SQL Server 2022

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create table RT_R78 (
  MONTH int,
  R78 int
);

insert into RT_R78 values
(1,       1),
(2,       3),
(3,       6),
(4,      10),
(5,      15),
(6,      21),
(7,      28),
(8,      36),
(9,      45),
(10,     55),
(11,     66),
(12,     78);
12 rows affected

select *, SUM(case when MONTH >= 4 then 1 else 0 end) over(order by MONTH ) as rn
from RT_R78


MONTH
R78
rn


1
1
0

2
3
0

3
6
0

4
10
1

5
15
2

6
21
3

7
28
4

8
36
5

9
45
6

10
55
7

11
66
8

12
78
9


with cte as (
  select *, SUM(case when MONTH >= 4 then 1 else 0 end) over(order by MONTH ) as rn
  from RT_R78
)
select c.month, r.R78
from cte c
inner join RT_R78 r on r.month = c.rn


month
R78


4
1

5
3

6
6

7
10

8
15

9
21

10
28

11
36

12
45