SQL monthly rolling sum - sql

Question

I am trying to calculate monthly balances of bank accounts from the following postgresql table, containing transactions:
# \d transactions
View "public.transactions"
Column | Type | Collation | Nullable | Default
--------+------------------+-----------+----------+---------
year | double precision | | |
month | double precision | | |
bank | text | | |
amount | numeric | | |
In "rolling sum" I mean that the sum should contain the sum of all transactions until the end of the given month from the beginning of time, not just all transactions in thegiven month.
I came up with the following query:
select
a.year, a.month, a.bank,
(select sum(b.amount) from transactions b
where b.year < a.year
or (b.year = a.year and b.month <= a.month))
from
transactions a
order by
bank, year, month;
The problem is that this contains as many rows for each of the months for each banks as many transactions were there. If more, then more, if none, then none.
I would like a query which contains exactly one row for each bank and month for the whole time interval including the first and last transaction.
How to do that?
An example dataset and a query can be found at https://rextester.com/WJP53830 , courtesy of #a_horse_with_no_name

Answer 1

You need to generate a list of months first, then you can outer join your transactions table to that list.
with all_years as (
select y.year, m.month, b.bank
from generate_series(2010, 2019) as y(year) --<< adjust here for your desired range of years
cross join generate_series(1,12) as m(month)
cross join (select distinct bank from transactions) as b(bank)
)
select ay.*, sum(amount) over (partition by ay.bank order by ay.year, ay.month)
from all_years ay
left join transactions t on (ay.year, ay.month, ay.bank) = (t.year::int, t.month::int, t.bank)
order by bank, year, month;
The cross join with all banks is necessary so that the all_years CTE will also contain a bank for each month row.
Online example: https://rextester.com/ZZBVM16426

Answer 2

Here is my suggestion in Oracle 10 SQL:
select a.year,a.month,a.bank, (select sum(b.amount) from
(select a.year as year,a.month as month,a.bank as bank,
sum(a.amount) as amount from transactions c
group by a.year,a.month,a.bank
) b
where b.year<a.year or (b.year=a.year and b.month<=a.month))
from transactions a order by bank, year, month;

Answer 3

Consider aggregating all transactions first by bank and month, then run a window SUM() OVER() for rolling monthly sum since earliest amount.
WITH agg AS (
SELECT t.year, t.month, t.bank, SUM(t.amount) AS Sum_Amount
FROM transactions t
GROUP BY t.year, t.month, t.bank
)
SELECT agg.year, agg.month, agg.bank,
SUM(agg.Sum_Amount) OVER (PARTITION BY agg.bank ORDER BY agg.year, agg.month) AS rolling_sum
FROM agg
ORDER BY agg.year, agg.month, agg.bank
Should you want YTD rolling sums, adjust the OVER() clause by adding year to partition:
SUM(agg.Sum_Amount) OVER (PARTITION BY agg.bank, agg.year ORDER BY agg.month)