How to group nodes with relationships in SQL - sql

Question

I have the following table which lists related nodes:
;WITH CTE AS
( SELECT *
FROM (VALUES (1,2)
,(2,1)
,(3,4)
,(3,5)
,(4,3)
,(4,5)
,(5,3)
,(5,4)
,(6,NULL)
,(7,NULL)
,(8,9)
,(9,8)
) AS ValuesTable(ID,RelatedID)
)
SELECT *
FROM CTE
How can I assign unique IDS (GUID or integer GroupID) to each group, So, 1 and 2 will be on the same group, 3, 4, 5 on a different group, 6 is alone in it's group and so is 7, and 8 and 9 are one more group?
My answer so far seems very cumbersome:
;WITH CTE AS
( SELECT *
FROM (VALUES (1,2)
,(2,1)
,(3,4)
,(3,5)
,(4,3)
,(4,5)
,(5,3)
,(5,4)
,(6,NULL)
,(7,NULL)
,(8,9)
,(9,8)
) AS ValuesTable(ID,RelatedID)
)
SELECT DENSE_RANK() OVER(ORDER BY CA.IDList) AS GroupID,
ID,
RelatedID
FROM CTE
CROSS APPLY (SELECT STUFF((SELECT ',' + CONVERT(NVARCHAR(255), ID)
FROM CTE AS CTEInner
WHERE CTEInner.ID = CTE.ID
OR CTEInner.ID = CTE.RelatedID
OR CTEInner.RelatedID = CTE.RelatedID
OR CTEInner.RelatedID = CTE.ID
FOR XML PATH(''),TYPE).value('(./text())[1]','NVARCHAR(MAX)'),1,1,'') AS IDList) AS CA
But it provides the correct answer:
GroupID ID RelatedID
1 1 2
1 2 1
2 3 4
2 3 5
2 4 3
2 4 5
2 5 3
2 5 4
3 6 NULL
4 7 NULL
5 8 9
5 9 8

Answer 1

Adding a unique number for each group is not hard but it does require a few steps.
The first step would be to select unique values for the groups - so for instance the group with (1, 2) and (2, 1) will contain only a single record - (1, 2).
The next step is to get rid of the records that creates multiple paths for the same relationship - in this case - (3, 4), (4, 5), (3, 5) - means that 5 is the related to both 3 and 4, but for the recursive cte to work, we only need a single relationship path - so either (3, 4), (4, 5) or (3, 4), (3, 5) but not both.
The next step is to create a recursive cte based on these unique values, so that each group can get it's unique number.
After that, you can select from the original cte joined to the recursive cte and get the unique group numbers:
;WITH CTE AS
( SELECT *
FROM (VALUES (1,2)
,(2,1)
,(3,4)
,(3,5)
,(4,3)
,(4,5)
,(5,3)
,(5,4)
,(6,NULL)
,(7,NULL)
,(8,9)
,(9,8)
) AS ValuesTable(ID,RelatedID)
)
, PreUniqueValues AS
(
SELECT MIN(ID) AS ID,
MAX(RelatedID) As RelatedID
FROM CTE AS B
GROUP BY (ID + ISNULL(RelatedID, 0)) + (ID * ISNULL(RelatedID, 0))
)
, UniqueValues AS
(
SELECT ID, MIN(RelatedID) As RelatedID
FROM PreUniqueValues
GROUP BY ID
)
, Recursive AS
(
SELECT ID, RelatedId, DENSE_RANK() OVER(ORDER BY ID) As GroupID
FROM UniqueValues AS T0
WHERE NOT EXISTS
(
SELECT 1
FROM UniqueValues AS T1
WHERE T1.ID = T0.RelatedID
)
UNION ALL
SELECT UV.ID, UV.RelatedID, GroupID
FROM UniqueValues As UV
JOIN Recursive As Re
ON UV.ID = Re.RelatedId
)
SELECT CTE.ID, CTE.RelatedID, GroupID
FROM CTE
JOIN Recursive
ON CTE.ID = Recursive.ID OR CTE.ID = ISNULL(Recursive.RelatedID, 0)
ORDER BY ID
Results:
ID RelatedID GroupID
1 2 1
2 1 1
4 3 2
4 5 2
5 3 2
5 4 2
6 NULL 3
7 NULL 4
8 9 5
9 8 5

Answer 2

This is a graph-walking problem and you would seem to need recursive CTEs. The logic looks like this:
WITH t AS (
SELECT *
FROM (VALUES (1,2)
,(2,1)
,(3,4)
,(3,5)
,(4,3)
,(4,5)
,(5,3)
,(5,4)
,(6,NULL)
,(7,NULL)
,(8,9)
,(9,8)
) AS ValuesTable(ID,RelatedID)
),
cte as (
select distinct id, id as relatedId, ',' + convert(varchar(max), id) + ',' as relatedIds
from t
union all
select cte.id, t.relatedId, cte.relatedIds + convert(varchar(max), t.relatedId) + ','
from cte join
t
on cte.relatedId = t.id
where cte.relatedId is not null and
cte.relatedIds not like '%,' + convert(varchar(max), t.relatedId) + ',%'
)
SELECT id, min(relatedId) as grp,
dense_rank() over (order by min(relatedId)) as grp_number
FROM cte
GROUP BY id;
Here is a db<>fiddle.