as part of a group, as per below.
ID | PID | CID
------ | ------ | ------
1 | NULL | 1
2 | 1 | 1
3 | 2 | 1
4 | 3 | 1
5 | NULL | 2
6 | 5 | 2
7 | 6 | 2
8 | NULL | 3
9 | NULL | 4
10 | 9 | 4

Answer 1

You're correct that you'd need a CTE for this.
You need to define the first part of the query to select the top level records (i.e. those which have no parent):
select ID, PID, ID
from #t
where PID is null
Then, for every row added to the resulting CTE (i.e. first of all for those records returned by the above query, then again for each new row added by this second part of the query, repeated for each addition until no new additions are made) you should add all records from the source table for which the parent id matches the previously added row's id.
select t.ID, t.PID, c.CID
from cte c
inner join #t t
on t.PID = c.ID
Aside from this logic, the only other thing to be aware of is that the CID column for the first expression takes the record's ID, whilst for those records returned by the second expression it takes the parent record's CID.
Full Code
--set up the demo data
declare #t table (ID int not null, PID int null)
insert #t
values (1, null)
, (2,1)
, (3,2)
, (4,3)
, (5,null)
, (6,5)
, (7,6)
, (8,null)
, (9,null)
, (10,9)
--actual demo
;with cte (ID, PID, CID) as (
--select out top most (ancestor) records; setting CID to ID (since they're the oldest ancestor in their own chain, given they don't have parents)
select ID, PID, ID
from #t
where PID is null
union all
--select each record that is a child of the record we previously selected, holding the ancestor as the parent record's ancestor
select t.ID, t.PID, c.CID
from cte c
inner join #t t
on t.PID = c.ID
)
select *
from CTE
order by ID

Answer 2

you have to use Common text expression With Row_Number Window function
CREATE TABLE #TblTemp(ID int,PID int)
INSERT INTO #TblTemp(ID ,PID ) VALUES (1,NULL),(2,1),(3,1),(4,3),(5,NULL),(6,5),(7,6),(8,NULL),(9,NULL),(10,9)
;WITH CTE (ID, PID, CID) AS (
SELECT ID, PID, ROW_NUMBER() OVER(ORDER BY ID) RN
FROM #TBLTEMP
WHERE PID IS NULL
UNION ALL
SELECT T.ID, T.PID, C.CID
FROM CTE C
INNER JOIN #TBLTEMP T
ON T.PID = C.ID
)
SELECT *
FROM CTE
ORDER BY ID

Answer 3

I will post some simple example
-- shows how to create a recursive grouping for wrongly linked or corrupted pieces of a parent/child groups
declare #t table (item varchar(2), tr int null, rc int null)
insert #t select 'a',1,9 -- no links 'a' - is group parent
insert #t select 'b',2,1 -- links to 'a'
insert #t select 'c',3,2 -- links to 'b'
insert #t select 'd',4,3 -- links to 'd'
insert #t select 'e',6,7 -- no links 'e' - is a different group
insert #t select 'f',8,2 -- links to 'c'
-- grn-group name based on a parent item name;
-- gid-group name based on a parent item id;
-- tr-transactionID ; rc-recursiveID;
-- rc_New-new recursiveID to use; rc_Old - original recursiveID
;with cte as
(
select grn=s.item, gid=s.tr, s.item, s.tr, rc_New= t.tr, rc_Old=s.rc from #t s
left join #t t on t.tr=s.rc where (t.tr is NULL or s.rc is NULL)
union all
select c.grn, c.gid,s.item, s.tr, rc_New=s.rc, rc_Old=s.rc
from cte c join #t s on s.rc=c.tr where s.rc is not NULL
)
select * from cte order by 2,3
option (MAXRECURSION 32767, FAST 100)