Reset cumulative sum column after threshold with groups - sql

Question

I need to calculate a cumulative sum (group based, column GroupNr) that resets after exceeding some number, in this example - 330.
Can this be done using a function or CTE? If so, how?
Current Table
GroupNr Name Sum Cumsum
1 Mary 0.00 0.00
1 Jane 179.00 179.00
1 Tom 106.00 285.00
1 Joseph 175.00 460.00
1 Arthur 253.00 713.00
2 Mary 0.00 0.00
2 Jane 365.00 365.00
2 Tom 365.00 730.00
2 Joseph 365.00 1095.00
2 Arthur 365.00 1460.00
Expected Table
GroupNr Name Sum Cumsum Resetcumsum
1 Mary 0.00 0.00 0.00
1 Jane 179.00 179.00 179.00
1 Tom 106.00 285.00 285.00
1 Joseph 175.00 460.00 460.00 -- Reset point
1 Arthur 253.00 713.00 253.00
2 Mary 0.00 0.00 0.00
2 Jane 365.00 365.00 365.00
2 Tom 365.00 730.00 365.00
2 Joseph 365.00 1095.00 365.00
2 Arthur 365.00 1460.00 365.00
Code for tables
CREATE TABLE Table1 (
GroupNr int,
Name varchar(7),
Sum numeric(14, 2),
Cumsum numeric(14, 2)
)
INSERT INTO Table1 (GroupNr, Name, Sum, Cumsum)
VALUES (1, 'Mary', 0, 0);
INSERT INTO Table1 (GroupNr, Name, Sum, Cumsum)
VALUES (1, 'Jane', 179, 179);
INSERT INTO Table1 (GroupNr, Name, Sum, Cumsum)
VALUES (1, 'Tom', 106, 285);
INSERT INTO Table1 (GroupNr, Name, Sum, Cumsum)
VALUES (1, 'Joseph', 175, 460);
INSERT INTO Table1 (GroupNr, Name, Sum, Cumsum)
VALUES (1, 'Arthur', 253, 713);
INSERT INTO Table1 (GroupNr, Name, Sum, Cumsum)
VALUES (2, 'Mary', 0, 0);
INSERT INTO Table1 (GroupNr, Name, Sum, Cumsum)
VALUES (2, 'Jane', 365, 365);
INSERT INTO Table1 (GroupNr, Name, Sum, Cumsum)
VALUES (2, 'Tom', 365, 730);
INSERT INTO Table1 (GroupNr, Name, Sum, Cumsum)
VALUES (2, 'Joseph', 365, 1095);
INSERT INTO Table1 (GroupNr, Name, Sum, Cumsum)
VALUES (2, 'Arthur', 365, 1460);

Answer 1

Capping a cumulative SUM by using standard SUM() OVER() is not possible due to threshold. One way to achieve such result is recursive CTE:
WITH cte_r AS (
SELECT t.*, ROW_NUMBER() OVER(PARTITION BY GroupNr ORDER BY (SELECT 1)) AS rn
FROM Table1 t
), cte AS (
SELECT GroupNr, Name, [Sum], [CumSum],
CAST([Sum] AS INT) AS ResetCumSum,
rn
FROM cte_r
WHERE rn = 1
UNION ALL
SELECT cte_r.GroupNr, cte_r.Name, cte_r.[Sum], cte_r.[CumSum],
CAST(CASE WHEN cte.ResetCumSum >= 330 THEN 0 ELSE cte.ResetCumSum END + cte_r.[Sum] AS INT)
AS ResetCumSum,
cte_r.rn
FROM cte
JOIN cte_r
ON cte.rn = cte_r.rn-1
AND cte.GroupNr = cte_r.GroupNr
)
SELECT GroupNr, Name, [Sum], [CumSum], ResetCumSum
FROM cte
ORDER BY GroupNr, rn;
Output:
db<>fiddle demo
Warning: Table by design is unordered set so to get stable result a order column is required(like unqiue id, timestamp). Here to emulate insert ROW_NUMBER() OVER(PARTITION BY GroupNr ORDER BY (SELECT 1)) AS rn was used but it is not stable.
Related:
Conditional SUM and the same using MATCH_RECOGNIZE - in my opinion the cleanest way
Extra:
Quirky UPDATE: Running Total until specific condition is true
Disclaimer: "DO NOT USE IT AT PRODUCTION!!!"
-- source table to be extended with id and Resetcumsum columns
CREATE CLUSTERED INDEX IX_ROW_NUM ON Table1(GroupNr, id);
DECLARE #running_total NUMERIC(14,2) = 0
,#prev_running_total NUMERIC(14,2) = 0
,#prev_GroupNr INT = 0;
UPDATE Table1
SET
#prev_running_total = #running_total
,#running_total = Resetcumsum = IIF(#prev_GroupNr != GroupNr
OR #running_total >= 330, 0, #running_total)
+ [Sum]
,#prev_GroupNr = GroupNr
FROM Table1 WITH(INDEX(IX_ROW_NUM))
OPTION (MAXDOP 1);
SELECT *
FROM Table1
ORDER BY id;
db<>fiddle demo - 2