SQL sum by month with the previous values - sql

Question

I have following data:
cohort activity counter
-----------------------------
2010-12 0 470
2010-12 1 2
2010-12 2 1
2010-12 3 1
2010-12 6 1
2011-01 0 550
2011-01 1 1
2011-01 6 1
I want to sum counter of different activities by month, so the final table looks like:
cohort activity counter sumResult
-------------------------------------------
2010-12 0 470 470
2010-12 1 2 472
2010-12 2 1 473
2010-12 3 1 474
2010-12 6 1 475
2011-01 0 550 550
2011-01 1 1 551
2011-01 6 1 552
I've tried to do it like this:
select
a.activity, a.counter, a.cohort,
(
select sum(b.counter)
from data_table as b
where b.cohort = a.cohort and b.counter >= a.counter
) as sumResult
from data_table as a;
GO;
but it gave me strange results as:
cohort activity counter sumResult
-------------------------------------------
2010-12 0 470 470
2010-12 1 2 472
2010-12 2 1 475
2010-12 3 1 475
2010-12 6 1 475
2011-01 0 550 550
2011-01 1 1 552
2011-01 6 1 552
What could be a problem?

Answer 1

Depends on your RDBMS , some(SQL Server,Oracle,Postgresql) of them will accept SUM() OVER() :
SELECT t.*,
SUM(t.counter) OVER(PARTITION BY t.cohort ORDER BY t.activity) as sumResult
FROM YourTable t
If it's another, that's a bit more complicated and can be dealt with JOINS

Answer 2

The normal way to do this uses the ANSI standard cumulative sum function:
select dt.*,
sum(dt.counter) over (partition by dt.cohort order by dt.counter desc)
from data_table dt
order by cohort, counter desc;
If you want to use a subquery, the you need a stable sort, and activity can give you one. You can use this in the cumulative sum syntax:
select dt.*,
sum(dt.counter) over (partition by dt.cohort order by dt.counter desc, dt.activity)
from data_table dt
order by cohort, counter desc, activity;
Or using a subquery:
select dt.*,
(select sum(dt2.counter)
from data_table dt2
where dt2.cohort = dt.cohort and
(dt2.counter > dt.counter or
dt2.counter = dt.counter and dt2.activity < dt.activity)
)
from data_table dt
order by cohort, counter desc, activity;