SQL Database Parent/Child recursion - sql

Question

Here is my table:
parent_id | child_id
--------------
1 | 2
1 | 3
1 | 4
2 | 5
2 | 6
5 | 8
8 | 9
9 | 5
I need to get all of the items under parent 2. I've found a few things similar to this, but but couldn't figure out how to make it work for my case. I keep getting maximum recursion limit reached. Here's what I have:
WITH CTE AS
(
SELECT gt.[child_id]
FROM [CHSPortal].[dbo].[company_adgroupstoadgroups] gt
WHERE gt.parent_id='2'
UNION ALL
SELECT g.[child_id]
FROM [CHSPortal].[dbo].[company_adgroupstoadgroups] g
INNER JOIN CTE g2 on g.parent_id=g2.child_id
)
select distinct child_id from CTE
The desired result is going to be: 2,3,4,5,6,8,9.
What modification do I need to make to get a list of all the items under child 2. I would also prefer 2 (the parent node) to be in the list. Any help would be appreciated.

Answer 1

First of all, there is a loop in your example (5|8, 8|9, 9|5), that is why you reach the maximum recursion limit.
Regarding the filtering question,below you can find an example for filtering by root node:
;WITH MTree (parent_id, child_id, LEVEL) AS (
SELECT t.parent_id , t.child_id, 0 AS LEVEL
FROM table_1 t
WHERE child_id = 2 --here you can filter the root node
UNION ALL
SELECT m.parent_id , m.child_id, LEVEL + 1
FROM Table_1 m
INNER JOIN MTree t ON t.child_id = m.parent_id
)
SELECT * FROM Mtree;

Answer 2

Not sure what's wrong with your query, aside from not relating to the sample data you provided, but this works just fine:
;WITH src AS (SELECT 1 AS parent_id, 2 AS child_id
UNION SELECT 1, 3
UNION SELECT 1, 4
UNION SELECT 2, 5
UNION SELECT 2, 6
UNION SELECT 5, 8
UNION SELECT 8, 9
UNION SELECT 9, 5)
,cte AS (SELECT *
FROM src
WHERE child_id = 2
UNION ALL
SELECT a.*
FROM src a
JOIN cte b
ON a.parent_id = b.child_id
)
SELECT TOP 100 *
FROM cte
--Limited to top 100 because of infinite recursion problem with sample data.