How to spread annual amount and then add by month in SQL - sum

Question

Currently I'm working with a table that looks like this:
Month | Transaction | amount
2021-07-01| Annual Membership Fee| 45
2021-08-01| Annual Membership Fee| 145
2021-09-01| Annual Membership Fee| 2940
2021-10-01| Annual Membership Fee| 1545
the amount on that table is the total monthly amount (ex. I have 100 customers who paid $15 for the annual membership, so my total monthly amount would be $1500).
However what I would like to do (and I have no clue how) is divide the amount by 12 and spread it into the future in order to have a monthly revenue per month. As an example for 2021-09-01 I would get the following:
$2490/12 = $207.5 (dollars per month for the next 12 months)
in 2021-09-01 I would only get $207.5 for that specific month.
On 2021-10-01 I would get $1545/12 = $128.75 plus $207.5 from the previous month (total = $336.25 for 2021-10-01)
And the same operation would repeat onwards. The last period that I would collect my $207.5 from 2021-09-01 would be in 2022-08-01.
I was wondering if someone could give me an idea of how to perform this in a SQL query/CTE?

Answer 1

Assuming all the months you care about exist in your table, I would suggest something like:
SELECT
month,
(SELECT SUM(m2.amount/12) FROM mytable m2 WHERE m2.month BETWEEN ADD_MONTHS(m1.month, -11) AND m1.month) as monthlyamount
FROM mytable m1
GROUP BY month
ORDER BY month
For each month that exists in the table, this sums 1/12th of the current amount plus the previous 11 months (using the add_months function). I think that's what you want.
A few notes/thoughts:
I'm assuming (based on the column name) that all the dates in the month column end on the 1st, so we don't need to worry about matching days or having the group by return multiple rows for the same month.
You might want to round the SUMs I did, since in some cases dividing by 12 might give you more digits after the decimal than you want for money (although, in that case, you might also have to consider remainders).
If you really only have one transaction per month (like in your example), you don't need to do the group by.
If the months you care about don't exist in your table, then this won't work, but you could do the same thing generating a table of months. e.g. If you have an amount on 2020-01-01 but nothing in 2020-02-01, then this won't return a row for 2021-02-01.

Answer 2

CTE = set up dataset
CTE_2 = pro-rate dataset
FINAL SQL = select future_cal_month,sum(pro_rated_amount) from cte_2 group by 1
with cte as (
select '2021-07-01' cal_month,'Annual Membership Fee' transaction ,45 amount
union all select '2021-08-01' cal_month,'Annual Membership Fee' transaction ,145 amount
union all select '2021-09-01' cal_month,'Annual Membership Fee' transaction ,2940 amount
union all select '2021-10-01' cal_month,'Annual Membership Fee' transaction ,1545 amount)
, cte_2 as (
select
dateadd('month', row_number() over (partition by cal_month order by 1), cal_month) future_cal_month
,amount/12 pro_rated_amount
from
cte
,table(generator(rowcount => 12)) v)
select
future_cal_month
, sum(pro_rated_amount)
from
cte_2
group by
future_cal_month