SQL Server 2017

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create table tbl (FiscalYear int, OrderDate date, OrderNumber int, SubTotal decimal(10,2), RunningTotal decimal(10,2));
insert into tbl values
(2011, '20110531', 1, 5000.02, null),
(2011, '20110531', 2, 1000.15, null),
(2011, '20110531', 3,  700.25, null),
(2011, '20110531', 4,  225.02, null),
(2011, '20110531', 5, 1258.25, null),
(2011, '20110531', 6, 1000.00, null),
(2011, '20110531', 7,  695.20, null),
(2011, '20110531', 8,  789.25, null),
(2011, '20110531', 9, 2125.02, null);
9 rows affected

;with x as 
(
    select FiscalYear,
           OrderDate,
           OrderNumber,
           SubTotal,
           sum(SubTotal) over (partition by FiscalYear 
                               order by FiscalYear, OrderDate, OrderNumber) as CumTotal
    from   tbl
)
update t
set    RunningTotal = CumTotal
from   tbl t
join   x
on     x.Fiscalyear = t.FiscalYear
and    x.OrderDate = t.OrderDate
and    x.OrderNumber = t.OrderNumber;
9 rows affected

select * from tbl;


FiscalYear
OrderDate
OrderNumber
SubTotal
RunningTotal


2011
31/05/2011 00:00:00
1
5000.02
5000.02

2011
31/05/2011 00:00:00
2
1000.15
6000.17

2011
31/05/2011 00:00:00
3
700.25
6700.42

2011
31/05/2011 00:00:00
4
225.02
6925.44

2011
31/05/2011 00:00:00
5
1258.25
8183.69

2011
31/05/2011 00:00:00
6
1000.00
9183.69

2011
31/05/2011 00:00:00
7
695.20
9878.89

2011
31/05/2011 00:00:00
8
789.25
10668.14

2011
31/05/2011 00:00:00
9
2125.02
12793.16

FiscalYear	OrderDate	OrderNumber	SubTotal	RunningTotal
2011	31/05/2011 00:00:00	1	5000.02	5000.02
2011	31/05/2011 00:00:00	2	1000.15	6000.17
2011	31/05/2011 00:00:00	3	700.25	6700.42
2011	31/05/2011 00:00:00	4	225.02	6925.44
2011	31/05/2011 00:00:00	5	1258.25	8183.69
2011	31/05/2011 00:00:00	6	1000.00	9183.69
2011	31/05/2011 00:00:00	7	695.20	9878.89
2011	31/05/2011 00:00:00	8	789.25	10668.14
2011	31/05/2011 00:00:00	9	2125.02	12793.16