Sql HierarchyId How do I get the last descendants? - sql

Question

Using t-sql hierarchy Id how do I get all of the rows that have no children (that is the last decendants)?
Say my table is structured like this:
Id,
Name,
HierarchyId
And has these rows:
1, Craig, /
2, Steve, /1/
3, John, /1/1/
4, Sam, /2/
5, Matt, /2/1/
6, Chris, /2/1/1/
What query would give me John and Chris?

Answer 1

Perhaps there are better ways but this seams to do the job.
declare #T table
(
ID int,
Name varchar(10),
HID HierarchyID
)
insert into #T values
(1, 'Craig', '/'),
(2, 'Steve', '/1/'),
(3, 'John', '/1/1/'),
(4, 'Sam', '/2/'),
(5, 'Matt', '/2/1/'),
(6, 'Chris', '/2/1/1/')
select *
from #T
where HID.GetDescendant(null, null) not in (select HID
from #T)
Result:
ID Name HID
----------- ---------- ---------------------
3 John 0x5AC0
6 Chris 0x6AD6
Update 2012-05-22
Query above will fail if node numbers is not in an unbroken sequence. Here is another version that should take care of that.
declare #T table
(
ID int,
Name varchar(10),
HID HierarchyID
)
insert into #T values
(1, 'Craig', '/'),
(2, 'Steve', '/1/'),
(3, 'John', '/1/1/'),
(4, 'Sam', '/2/'),
(5, 'Matt', '/2/1/'),
(6, 'Chris', '/2/1/2/') -- HID for this row is changed compared to above query
select *
from #T
where HID not in (select HID.GetAncestor(1)
from #T
where HID.GetAncestor(1) is not null)

Answer 2

Since you only need leafs and you don't need to get them from a specific ancestor, a simple non-recursive query like this should do the job:
SELECT * FROM YOUR_TABLE PARENT
WHERE
NOT EXISTS (
SELECT * FROM YOUR_TABLE CHILD
WHERE CHILD.HierarchyId = PARENT.Id
)
In plain English: select every row without a child row.
This assumes your HierarchyId is a FOREIGN KEY towards the Id, not the whole "path" as presented in your example. If it isn't, this is probably the first thing you should fix in your database model.
--- EDIT ---
OK, here is the MS SQL Server-specific query that actually works:
SELECT * FROM YOUR_TABLE PARENT
WHERE
NOT EXISTS (
SELECT * FROM YOUR_TABLE CHILD
WHERE
CHILD.Id <> PARENT.Id
AND CHILD.HierarchyId.IsDescendantOf(PARENT.HierarchyId) = 1
)
Note that the IsDescendantOf considers any row a descendant of itself, so we also need the CHILD.Id <> PARENT.Id in the condition.

Answer 3

Hi I use this one and works perfectly for me.
CREATE TABLE [dbo].[Test]([Id] [hierarchyid] NOT NULL, [Name] [nvarchar](50) NULL)
DECLARE #Parent AS HierarchyID = CAST('/2/1/' AS HierarchyID) -- Get Current Parent
DECLARE #Last AS HierarchyID
SELECT #Last = MAX(Id) FROM Test WHERE Id.GetAncestor(1) = #Parent -- Find Last Id for this Parent
INSERT INTO Test(Id,Name) VALUES(#Parent.GetDescendant(#Last, NULL),'Sydney') -- Insert after Last Id