Find loops within data using SQL Server - sql-server

Question

I have the following data to find the loop.
Table:
CREATE TABLE tblLoop
(
person1 varchar(20),
person2 varchar(20)
);
INSERT INTO tblLoop VALUES('A','B'),('A','C'),('A','D'),
('B','E'),('B','F'),
('D','G'),('D','H'),
('F','i'),
('G','J'),
('i','A'),
('J','D');
Edit: Added some more values
INSERT INTO tblLoop VALUES('X','Y'),('X','Z'),('Z','X'),('Y','W');
Records Look like:
Note: There is possibility of multiple trees like above we need to find all trees loop data.
Requirement: I need to find the persons which forms a loop. For an example in the given data we found 2 Loop's:
Loop 1: A connected with B connected with F connected with i connected with A.
Loop 2: A connected with D connected with G connected with J connected with D.
Expected result:
LoopFound
--------------------
A->B->F->i->A
A->D->G->J->D
X->Z->X
My try:
;WITH CTE_Loop AS
(
SELECT t.Person1,t.Person2,
CONVERT(VARCHAR(500), t.Person1 + '->' + t.Person2) AS [Loop],
0 AS FoundFlag
FROM tblLoop t
UNION ALL
SELECT t.Person1,t.Person2,
CONVERT(VARCHAR(500), cte.[Loop] +'->'+t.Person2) AS [Loop],
CASE WHEN CHARINDEX(t.Person2, cte.[Loop]) != 0 THEN 1 ELSE 0 END AS FoundFlag
FROM CTE_Loop cte
INNER JOIN tblLoop t ON t.Person1 = cte.Person2
WHERE cte.FoundFlag = 0 AND t.Person1 <> '-' AND t.Person2 <> '-'
)
SELECT [Loop] AS LoopFound
FROM CTE_Loop
WHERE FoundFlag = 1
GROUP BY [Loop];

Answer 1

SELECT
CONCAT_WS('->',t1.person1,t1.person2,t2.person2 ,t3.person2,t4.person2)
FROM tblLoop t1
left join tblLoop t2 on t1.person2 = t2.person1
left join tblLoop t3 on t2.person2 = t3.person1
left join tblLoop t4 on t3.person2 = t4.person1
where t1.person1 = 'A'AND t4.person2 IS NOT NULL

Answer 2

You could get the start&end of the loops assuming a hierarchy of some sort(eg. A comes before B which is before C etc) and exclude rows whose person2 does not appear as person1 (i.e the ends of the relation/chain)...
--get loops
select distinct a.person1, b.person1, b.person2 --a.person1, b.person1 :: loop range(start-end)
from tblLoop as a
join tblLoop as b on a.person1 = b.person2 and a.person1 <= b.person1 --assume a hierarchy by name (A is on a higher/or equal level than B etc)
where exists(select * from tblLoop as d where d.person1 = a.person2) --exclude dead ends (person2 which is not person1);
..and inject that into the cte. You could work out overlaps and loops without the same start&end.
(for eg. A->D->G->J->D or D->G->J->D is a loop?)
;WITH CTE_Loop AS
(
SELECT t.Person1,t.Person2,
CONVERT(VARCHAR(500), t.Person1 + '->' + t.Person2) AS [Loop],
0 AS FoundFlag
FROM tblLoop t
where t.person1 in
(
select distinct a.person1--, b.person1, b.person2 --a.person1, b.person1 :: loop range(start-end)
from tblLoop as a
join tblLoop as b on a.person1 = b.person2 and a.person1 <= b.person1 --assume a hierarchy by name (A is on a higher/or equal level than B etc)
where exists(select * from tblLoop as d where d.person1 = a.person2) --exclude dead ends (person2 which is not person1)
)
UNION ALL
SELECT t.Person1,t.Person2,
CONVERT(VARCHAR(500), cte.[Loop] +'->'+t.Person2) AS [Loop],
CASE WHEN CHARINDEX(t.Person2, cte.[Loop]) != 0 THEN 1 ELSE 0 END AS FoundFlag
FROM CTE_Loop cte
INNER JOIN tblLoop t ON t.Person1 = cte.Person2
WHERE cte.FoundFlag = 0 AND t.Person1 <> '-' AND t.Person2 <> '-'
)
SELECT [Loop] AS LoopFound
FROM CTE_Loop
WHERE FoundFlag = 1
GROUP BY [Loop];