running total starting from a date column - sql

Question

I'm trying to get a running total as of a date. This is the data I have
Date
transaction Amount
End of Week Balance
jan 1
5
100
jan 2
3
100
jan 3
4
100
jan 4
3
100
jan 5
1
100
jan 6
3
100
I would like to find out what the daily end balance is. My thought is to get a running total from each day to the end of the week and subtract it from the end of week balance, like below
Date
transaction Amount
Running total
End of Week Balance
Balance - Running total
jan 1
5
19
100
86
jan 2
3
14
100
89
jan 3
4
11
100
93
jan 4
3
7
100
96
jan 5
1
4
100
97
jan 6
3
3
100
100
I can use
SUM(transactionAmount) OVER (Order by Date)
to get a running total, is there a way to specify that I only want the total of transactions that have taken place after the date?

Answer 1

You can use sum() as a window function, but accumulate in reverse:
select t.*,
(end_of_week_balance -
sum(transactionAmount) over (order by date desc)
)
from t;

Answer 2

If you have this example:
1> select i, sum(i) over (order by i) S from integers where i<10;
2> go
i S
----------- -----------
1 1
2 3
3 6
4 10
5 15
6 21
7 28
8 36
9 45
you can also do:
1> select i, sum(case when i>3 then i else 0 end) over (order by i) S from integers where i<10;
2> go
i S
----------- -----------
1 0
2 0
3 0
4 4
5 9
6 15
7 22
8 30
9 39