Get cumulative sum that reset for each year - sql

Question

Please consider this table:
Year Month Value YearMonth
2011 1 70 201101
2011 1 100 201101
2011 2 200 201102
2011 2 50 201102
2011 3 80 201103
2011 3 250 201103
2012 1 100 201201
2012 2 200 201202
2012 3 250 201203
I want to get a cumulative sum based on each year. For the above table I want to get this result:
Year Month Sum
-----------------------
2011 1 170
2011 2 420 <--- 250 + 170
2011 3 750 <--- 330 + 250 + 170
2012 1 100
2012 2 300 <--- 200 + 100
2012 3 550 <--- 250 + 200 + 100
I wrote this code:
Select c1.YearMonth, Sum(c2.Value) CumulativeSumValue
From #Tbl c1, #Tbl c2
Where c1.YearMonth >= c2.YearMonth
Group By c1.YearMonth
Order By c1.YearMonth Asc
But its CumulativeSumValue is calculated twice for each YearMonth:
YearMonth CumulativeSumValue
201101 340 <--- 170 * 2
201102 840 <--- 420 * 2
201103 1500
201201 850
201202 1050
201203 1300
How can I achieve my desired result?
I wrote this query:
select year, (Sum (aa.[Value]) Over (partition by aa.Year Order By aa.Month)) as 'Cumulative Sum'
from #Tbl aa
But it returned multiple records for 2011:
Year Cumulative Sum
2011 170
2011 170
2011 420
2011 420
2011 750
2011 750
2012 100
2012 300
2012 550

Answer 1

You are creating a cartesian product here. In your ANSI-89 implicit JOIN (you really need to stop using those and switch to ANSI-92 syntax) you are joining on c1.YearMonth >= c2.YearMonth.
For your first month you have two rows with the same value of the year and month, so each of those 2 rows joins to the other 2; this results in 4 rows:
Year
Month
Value1
Value2
2011
1
70
70
2011
1
70
100
2011
1
100
70
2011
1
100
100
When you SUM this value you get 340, not 170, as you have 70+70+100+100.
Instead of a triangular JOIN however, you should be using a windowed SUM. As you want to also get aggregate nmonths into a single rows, you'll need to also aggregate inside the windowed SUM like so:
SELECT V.YearMonth,
SUM(SUM(V.Value)) OVER (PARTITION BY Year ORDER BY V.YearMonth) AS CumulativeSum
FROM (VALUES (2011, 1, 70, 201101),
(2011, 1, 100, 201101),
(2011, 2, 200, 201102),
(2011, 2, 50, 201102),
(2011, 3, 80, 201103),
(2011, 3, 250, 201103),
(2012, 1, 100, 201201),
(2012, 2, 200, 201202),
(2012, 3, 250, 201203)) V (Year, Month, Value, YearMonth)
GROUP BY V.YearMonth,
V.Year;