I need help on recursive CTE in SQL server

Question

2.00/5 (3 votes)

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Id is unique. NextID stores the noxt ID value. When NextID value is Null, there the path Ends.

Input:

ID	NextID
1	5
2	Null
3	6
4	7
5	8
6	9
7	Null
8	Null
9	10
10	Null

Output:

a. 1 --> 5 --> 8 
b. 2
C. 3 -->6 -->9 -->10
d. 4-->7
e. 8

Sample code to run:

SQL

INSERT Data VALUES
    ('1','5'),
    ('2','Null'),
    ('3','6'),
    ('4','7'),
    ('5','8'),
    ('6','9'),
    ('7','Null'),
    ('8','Null'),
    ('9','10'),
    ('10','Null')

What I have tried:

SQL

;WITH RecursivePaths AS 
(
    SELECT ID, NextID, CAST(ID AS VARCHAR(MAX)) AS Path 
    FROM Data 
    WHERE ID = 3 -- Starting point, adjust as needed

    UNION ALL

    SELECT t.ID, t.NextID, r.Path + ' --> ' + CAST(t.ID AS VARCHAR(MAX)) 
    FROM Data t 
    JOIN RecursivePaths r ON t.ID = r.NextID 
    WHERE t.NextID IS NOT NULL 
)
SELECT Path FROM RecursivePaths

Here I have to give starting ID which is not desired.

Posted 16-Nov-23 0:35am

smaranika

Updated 16-Nov-23 0:37am

Richard Deeming

v2

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Comments

Richard Deeming 16-Nov-23 6:40am

Why are 5, 6, 7, 9, and 10 not included as starting points in your expected output?

If that's just an oversight in your question, then you just need to remove the WHERE ID = 3 line.

Oh, and also correct the condition in the recursive part: WHERE r.NextID Is Not Null

smaranika 16-Nov-23 6:51am

Hi,

I am getting all possible paths.
But I need only the below paths as output.
a. 1 --> 5 --> 8
b. 2
C. 3 -->6 -->9 -->10
d. 4-->7
e. 8

Refer : https://dbfiddle.uk/on1WyfUp

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Richard Deeming · Accepted Answer · 2023-11-16T00:59:00

To get the list of paths without including any sub-paths, you could try the following:

1. Reverse the recursion - start with the records with no NextID and work backwards.

2. Find the longest path for each "leaf" node.

Eg:

SQL

WITH RecursivePaths As
(
    SELECT
        ID,
        ID As Leaf,
        CAST(ID AS VARCHAR(MAX)) AS Path
    FROM
        Data
    WHERE
        NextID Is Null

    UNION ALL

    SELECT
        D.ID,
        R.Leaf,
        CAST(D.ID AS VARCHAR(MAX)) + ' --> ' + R.Path
    FROM
        RecursivePaths As R
        INNER JOIN Data As D ON D.NextID = R.ID
),
SortedPaths As
(
    SELECT
        Leaf,
        Path,
        ROW_NUMBER() OVER (PARTITION BY Leaf ORDER BY Len(Path) DESC) As RN
    FROM
        RecursivePaths
)
SELECT
    Path
FROM
    SortedPaths
WHERE
    RN = 1
;