SQL Server Cumulative Sum breaks - sql-server

Question

I have one Amount field and I want to build the following table (CumulativeSum and GroupNo columns):
| Id | Amount | CumulativeSum | GroupNo |
| 1 | 1000 | 1000 | 0 |
| 2 | 2000 | 3000 | 0 |
| 3 | 1000 | 4000 | 0 |
| 4 | 3000 | 3000 | 1 |
| 5 | 2000 | 5000 | 1 |
| 6 | 3000 | 3000 | 2 |
| 7 | 1000 | 4000 | 2 |
| 8 | 4000 | 4000 | 3 |
| 9 | 2000 | 2000 | 4 |
Note: when ever the CumulativeSum becomes grater than 5000, I need it to start summing from 0 with a new Group Number (GroupNo).
For Example the sum of rows 1, 2 and 3 equals 4000. The amount of row 4 is 3000 and 4000 + 3000 = 7000 and it is grater than 5000 so it must break and start from 0 again.

Answer 1

The easiest way to do this is with a recursive CTE. These are effectively loops, where the next value is calculated based on previous values. Here's a link to a basic example and writeup of how they work: https://www.sqlservertutorial.net/sql-server-basics/sql-server-recursive-cte/
In this case, it checks if the previous value + the current value is over 5000; if so, it resets the counter and adds 1 to the GroupNo; otherwise it just adds the current value to CumulativeSum.
CREATE TABLE #Amts (ID int PRIMARY KEY, Amount int);
INSERT INTO #Amts (ID, Amount)
VALUES
(1, 1000), -- | 1000 | 0 |
(2, 2000), -- | 3000 | 0 |
(3, 1000), -- | 4000 | 0 |
(4, 3000), -- | 3000 | 1 |
(5, 2000), -- | 5000 | 1 |
(6, 3000), -- | 3000 | 2 |
(7, 1000), -- | 4000 | 2 |
(8, 4000), -- | 4000 | 3 |
(9, 2000); -- | 2000 | 4 |
WITH RunningTotals AS
(SELECT A.ID, A.Amount, A.Amount AS CumulativeSum, 0 AS GroupNo
FROM #Amts A
WHERE ID = 1
UNION ALL
SELECT RT.ID + 1,
A.Amount,
CASE WHEN RT.CumulativeSum + A.Amount > 5000 THEN A.Amount
ELSE RT.CumulativeSum + A.Amount END,
CASE WHEN RT.CumulativeSum + A.Amount > 5000 THEN RT.GroupNo + 1
ELSE RT.GroupNo END
FROM RunningTotals RT
INNER JOIN #Amts A ON RT.ID + 1 = A.ID
)
SELECT *
FROM RunningTotals
OPTION (MAXRECURSION 1000);
Results are as follows
ID Amount CumulativeSum GroupNo
1 1000 1000 0
2 2000 3000 0
3 1000 4000 0
4 3000 3000 1
5 2000 5000 1
6 3000 3000 2
7 1000 4000 2
8 4000 4000 3
9 2000 2000 4
Notes
This currently requires the IDs to be consecutive and starting with 1. If not, you may need to make an initial CTE with ROW_NUMBER() OVER (ORDER BY Id) to get this
I've put MAXRECURSION 1000 on there - that means it will do this looping 1000 times. If you have a larger table than this, you'll need to increase that number.
Update: fixed bug in code ( RT.GroupNo + 1 was in wrong spot). Also added 'Amount' column to output.