Sql Server 2014 - Deep Recursive Parent-Child Self Join - sql

Question

I am trying to build a deep recursive self-join query. Having the table like:
Id | ParentId
1 | NULL
2 | 1
3 | 1
4 | 2
5 | 3
6 | 8
7 | 9
For Id 1 my query should be fetching 1,2,3,4,5 since they are either the children of 1 or children of the children of 1. In the given example 6 and 7 should not be included in the query result.
I tried using CTE but I am getting tons of duplicates:
WITH CTE AS (
SELECT Id, ParentId
FROM dbo.Table
WHERE ParentId IS NULL
UNION ALL
SELECT t.Id, t.ParentId
FROM dbo.Table t
INNER JOIN CTE c ON t.ParentId = c.Id
)
SELECT * FROM CTE
Ideas?

Answer 1

You can try to use DISTINCT to filter duplicate rows.
;WITH CTE AS (
SELECT Id, ParentId
FROM T
WHERE ParentId IS NULL
UNION ALL
SELECT t.Id, t.ParentId
FROM T
INNER JOIN CTE c ON t.ParentId = c.Id
)
SELECT DISTINCT Id, ParentId
FROM CTE

Answer 2

Try the following query with CTE where you can set parentId by #parentID:
DECLARE #parentID INT = 1
;WITH cte AS
(
SELECT
t.ID
, t.ParentId
FROM #table t
),
cteParent AS
(
SELECT
t.ID
, t.ParentId
FROM #table t
WHERE t.ParentId IN (SELECT t1.ID FROM #table t1 WHERE T1.ParentId = #parentID)
)
SELECT
DISTINCT c1.ID
, c1.ParentId
FROM cte c1
INNER JOIN cte c2 ON c2.ParentId = c1.ID
UNION ALL
SELECT *
FROM cteParent
And the sample data:
DECLARE #table TABLE
(
ID INT
, ParentId INT
)
INSERT INTO #table
(
ID,
ParentId
)
VALUES
(1, NULL )
, (2, 1 )
, (3, 1 )
, (4, 2 )
, (5, 3 )
, (6, 8 )
, (7, 9 )
OUTPUT:
ID ParentId
1 NULL
2 1
3 1
4 2
5 3

Answer 3

I don't see duplicates.
Your code returns the following on the data you provided:
Id ParentId
1
2 1
3 1
5 3
4 2
which is what you want.
Here is a db<>fiddle.
Here is the code:
WITH t as (
SELECT *
FROM (VALUES (1, NULL), (2, 1), (3, 1), (4, 2), (5, 3), (6, 8), (7, 9)
) v(id, parentId)
),
CTE AS (
SELECT Id, ParentId
FROM t
WHERE ParentId IS NULL
UNION ALL
SELECT t.Id, t.ParentId
FROM t
INNER JOIN CTE c ON t.ParentId = c.Id
)
SELECT *
FROM CTE;
If you are getting duplicates in your actual result set, then you presumably have duplicates in your original table. I would recommend removing them before doing the recursive logic:
with t as (
select distinct id, parentid
from <your query>
),
. . .
Then run the recursive logic.

Answer 4

Try this sql script which result Parent Child Hierarchy
;WITH CTE(Id , ParentId)
AS
(
SELECT 1 , NULL UNION ALL
SELECT 2 , 1 UNION ALL
SELECT 3 , 1 UNION ALL
SELECT 4 , 2 UNION ALL
SELECT 5 , 3 UNION ALL
SELECT 6 , 8 UNION ALL
SELECT 7 , 9
)
,Cte2
AS
(
SELECT Id ,
ParentId ,
CAST('\'+ CAST(Id AS VARCHAR(MAX))AS VARCHAR(MAX)) AS [Hierarchy]
FROM CTE
WHERE ParentId IS NULL
UNION ALL
SELECT c1.Id ,
c1.ParentId ,
[Hierarchy]+'\'+ CAST(c1.Id AS VARCHAR(MAX)) AS [Hierarchy]
FROM Cte2 c2
INNER JOIN CTE c1
ON c1.ParentId = c2.Id
)
SELECT Id,
RIGHT([Hierarchy],LEN([Hierarchy])-1) AS ParentChildHierarchy
FROM Cte2
GO
Result
Id ParentChildHierarchy
-------------------------
1 1
2 1\2
3 1\3
5 1\3\5
4 1\2\4

Answer 5

This query will help you
CREATE TABLE #table( ID INT, ParentId INT )
INSERT INTO #table(ID,ParentId)
VALUES (1, NULL ), (2, 1 ), (3, 1 ), (4, 2 ), (5, 3 ), (6, 8 ), (7, 9 )
;WITH CTE AS (
SELECT ID FROM #table WHERE PARENTID IS NULL
UNION ALL
SELECT T.ID FROM #table T
INNER JOIN #table T1 ON T.PARENTID =T1.ID
) SELECT * FROM CTE