SQL- Cumulative sum based on condition - sql

Question

I have a scenario in which I have to calculate the counter based on below data. If the status is A, B,C than counter should be 0 which is working fine.
If STATUS is D counter should do a cumulative sum with the exception that if status is changed in between(like in 201907) , the counter should reset again and sum should start again with 1,2,3 and so on. Any possible help is appreciated on same.
Input - 3 columns - Customer_No, Date, Status
CUSTOMER_NO Date STATUS
1234 201901 A
1234 201902 B
1234 201903 C
1234 201904 D
1234 201905 D
1234 201906 D
1234 201907 C
1234 201908 D
1234 201910 D
1234 201911 D
1234 201912 D
expected Output - Input columns + Counter Column
CUSTOMER_NO Date STATUS COUNTER
----------------------------------------
1234 201901 A 0
1234 201902 B 0
1234 201903 C 0
1234 201904 D 1
1234 201905 D 2
1234 201906 D 3
1234 201907 C 0
1234 201908 D 1
1234 201910 D 2
1234 201911 D 3
1234 201912 D 4
Sample data
Thanks

Answer 1

You can create a numbering like a serial number for the ordering purpose using the ROW_NUMBER() function as shown below.
create table SampleData(CUSTOMER_NO int
, STATUS char(1)
, COUNTER int)
insert into SampleData Values
(1234, 'A', 0),
(1234, 'B', 0),
(1234, 'C', 0),
(1234, 'D', 1),
(1234, 'D', 2),
(1234, 'D', 3),
(1234, 'C', 0),
(1234, 'D', 1),
(1234, 'D', 2),
(1234, 'D', 3),
(1234, 'D', 4)
;with cte as(
Select *
, ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) AS RN
from SampleData
)
select CUSTOMER_NO
, STATUS
, COUNTER
, (SELECT SUM(case STATUS when 'D' then Counter else 0 end) FROM cte t2 WHERE t2.RN <= cte.RN) AS Needed
from cte
Live db<>fiddle demo.

Answer 2

This is a similar approach this Gordon's, however, uses CTEs and ROW_NUMBER to make the islands first, and then 0's if there is only 1 row in that island using a windowed COUNT and a CASE expression:
WITH Grps AS(
SELECT ID,
CUSTOMER_NO,
[STATUS],
ROW_NUMBER() OVER (PARTITION BY CUSTOMER_NO ORDER BY ID) -
ROW_NUMBER() OVER (PARTITION BY CUSTOMER_NO, [STATUS] ORDER BY ID) AS Grp
FROM (VALUES(1,1234,'A'),
(2,1234,'B'),
(3,1234,'C'),
(4,1234,'D'),
(5,1234,'D'),
(6,1234,'D'),
(7,1234,'C'),
(8,1234,'D'),
(9,1234,'D'),
(10,1234,'D'),
(11,1234,'D'))V(ID,CUSTOMER_NO,[STATUS]))
SELECT ID,
CUSTOMER_NO,
[STATUS],
Grp,
CASE WHEN COUNT(ID) OVER (PARTITION BY CUSTOMER_NO, [STATUS], Grp) = 1 THEN 0
ELSE ROW_NUMBER() OVER (PARTITION BY CUSTOMER_NO, [STATUS], Grp ORDER BY ID) - 1
END AS [COUNTER]
FROM Grps;
As Gordon mentioned, as well, if you don't have some kind of sequential ID/Key, you can't do this with your data. You will need to implement some kind of sequential ID, and hope that your data retains it's "insert order".

Answer 3

This is a variant of a gaps-and-islands problem. For this particular incarnation, you can identify the islands by counting the number of non-D statuses before a given row.
After identifying the groups, use case and row_number():
select t.*,
(case when status = 'D'
then row_number() over (partition by customer_no, grp, status order by date)
else 0
end) as counter
from (select t.*,
sum(case when status <> 'D' then 1 else 0 end) over (partition by customer_no order by date) as grp
from t
) t