SQL Self Recursive Join with Grouping - sql

Question

In SQL Server, I have the following table:
Name New_Name
---------------
A B
B C
C D
G H
H I
Z B
I want to create a new table that links all the names that are related to a single new groupID
GroupID Name
-------------
1 A
1 B
1 C
1 D
1 Z
2 G
2 H
2 I
I'm a bit stuck on this can can't figure out how to do it apart from a bunch of joins. But I would like to do it properly.
Edited the question to allow grouping from two different start points, A and Z into one group.

Answer 1

Since you've changed the question, I'm updating the answer. Please note that answer is same in sense of logical structure. All it does differently is that instead of going from G to I in calculating levels, answer now calculates from I to G.
Working demo link
;with cte as
(
select
t1.Name as Name, row_number() over (order by t1.Name) r,
t1.New_Name as New_Name,
1 as level
from yt t1 left join yt t2
on t1.New_Name=t2.name
where t2.name is null
union all
select
yt.Name as Name, r,
yt.New_Name as New_Name,
c.level+1 as level
from cte c join yt
on yt.New_Name=c.Name
),
cte2 as
(
select r as group_id, Name from cte
union
select c1.r as group_id, c1.New_name as Name from cte c1
where level = (select min(level) from cte c2 where c2.r=c1.r)
)
select * from cte2;
Below is old answer.
You can try below CTE based query:
create table yt (Name varchar(10), New_Name varchar(10));
insert into yt values
('A','B'),
('B','C'),
('C','D'),
('G','H'),
('H','I');
;with cte as
(
select
t1.Name as Name, row_number() over (order by t1.Name) r,
t1.New_Name as New_Name,
1 as level
from yt t1 left join yt t2
on t1.Name=t2.New_name
where t2.new_name is null
union all
select
yt.Name as Name, r,
yt.New_Name as New_Name,
c.level+1 as level
from cte c join yt
on yt.Name=c.New_Name
),
cte2 as
(
select r as group_id, Name from cte
union
select c1.r as group_id, c1.New_name as Name from cte c1
where level = (select max(level) from cte c2 where c2.r=c1.r)
)
select * from cte2;
see working demo

Answer 2

Little bit complex but working.
DECLARE #T TABLE (Name VARCHAR(2), New_Name VARCHAR(2))
INSERT INTO #T
VALUES
('A','B'),
('B','C'),
('C','D'),
('G','H'),
('H','I'),
('Z','B')
;WITH CTE AS
(
SELECT * , RN = ROW_NUMBER() OVER(ORDER BY Name) FROM #T
)
,CTE2 AS (SELECT T1.RN, T1.Name Name1, T1.New_Name New_Name1,
X.Name Name2, X.New_Name New_Name2,
FLAG = CASE WHEN X.Name IS NULL THEN 1 ELSE 0 END
FROM CTE T1
OUTER APPLY (SELECT * FROM CTE T2 WHERE T2.RN > T1.RN
AND (T2.Name IN (T1.Name , T1.New_Name)
OR T2.New_Name IN (T1.Name , T1.New_Name)
)) AS X
)
,CTE3 AS (SELECT *,
GroupID = ROW_NUMBER() OVER (ORDER BY RN) -
ROW_NUMBER() OVER (PARTITION BY Flag ORDER BY RN) +1
FROM CTE2
)
SELECT
DISTINCT GroupID, Name
FROM
(SELECT * FROM CTE3 WHERE Name2 IS NOT NULL) SRC
UNPIVOT ( Name FOR COL IN ([Name1], [New_Name1], [Name2], [New_Name2])) UNPVT
Result
GroupID Name
-------------------- ----
1 A
1 B
1 C
1 D
1 Z
2 G
2 H
2 I