SQL recursive logic

Question

I have a situation where I need to configure existing client data to address a problem where our application was not correctly updating IDs in a table when it should have been.

Here's the scenario. We have a parent table, where rows can be inserted that effectively replace existing rows; the replacement can be recursive. We also have a child table, which has a field that points to the parent table. In existing data, the child table could be pointing at rows that have been replaced, and I need to correct that. I can't simply update each row to the replacing row, however, because that row could have been replaced as well, and I need the latest row to be reflected.

I was trying to find a way to write a CTE that would accomplish this for me, but I'm struggling to find a query that finds what I'm actually looking for. Here's a sample of the tables that I'm working with; the 'ShouldBe' column is what I'd like my update query to end up with, taking into account the recursive replacement of some of the rows.

DECLARE @parent TABLE (SampleID int, 
                   SampleIDReplace int,
                   GroupID char(1))

INSERT INTO @parent (SampleID, SampleIDReplace, GroupID)
VALUES (1, -1, 'A'), (2, 1, 'A'), (3, -1, 'A'), 
       (4, -1, 'A'), (5, 4, 'A'), (6, 5, 'A'),
       (7, -1, 'B'), (8, 7, 'B'), (9, 8, 'B')


DECLARE @child TABLE (ChildID int, ParentID int)
INSERT INTO @child (ChildID, ParentID)
VALUES (1, 4), (2, 7), (3, 1), (4, 3)

Desired results in child table, after the update script has been applied:

ChildID     ParentID    ParentID_ShouldBe
1           4           6 (4 replaced by 5, 5 replaced by 6)
2           7           9 (7 replaced by 8, 8 replaced by 9)
3           1           2 (1 replaced by 2)
4           3           3 (unchanged, never replaced)

Answer 1

The following returns what you are looking for:

with cte as (
    select sampleid, sampleidreplace, 1 as num
    from @parent
    where sampleidreplace <> -1
    union all
    select p.sampleid, cte.sampleidreplace, cte.num+1
    from @parent p join
         cte
         on p.sampleidreplace = cte.sampleId
)
select c.*, coalesce(p.sampleid, c.parentid)
from @child c left outer join
     (select ROW_NUMBER() over (partition by sampleidreplace order by num desc) as seqnum, *
      from cte
     ) p
     on c.ParentID = p.SampleIDReplace and p.seqnum = 1

The recursive part keeps track of every correspondence (4-->5, 4-->6). The addition number is a "generation" count. We actually want the last generation. This is identified by using the row_number() function, ordering by the num in decreasing order -- hence the p.seqnum = 1.