Question

I have following table:

ID     ParentID
1         NULL
2          1
3          2
4          NULL
5          4
6          5
7          3

I want to find the first ID of a specific child ID.
Example: ID=7 and the result is 1
ID=6 and the result is 4

How to do it?

Answer 1

You need to do a bit of recursive CTE magic to solve this one.

Given the data in a table variable thusly:

declare @data table(id int, parentid int)
insert into @data
  select 1, null
  union select 2,1
  union select 3,2
  union select 4, null
  union select 5,4
  union select 6,5
  union select 7,3

The following should do the trick:

;with recursiveTable as
(
    select d.id, d.parentId, 0 as depth
    from @data d
    where d.id=6 -- Parameterize this
    union all

    select d.id, d.parentid, r.depth-1
    from @data d
    inner join recursiveTable r
    on d.id = r.parentId
)
select top 1 id 
from recursiveTable 
order by depth

Plugging 6 in as above returns 4. Changing this to 7 returns 1 as requested.