SQL Server (terminal result) hierarchy map - sql

Question

In SQL Server 2016, I have a table with the following chaining structure:
dbo.Item
OriginalItem
ItemID
NULL
7
1
2
NULL
1
5
6
3
4
NULL
8
NULL
5
9
11
2
3
EDIT NOTE: Bold numbers were added as a response to #lemon comments below
Importantly, this example is a trivialized version of the real data, and the neatly ascending entries is not something that is present in the actual data, I'm just doing that to simplify the understanding.
I've constructed a query to get what I'm calling the TerminalItemID, which in this example case is ItemID 4, 6, and 7, and populated that into a temporary table #TerminalItems, the resultset of which would look like:
#TerminalItems
TerminalItemID
4
6
7
8
11
What I need, is a final mapping table that would look something like this (using the above example -- note that it also contains for 4, 6, and 7 mapping to themselves, this is needed by the business logic):
#Mapping
ItemID
TerminalItemID
1
4
2
4
3
4
4
4
5
6
6
6
7
7
8
8
9
11
11
11
What I need help with is how to build this last #Mapping table. Any assistance in this direction is greatly appreciated!

Answer 1

This should do:
with MyTbl as (
select *
from (values
(NULL, 1 )
,(1, 2 )
,(2, 3 )
,(3, 4 )
,(NULL, 5 )
,(5, 6 )
,(NULL, 7 )
) T(OriginalItem, ItemID)
)
, TerminalItems as (
/* Find all leaf level items: those not appearing under OriginalItem column */
select LeafItem=ItemId, ImmediateOriginalItem=M.OriginalItem
from MyTbl M
where M.ItemId not in
(select distinct OriginalItem
from MyTbl AllParn
where OriginalItem is not null
)
), AllLevels as (
/* Use a recursive CTE to find and report all parents */
select ThisItem=LeafItem, ParentItem=ImmediateOriginalItem
from TerminalItems
union all
select ThisItem=AL.ThisItem, M.OriginalItem
from AllLevels AL
inner join
MyTbl M
on M.ItemId=AL.ParentItem
)
select ItemId=coalesce(ParentItem,ThisItem), TerminalItemId=ThisItem
from AllLevels
order by 1,2
Beware of the MAXRECURSION setting; by default SQLServer iterates through recursion 100 times; this would mean that the depth of your tree can be 100, max (the maximum number of nodes between a terminal item and its ultimate original item). This can be increased by OPTION(MAXRECURSION nnn) where nnn can be adjusted as needed. It can also be removed entirely by using 0 but this is not recommended because your data can cause infinite loops.

Answer 2

This is a typical gaps-and-islands problem and can also be carried out without recursion in three steps:
assign 1 at the beginning of each partition
compute a running sum over your flag value (generated at step 1)
extract the max "ItemID" on your partition (generated at step 2)
WITH cte1 AS (
SELECT *, CASE WHEN OriginalItem IS NULL THEN 1 ELSE 0 END AS changepartition
FROM Item
), cte2 AS (
SELECT *, SUM(changepartition) OVER(ORDER BY ItemID) AS parts
FROM cte1
)
SELECT ItemID, MAX(ItemID) OVER(PARTITION BY parts) AS TerminalItemID
FROM cte2
Check the demo here.
Assumption: Your terminal id items correspond to the "ItemID" value preceding a NULL "OriginalItem" value.
EDIT: "Fixing orphaned records."
The query works correctly when records are not orphaned. The only way to deal them, is to get missing records back, so that the query can work correctly on the full data.
This is carried out by an extra subquery (done at the beginning), that will apply a UNION ALL between:
the available records of the original table
the missing records
WITH fix_orphaned_records AS(
SELECT * FROM Item
UNION ALL
SELECT NULL AS OriginalItem,
i1.OriginalItem AS ItemID
FROM Item i1
LEFT JOIN Item i2 ON i1.OriginalItem = i2.ItemID
WHERE i1.OriginalItem IS NOT NULL AND i2.ItemID IS NULL
), cte AS (
...
Missing records correspond to "OriginalItem" values that are never found within the "ItemID" field. A self left join will uncover these missing records.
Check the demo here.

Answer 3

You can use a recursive CTE to compute the last item in the sequence. For example:
with
n (orig_id, curr_id, lvl) as (
select itemid, itemid, 1 from item
union all
select n.orig_id, i.itemid, n.lvl + 1
from n
join item i on i.originalitem = n.curr_id
)
select *
from (
select *, row_number() over(partition by orig_id order by lvl desc) as rn from n
) x
where rn = 1
Result:
orig_id curr_id lvl rn
-------- -------- ---- --
1 4 4 1
2 4 3 1
3 4 2 1
4 4 1 1
5 6 2 1
6 6 1 1
7 7 1 1
See running example at db<>fiddle.