SQL Server 2017

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CREATE TABLE [dbo].[Warehouse](
        [ID] [int] NOT NULL
        ,[Flag] [nvarchar](1) NULL
	,[ArtNo] [nvarchar](30) NOT NULL
	,[Amount] [float] NULL
) ON [PRIMARY]
INSERT INTO [dbo].[Warehouse]
           ([ID]
           ,[Flag]
           ,[ArtNo]
           ,[Amount])
     VALUES
           (1,'U','A1000',-100),
           (2,'U','B2000',-5),
           (3,'V','B2000',900),
           (4,'U','B2000',-10),
           (5,'I','B2000',50),
           (6,'U','B2000',-20),
           (7,'U','A1000',-50),
           (8,'I','A1000',1000),
           (9,'V','A1000',3600),
           (10,'U','A1000',-500),
           (11,'U','A1000',-100),
           (12,'U','A1000',-2000),
           (13,'I','A1000',2000),
           (14,'U','A1000',-1000),
           (15,'I','C3000',10000),
           (16,'U','C3000',-4000),
           (17,'U','B2000',-5),
           (18,'U','B2000',-5),
           (19,'I','B2000',40),
           (20,'V','B2000',200),
           (21,'U','A1000',-500),
           (22,'U','B2000',-50),
           (23,'U','C3000',-1000)
23 rows affected

with cte as
 (
   select *,
      -- find the latest 'V' ID per ArtNo
      max(case when Flag = 'V' then ID end) 
      over (partition by ArtNo) as Last_V_ID
   from [dbo].[Warehouse]
 )
select *,
   -- cumulative sum, but ignore all rows before the latest 'V' ID
   -- includes rows when there's no 'V' ID for this ArtNo
   COALESCE(sum(case when ID < Last_V_ID then null else Amount end)
   over (partition by ArtNo
         order by ID
         rows unbounded preceding)
   -- calculate in-stock based on last 'V' ID, discarding all previous 'V' rows
  ,sum(case when (ID < Last_V_ID and Flag <> 'V')  then -Amount 
            when ID = Last_V_ID then Amount 
       end)
   over (partition by ArtNo
         order by ID 
         rows between 1 following and unbounded following),sum(case when ID < Last_V_ID then null else Amount end)
   over (partition by ArtNo
         order by ID
         rows unbounded preceding)) AS Cummulative_Amount
from cte
order by ArtNo, ID


ID
Flag
ArtNo
Amount
Last_V_ID
Cummulative_Amount


1
U
A1000
-100
9
2650

7
U
A1000
-50
9
2600

8
I
A1000
1000
9
3600

9
V
A1000
3600
9
3600

10
U
A1000
-500
9
3100

11
U
A1000
-100
9
3000

12
U
A1000
-2000
9
1000

13
I
A1000
2000
9
3000

14
U
A1000
-1000
9
2000

21
U
A1000
-500
9
1500

2
U
B2000
-5
20
150

3
V
B2000
900
20
150

4
U
B2000
-10
20
140

5
I
B2000
50
20
190

6
U
B2000
-20
20
170

17
U
B2000
-5
20
165

18
U
B2000
-5
20
160

19
I
B2000
40
20
200

20
V
B2000
200
20
200

22
U
B2000
-50
20
150

15
I
C3000
10000
null
10000

16
U
C3000
-4000
null
6000

23
U
C3000
-1000
null
5000


Warning: Null value is eliminated by an aggregate or other SET operation.

ID	Flag	ArtNo	Amount	Last_V_ID	Cummulative_Amount
1	U	A1000	-100	9	2650
7	U	A1000	-50	9	2600
8	I	A1000	1000	9	3600
9	V	A1000	3600	9	3600
10	U	A1000	-500	9	3100
11	U	A1000	-100	9	3000
12	U	A1000	-2000	9	1000
13	I	A1000	2000	9	3000
14	U	A1000	-1000	9	2000
21	U	A1000	-500	9	1500
2	U	B2000	-5	20	150
3	V	B2000	900	20	150
4	U	B2000	-10	20	140
5	I	B2000	50	20	190
6	U	B2000	-20	20	170
17	U	B2000	-5	20	165
18	U	B2000	-5	20	160
19	I	B2000	40	20	200
20	V	B2000	200	20	200
22	U	B2000	-50	20	150
15	I	C3000	10000	null	10000
16	U	C3000	-4000	null	6000
23	U	C3000	-1000	null	5000