Writing a recursive SQL query on a self-referencing table - sql

Question

I have a database with a table called Items, that contains these columns:
ID - primary key, uniqueidentifier
Name - nvarchar(256)
ParentID - uniqueidentifier
The name field can be used to build out a path to the item, by iterating through each ParentId until it equals '11111111-1111-1111-1111-111111111111', which is a root item.
So if you had a table that had rows like
ID Name ParentID
-------------------------------------------------------------------------------------
11111111-1111-1111-1111-111111111112 grandparent 11111111-1111-1111-1111-111111111111
22222222-2222-2222-2222-222222222222 parent 11111111-1111-1111-1111-111111111112
33333333-3333-3333-3333-333333333333 widget 22222222-2222-2222-2222-222222222222
So if I looked up an item with id '33333333-3333-3333-3333-333333333333' in the example above, i'd want the path
/grandparent/parent/widget
returned. i've attempted to write a CTE, as it looks like that's how you'd normally accomplish something like this - but as I don't do very much SQL, I can't quite figure out where i'm going wrong. I've looked at some examples, and this is as close as I seem to be able to get - which only returns the child row.
declare #id uniqueidentifier
set #id = '10071886-A354-4BE6-B55C-E5DBCF633FE6'
;with ItemPath as (
select a.[Id], a.[Name], a.ParentID
from Items a
where Id = #id
union all
select parent.[Id], parent.[Name], parent.ParentID
from Items parent
inner join ItemPath as a
on a.Id = parent.id
where parent.ParentId = a.[Id]
)
select * from ItemPath
I have no idea how i'd declare a local variable for the path and keep appending to it in the recursive query. i was going to try to at least get all the rows to the parent before going after that. if anyone could help with that as well - i'd appreciate it.

Answer 1

well here's working solution
SQL FIDDLE EXAMPLE
declare #id uniqueidentifier
set #id = '33333333-3333-3333-3333-333333333333'
;with ItemPath as
(
select a.[Id], a.[Name], a.ParentID
from Items a
where Id = #id
union all
select parent.[Id], parent.[Name] + '/' + a.[Name], parent.ParentID
from ItemPath as a
inner join Items as parent on parent.id = a.parentID
)
select *
from ItemPath
where ID = '11111111-1111-1111-1111-111111111112'
I don't like it much, I think better solution will be to do it other way around. Wait a minute and I try to write another query :)
UPDATE here it is
SQL FIDDLE EXAMPLE
create view vw_Names
as
with ItemPath as
(
select a.[Id], cast(a.[Name] as nvarchar(max)) as Name, a.ParentID
from Items a
where Id = '11111111-1111-1111-1111-111111111112'
union all
select a.[Id], parent.[Name] + '/' + a.[Name], a.ParentID
from Items as a
inner join ItemPath as parent on parent.id = a.parentID
)
select *
from ItemPath
and now you can use this view
declare #id uniqueidentifier
set #id = '33333333-3333-3333-3333-333333333333'
select *
from vw_Names where Id = #id

Answer 2

I needed a slightly different version of this answer as I wanted to produce a list of all lineages in the tree. I also wanted to know the depth of each node. I added a temporary table of top level parents that I could loop through and a temporary table to build the result set.
use Items
Select *
Into #Temp
From Items
where ParentID=0
Declare #Id int
create table #Results
(
Id int,
Name nvarchar(max),
ParentId int,
Depth int
)
While (Select Count(*) From #Temp) > 0
Begin
Select Top 1 #Id = Id From #Temp
begin
with ItemPath as
(
select a.[Id], cast(a.[Name] as nvarchar(max))as Name, a.ParentID ,1 as
Depth
from Items a
where a.ID = #id
union all
select a.[Id], parent.[Name] + '/' + a.[Name], a.ParentID, 1 + Depth
from Items as a
inner join ItemPath as parent on parent.id = a.parentID
)
insert into #Results
select *
from ItemPath
end
Delete #Temp Where Id = #Id
End
drop table #Temp
select * from #Results
drop table #Results
If we start from the following table...
Id Name ParentID
1 Fred 0
2 Mary 0
3 Baker 1
4 Candle 2
5 Stick 4
6 Maker 5
We would get this result table.
Id Name ParentID Depth
1 Fred 0 1
2 Mary 0 1
3 Fred/Baker 1 2
4 Mary/Candle 2 2
5 Mary/Candle/Stick 4 3
6 Mary/Candle/Stick/Maker 5 4