Finding a Top Level Parent in SQL - sql

Question

I have got two tables as following
Table Person
Id Name
1 A
2 B
3 C
4 D
5 E
Table RelationHierarchy
ParentId ChildId
2 1
3 2
4 3
This will form a tree like structure
D
|
C
|
B
|
A
ParentId and ChildId are foreign keys of Id column of Person Table
I need to write SQL that Can fetch me Top Level Parent i-e Root. Can anyone suggest any SQL that can help me accomplish this

Answer 1

You can use recursive CTE to achieve that:
DECLARE #childID INT
SET #childID = 1 --chield to search
;WITH RCTE AS
(
SELECT *, 1 AS Lvl FROM RelationHierarchy
WHERE ChildID = #childID
UNION ALL
SELECT rh.*, Lvl+1 AS Lvl FROM dbo.RelationHierarchy rh
INNER JOIN RCTE rc ON rh.CHildId = rc.ParentId
)
SELECT TOP 1 id, Name
FROM RCTE r
inner JOIN dbo.Person p ON p.id = r.ParentId
ORDER BY lvl DESC
SQLFiddle DEMO
EDIT - for updated request for top level parents for all children:
;WITH RCTE AS
(
SELECT ParentId, ChildId, 1 AS Lvl FROM RelationHierarchy
UNION ALL
SELECT rh.ParentId, rc.ChildId, Lvl+1 AS Lvl
FROM dbo.RelationHierarchy rh
INNER JOIN RCTE rc ON rh.ChildId = rc.ParentId
)
,CTE_RN AS
(
SELECT *, ROW_NUMBER() OVER (PARTITION BY r.ChildID ORDER BY r.Lvl DESC) RN
FROM RCTE r
)
SELECT r.ChildId, pc.Name AS ChildName, r.ParentId, pp.Name AS ParentName
FROM CTE_RN r
INNER JOIN dbo.Person pp ON pp.id = r.ParentId
INNER JOIN dbo.Person pc ON pc.id = r.ChildId
WHERE RN =1
SQLFiddle DEMO
EDIT2 - to get all persons change JOINS a bit at the end:
SELECT pc.Id AS ChildID, pc.Name AS ChildName, r.ParentId, pp.Name AS ParentName
FROM dbo.Person pc
LEFT JOIN CTE_RN r ON pc.id = r.CHildId AND RN =1
LEFT JOIN dbo.Person pp ON pp.id = r.ParentId
SQLFiddle DEMo

Answer 2

I've used this pattern to associate items in a hierarchy with the item's root node.
Essentially recursing the hierarchies maintaining the values of the root node as additional columns appended to each row. Hope this helps.
with allRows as (
select ItemId, ItemName, ItemId [RootId],ItemName [RootName]
from parentChildTable
where ParentItemId is null
union all
select a1.ItemId,a1.ItemName,a2.[RootId],a2.[RootName]
from parentChildTable a1
join allRows a2 on a2.ItemId = a1.ParentItemId
)
select * from allRows

Answer 3

To find all top-level parents, use a query like:
select p.Name
from Person p
where not exists
(select null
from RelationHierarchy r
where r.ChildId = p.Id)
SQLFiddle here.
To find the top-level parent of a specific child, use:
with cte as
(select t.ParentId TopParent, t.ChildId
from RelationHierarchy t
left join RelationHierarchy p on p.ChildId = t.ParentId
where p.ChildId is null
union all
select t.TopParent TopParent, c.ChildId
from cte t
join RelationHierarchy c on t.ChildId = c.ParentId)
select p.name
from cte h
join Person p on h.TopParent = p.Id
where h.ChildId=3 /*or whichever child is required*/
SQLFiddle here.

Answer 4

Try this.
The recursive CTE will find the person and walk up the hierarchy until it finds no parent.
-- This CTE will find the ancestors along with a measure of how far up
-- the hierarchy each ancestor is from the selected person.
with ancestor as (
select ParentId as AncestorId, 0 as distance
from RelationHierarchy
where CHildId = ?
union all
select h.ParentId, a.distance + 1
from ancestor a inner join RelationHierarchy rh on a.AncestorId = rh.ChildId
)
select AncestorId
from ancestor
where distance = (select max(distance) from ancestor)

Answer 5

Something like this will work for above example:
SELECT ParentId FROM RelationHierarchy
WHERE ParentId NOT IN (SELECT CHildId FROM RelationHierarchy)

Answer 6

The only way you can do this in "standard" SQL is to assume a maximum depth for the tree, and then do joins for each level. The following gets the top level id:
select rh1.ChildId,
coalesce(rh4.parentid, rh3.parentid, rh2.parentid, rh1.parentid) as topLevel
from RelationshipHierarchy rh1 left outer join
RelationshipHierarchy rh2
on rh1.parentId = rh2.childId left outer join
RelationshipHierarchy rh3
on rh2.parentId = rh3.childId left outer join
RelationshipHierarchy rh4
on rh3.parentId = rh4.childId;
If you want the name, you can just join it in:
select rh1.ChildId,
coalesce(rh4.parentid, rh3.parentid, rh2.parentid, rh1.parentid) as topLevel,
p.name
from RelationshipHierarchy rh1 left outer join
RelationshipHierarchy rh2
on rh1.parentId = rh2.childId left outer join
RelationshipHierarchy rh3
on rh2.parentId = rh3.childId left outer join
RelationshipHierarchy rh4
on rh3.parentId = rh4.childId left outer join
Person p
on p.id = coalesce(rh4.parentid, rh3.parentid, rh2.parentid, rh1.parentid);

Answer 7

Get all top parents using path
path format: rootId/.../parentId/nodeId/
select t1.path from nodes t1 inner join nodes t2
on t1.path like t2.path+'%'
group by t1.path
having len(t1.path)-len(replace(t1.path, '/', ''))
=min(len(t2.path)-len(replace(t2.path, '/', '')))

Answer 8

Give this a go:
select id,name
from person p
where not exists
(
select 1
from relationhierarchy r
where r.childid= p.id
)
and exists
(
select 1
from relationhierarchy r
where r.parentid= p.id
)
It is not enough to just see if a child id exists as in your example E is present in the person table but not in the relationshiphierarchy table.

Answer 9

WITH CTE_MyTable AS (
SELECT Id, ParentId, NULL As RootParent, 1 As Lvl
FROM dbo.Ministry
UNION ALL
SELECT a.id, b.ParentId, a.ParentId As RootParent, Lvl + 1
FROM CTE_MyTableAS a INNER JOIN
dbo.MyTableAS b ON a.ParentId = b.Id
)
, CTE_Ministry_RN AS (
SELECT Id, RootParent, ROW_NUMBER() OVER (PARTITION BY Id ORDER BY Lvl DESC) RN
FROM CTE_Ministry
)
SELECT Id, ISNULL(RootParent, Id) As RootParent
FROM CTE_Ministry_RN
WHERE RN = 1