Eliminate similar cycles in directed graph - sql-server

Question

I have the following data:
Table:
CREATE TABLE tblLoop
(
person1 varchar(20),
person2 varchar(20),
ColDate date,
);
INSERT INTO tblLoop VALUES('A','B','2020-01-01'),('A','C','2020-01-01'),('A','D','2020-01-01'),
('B','E','2020-01-02'),('B','F','2020-01-02'),
('D','G','2020-01-03'),('D','H','2020-01-03'),
('F','i','2020-01-04'),
('G','J','2020-01-05'),
('i','A','2020-01-06'),
('J','D','2020-01-07'),
('X','Y','2020-01-08'),('X','Z','2020-01-08'),
('Z','X','2020-01-09'),
('Y','W','2020-01-09');
Records Look like:
Requirement: I need to find the persons which forms a cycle. For an example in the given data we found 3 Cycle's:
Cycle 1: A connected with B connected with F connected with i connected with A.
Cycle 2: A connected with D connected with G connected with J connected with D.
Cycle 3: X connected with Z connected with X.
Expected result:
LoopFound
--------------------
A->B->F->i->A
A->D->G->J->D
X->Z->X
My try:
;WITH CTE AS
(
SELECT Person1, Person2,
CONVERT(VARCHAR(MAX), (','+ Person1+ ','+ Person2+ ',')) AS nodes, 1 AS lev,
(CASE WHEN Person1 = Person2 THEN 1 ELSE 0 END) AS has_cycle
FROM tblLoop e
UNION ALL
SELECT cte.Person1, e.Person2,
CONVERT(VARCHAR(MAX), (cte.nodes+ e.Person2+ ',')), lev + 1,
(CASE WHEN cte.nodes LIKE ('%,'+ e.Person2+ ',%') THEN 1 ELSE 0 END) AS has_cycle
FROM CTE
JOIN tblLoop e ON e.Person1 = cte.Person2
WHERE cte.has_cycle = 0
)
SELECT *
FROM CTE
WHERE has_cycle = 1;
NOTE: Getting multiple combination of cycles from above query.

Answer 1

As per the comment by Kevin, the answer lies in including a flag to indicate that a given node is a valid start point, and can be included in the query:
In this instance, 'A' and 'X' are given as start points, therefore we will mark all records where the originator is either 'A' or 'X':
CREATE TABLE #tblLoop
(
person1 varchar(20),
person2 varchar(20),
ColDate date,
isRoot INT
);
INSERT INTO #tblLoop VALUES('A','B','2020-01-01',1),
('A','C','2020-01-01',1),
('A','D','2020-01-01',1),
('B','E','2020-01-02',0),
('B','F','2020-01-02',0),
('D','G','2020-01-03',0),
('D','H','2020-01-03',0),
('F','i','2020-01-04',0),
('G','J','2020-01-05',0),
('i','A','2020-01-06',0),
('J','D','2020-01-07',0),
('X','Y','2020-01-08',1),
('X','Z','2020-01-08',1),
('Z','X','2020-01-09',0),
('Y','W','2020-01-09',0);
Then the following query can be amended as follows:
;WITH CTE AS
(
SELECT Person1, Person2, isRoot,
CONVERT(VARCHAR(MAX), (','+ Person1+ ','+ Person2+ ',')) AS nodes, 1 AS lev,
(CASE WHEN Person1 = Person2 THEN 1 ELSE 0 END) AS has_cycle
FROM #tblLoop e
UNION ALL
SELECT cte.Person1, e.Person2, cte.isRoot,
CONVERT(VARCHAR(MAX), (cte.nodes+ e.Person2+ ',')), lev + 1,
(CASE WHEN cte.nodes LIKE ('%,'+ e.Person2+ ',%') THEN 1 ELSE 0 END) AS has_cycle
FROM CTE
JOIN #tblLoop e ON e.Person1 = cte.Person2
WHERE cte.has_cycle = 0
)
SELECT *
FROM CTE
WHERE has_cycle = 1
AND isRoot = 1
Credit to Kevin for the idea, this is just a working implementation of it.

Answer 2

I have tried the following two steps:
Step 1: In this step, Find the start node's of the graph.
--Create table to store start nodes
IF OBJECT_ID('dbo.Temp_tblLoop', 'U') IS NOT NULL
BEGIN
DROP TABLE dbo.Temp_tblLoop;
END
--Query to find start nodes.
;WITH CTE AS
(
SELECT t.*
FROM tblLoop t
WHERE person1 IN (SELECT person2 FROM tblLoop t2 WHERE t.ColDate<= t2.ColDate) OR
person2 IN (SELECT person1 FROM tblLoop t3 WHERE t.ColDate<= t3.ColDate)
)
SELECT DISTINCT person1 INTO Temp_tblLoop
FROM CTE
WHERE person1 NOT IN (SELECT person2 FROM CTE);
Step 2: Find out the cycles in the graph (Excluding Similar Cycles)
;WITH CTE AS
(
SELECT Person1, Person2,
CONVERT(VARCHAR(MAX), (Person1+ '->'+ Person2)) AS nodes, 1 AS lev,
(CASE WHEN Person1 = Person2 THEN 1 ELSE 0 END) AS has_cycle
FROM tblLoop e WHERE person1 IN (SELECT person1 FROM Temp_tblLoop)
UNION ALL
SELECT cte.Person1, e.Person2,
CONVERT(VARCHAR(MAX), (cte.nodes+'->'+ e.Person2)), lev + 1,
(CASE WHEN CHARINDEX(e.Person2, cte.nodes) != 0 THEN 1 ELSE 0 END) AS has_cycle
FROM CTE
JOIN tblLoop e ON e.Person1 = cte.Person2
WHERE cte.has_cycle = 0
)
SELECT Person1 AS Start, Person2 AS [End],
nodes AS Links,
lev AS Levels
FROM CTE
WHERE has_cycle = 1;
Let me your reviews, if anything needs to be done for betterment.