DECLARE @tab TABLE (Id INT, Parent_id INT)
INSERT INTO @tab
SELECT 1, NULL UNION ALL 
SELECT 2, 1 UNION ALL
SELECT 3, 1 UNION ALL
SELECT 4, 2 UNION ALL
SELECT 5, 3

;with cte AS 
(
    SELECT Id,[PATH]=CAST('/' AS VARCHAR),Parent_id FROM @tab WHERE Parent_id IS NULL
    UNION ALL
    SELECT a.Id,[PATH]=CAST(c.[PATH]+CAST(a.Id AS VARCHAR)+'/' AS VARCHAR),
            a.Parent_id FROM @tab a JOIN  cte c ON a.Parent_id=c.Id
)
SELECT id,PATH FROM cte ORDER BY id 

--test:
/*
id          PATH
----------- ------------------------------
1           /
2           /2/
3           /3/
4           /2/4/
5           /3/5/

(5 行受影响)
*/