Combine Parent-Child Rows - TSQL - sql-server

Question

lI am trying to flatten/combine rows from a table with a parent-child hierarchy. I'm trying to identify the beginning and the end of each 'link' - so if a is linked to b, b is linked to c, and then c is linked to d, I want the output to link a to d.
I'm trying my best to avoid using a procedure with loops, so any advice would be much appreciated!
The original dataset and the required output is as follows:
personID | form | linkedform
---------|---------|---------
1 | a | b
1 | b | c
1 | c | d
1 | d | NULL
2 | e | f
2 | f | g
2 | g | NULL
2 | h | i
2 | i | NULL
3 | j | NULL
3 | k | l
3 | l | NULL
Desired output:
personID | form | linkedform
---------|---------|---------
1 | a | d
2 | e | g
2 | h | i
3 | j | NULL
3 | k | l
Each personID can have multiple links, and a link can be made of just one or multiple forms.

Answer 1

-- use a recursive cte to build the hierarchy
-- start with [linkedform] = null and work your way up
;WITH cte AS
(
SELECT *, [form] AS [root],
1 AS [Level]
FROM Table1
WHERE [linkedform] IS NULL
UNION ALL
SELECT t1.*,
[root],
[Level] + 1
FROM Table1 t1
JOIN cte ON cte.form = t1.linkedform
)
-- level 1 will be the last element, use row_number to get the first element
-- join the two together based on last and first level, that have the same personid and root ([linkedform] = null)
SELECT cte.personId,
cte2.form,
cte.form
FROM cte
JOIN ( SELECT *,
ROW_NUMBER() OVER (PARTITION BY personId, [root] ORDER BY Level DESC) Rn
FROM cte) cte2
ON cte2.Rn = cte.Level
AND cte2.personId = cte.personId
AND cte2.root = cte.root
WHERE cte.[Level] = 1
ORDER BY cte.personId, cte2.form