Creating a conditonal ROW_NUMBER() Partition clause based on previous row value - sql-server

Question

I have a table that looks like this:
+----------------+--------+
| EvidenceNumber | ID |
+----------------+--------+
| 001 | 8 |
| 001.A | 8 |
| 001.A.01 | 8 |
| 001.A.02 | 8 |
| 001.B | 8 |
| 001.C | 8 |
| 001.D | 8 |
| 001.E | 8 |
| 001.F | 8 |
| 001.G | 8 |
| 001.G.01 | 8 |
+----------------+--------+
If 001 were a bag, inside of it was 001.A, 001.B, and so on through to 001.G
In the output above, 001.A was another bag, and that bag contained 001.A.01 and 001.A.02. The same thing can be seen with 001.G.01.
Every entry in this table is either a bag or an item. I am only interested in counting the amount of items per ID.
Since 001.A.01 and 001.A.02 is the last we see of the "001.A's" we know A.01 and A.02 were items.
Since we see 001.B only once, that was an item as well.
001.G was a bag, but 001.G.01 was an item.
The above output is showing 8 items and 3 bags.
I feel like Row_number and the Partition clause is the perfect tool for the job, but I can't find a way to partition based on a clause that uses a previous row's value.
Maybe something like that isn't even necessary here, but I pictured it like:
{001} -- variable
{001}.A -- variable seen again, obviously 001 was a bag. Create new variable {001.A} and move on.
{001.A}.01 -- same thing.
{001.A.01} -- Unique variable. This is a final step. This is a bag and should be Row number 1.
Obviously, the below code is just making "ItemNum" 1 for each item since there are not duplicates.
SELECT
ROW_NUMBER() OVER(Partition BY EvidenceNumber ORDER BY EvidenceNumber) AS ItemNum,
EvidenceNumber,
ID
FROM EVIDENCE
WHERE ID = '18'
ORDER BY EvidenceNumber
+---------+----------------+--------+
| ItemNum | EvidenceNumber | ID |
+---------+----------------+--------+
| 1 | 001 | 8 |
| 1 | 001.A | 8 |
| 1 | 001.A.01 | 8 |
| 1 | 001.A.02 | 8 |
| 1 | 001.B | 8 |
| 1 | 001.C | 8 |
| 1 | 001.D | 8 |
| 1 | 001.E | 8 |
| 1 | 001.F | 8 |
| 1 | 001.G | 8 |
| 1 | 001.G.01 | 8 |
+---------+----------------+--------+
Ideally, it would partition on the items only, so in this case:
+---------+----------------+----+
| ItemNum | EvidenceNumber | ID |
+---------+----------------+----+
| 0 | 001 | 8 |
| 0 | 001.A | 8 |
| 1 | 001.A.01 | 8 |
| 2 | 001.A.02 | 8 |
| 3 | 001.B | 8 |
| 4 | 001.C | 8 |
| 5 | 001.D | 8 |
| 6 | 001.E | 8 |
| 7 | 001.F | 8 |
| 0 | 001.G | 8 |
| 8 | 001.G.01 | 8 |
+---------+----------------+----+

Answer 1

I don't think window functions alone are the best approach. Instead:
select t.*,
(case when exists (select 1
from evidence t2
where t2.caseid = t.caseid and
t2.EvidenceNumber like t.EvidenceNumber + '.%'
)
then 0 else 1
end) as is_item
from evidence t ;
Then sum these up using another subquery:
select t.*,
sum(is_item) over (partition by caseid order by EvidenceNumber) as item_counter
from (select t.*,
(case when exists (select 1
from evidence t2
where t2.caseid = t.caseid and
t2.EvidenceNumber like t.EvidenceNumber + '.%'
)
then 0 else 1
end) as is_item
from evidence t
) t;

Answer 2

trick with Lead and Row_Number:
DECLARE #Table TABLE (
EvidenceNumber varchar(64),
Id int
)
INSERT INTO #Table VALUES
('001',8),
('001.A',8),
('001.A.01',8),
('001.A.02',8),
('001.B',8),
('001.C',8),
('001.D',8),
('001.E',8),
('001.F',8),
('001.G',8),
('001.G.01',8);
WITH CTE AS (
SELECT
[IsBag] = PATINDEX(EvidenceNumber+'%',
IsNull(LEAD(EvidenceNumber) OVER (ORDER BY EvidenceNumber),0)
),
[EvidenceNumber],
[Id]
FROM
#Table
)
SELECT
[NumItem] = IIF(IsBag = 0,ROW_NUMBER() OVER (PARTITION BY [ISBag] order by [IsBag]),0),
[EvidenceNumber],
[Id]
FROM
CTE
ORDER BY EvidenceNumber