Sql Server CTE Parent Child recursive

Question

I have following table structure:

create table Test(
  ParentId int,
  ChildId int
 )

insert into Test(ParentId, ChildId)
select 1, NULL
union
select 1, 2
union         


        

Answer 1

As #Mikael Eriksson said: "You have a circular references in your data. 5 is parent to 6 and 6 is parent to 5."

Also in recursive part you output ParentId from previous step, not from just found rows.

declare @Test table (ParentId int, ChildId int)

insert into @Test (ParentId, ChildId)
select 1, null
union all
select 1, 2
union all
select 1, 3
union all
select 2, null
union all
select 2, 5
union all
select 5, 6
union all
--select 6, 5
--union all
select 6, 8

declare @id int = 2

;with MyCTE as (
    select ParentId, ChildId
    from @test
    where ParentId = @id

    union all

    select t2.ParentId, t2.ChildId
    from MyCTE t1
    inner join @Test t2 on t1.ChildId = t2.ParentId
)

select * from MyCTE

Another thing I didn't understand is why do you have such rows where ChildId is null and ParentId is not null. How can it be possible?

Does it mean that you have unknown items whose parent is known?