Simplest way to do a recursive self-join?
-
20-09-2019 - |
Question
What is the simplest way of doing a recursive self-join in SQL Server? I have a table like this:
PersonID | Initials | ParentID
1 CJ NULL
2 EB 1
3 MB 1
4 SW 2
5 YT NULL
6 IS 5
And I want to be able to get the records only related to a hierarchy starting with a specific person. So If I requested CJ's hierarchy by PersonID=1 I would get:
PersonID | Initials | ParentID
1 CJ NULL
2 EB 1
3 MB 1
4 SW 2
And for EB's I'd get:
PersonID | Initials | ParentID
2 EB 1
4 SW 2
I'm a bit stuck on this can can't think how to do it apart from a fixed-depth response based on a bunch of joins. This would do as it happens because we won't have many levels but I would like to do it properly.
Thanks! Chris.
Solution
WITH q AS
(
SELECT *
FROM mytable
WHERE ParentID IS NULL -- this condition defines the ultimate ancestors in your chain, change it as appropriate
UNION ALL
SELECT m.*
FROM mytable m
JOIN q
ON m.parentID = q.PersonID
)
SELECT *
FROM q
By adding the ordering condition, you can preserve the tree order:
WITH q AS
(
SELECT m.*, CAST(ROW_NUMBER() OVER (ORDER BY m.PersonId) AS VARCHAR(MAX)) COLLATE Latin1_General_BIN AS bc
FROM mytable m
WHERE ParentID IS NULL
UNION ALL
SELECT m.*, q.bc + '.' + CAST(ROW_NUMBER() OVER (PARTITION BY m.ParentID ORDER BY m.PersonID) AS VARCHAR(MAX)) COLLATE Latin1_General_BIN
FROM mytable m
JOIN q
ON m.parentID = q.PersonID
)
SELECT *
FROM q
ORDER BY
bc
By changing the ORDER BY
condition you can change the ordering of the siblings.
OTHER TIPS
Using CTEs you can do it this way
DECLARE @Table TABLE(
PersonID INT,
Initials VARCHAR(20),
ParentID INT
)
INSERT INTO @Table SELECT 1,'CJ',NULL
INSERT INTO @Table SELECT 2,'EB',1
INSERT INTO @Table SELECT 3,'MB',1
INSERT INTO @Table SELECT 4,'SW',2
INSERT INTO @Table SELECT 5,'YT',NULL
INSERT INTO @Table SELECT 6,'IS',5
DECLARE @PersonID INT
SELECT @PersonID = 1
;WITH Selects AS (
SELECT *
FROM @Table
WHERE PersonID = @PersonID
UNION ALL
SELECT t.*
FROM @Table t INNER JOIN
Selects s ON t.ParentID = s.PersonID
)
SELECT *
FROm Selects
The Quassnoi query with a change for large table. Parents with more childs then 10: Formating as str(5) the row_number()
WITH q AS ( SELECT m.*, CAST(str(ROW_NUMBER() OVER (ORDER BY m.ordernum),5) AS VARCHAR(MAX)) COLLATE Latin1_General_BIN AS bc FROM #t m WHERE ParentID =0 UNION ALL SELECT m.*, q.bc + '.' + str(ROW_NUMBER() OVER (PARTITION BY m.ParentID ORDER BY m.ordernum),5) COLLATE Latin1_General_BIN FROM #t m JOIN q ON m.parentID = q.DBID ) SELECT * FROM q ORDER BY bc
SQL 2005 or later, CTEs are the standard way to go as per the examples shown.
SQL 2000, you can do it using UDFs -
CREATE FUNCTION udfPersonAndChildren
(
@PersonID int
)
RETURNS @t TABLE (personid int, initials nchar(10), parentid int null)
AS
begin
insert into @t
select * from people p
where personID=@PersonID
while @@rowcount > 0
begin
insert into @t
select p.*
from people p
inner join @t o on p.parentid=o.personid
left join @t o2 on p.personid=o2.personid
where o2.personid is null
end
return
end
(which will work in 2005, it's just not the standard way of doing it. That said, if you find that the easier way to work, run with it)
If you really need to do this in SQL7, you can do roughly the above in a sproc but couldn't select from it - SQL7 doesn't support UDFs.
Check following to help the understand the concept of CTE recursion
DECLARE
@startDate DATETIME,
@endDate DATETIME
SET @startDate = '11/10/2011'
SET @endDate = '03/25/2012'
; WITH CTE AS (
SELECT
YEAR(@startDate) AS 'yr',
MONTH(@startDate) AS 'mm',
DATENAME(mm, @startDate) AS 'mon',
DATEPART(d,@startDate) AS 'dd',
@startDate 'new_date'
UNION ALL
SELECT
YEAR(new_date) AS 'yr',
MONTH(new_date) AS 'mm',
DATENAME(mm, new_date) AS 'mon',
DATEPART(d,@startDate) AS 'dd',
DATEADD(d,1,new_date) 'new_date'
FROM CTE
WHERE new_date < @endDate
)
SELECT yr AS 'Year', mon AS 'Month', count(dd) AS 'Days'
FROM CTE
GROUP BY mon, yr, mm
ORDER BY yr, mm
OPTION (MAXRECURSION 1000)