Using Sum() with multiple where clauses - sql

Question

I'm pretty new to this, so forgive if this has been posted (I had no idea what to even search on).
I have 2 tables, Accounts and Usage
AccountID AccountStartDate AccountEndDate
-------------------------------------------
1 12/1/2012 12/1/2013
2 1/1/2013 1/1/2014
UsageId AccountID EstimatedUsage StartDate EndDate
------------------------------------------------------
1 1 10 1/1 1/31
2 1 11 2/1 2/29
3 1 23 3/1 3/31
4 1 23 4/1 4/30
5 1 15 5/1 5/31
6 1 20 6/1 6/30
7 1 15 7/1 7/31
8 1 12 8/1 8/31
9 1 14 9/1 9/30
10 1 21 10/1 10/31
11 1 27 11/1 11/30
12 1 34 12/1 12/31
13 2 13 1/1 1/31
14 2 13 2/1 2/29
15 2 28 3/1 3/31
16 2 29 4/1 4/30
17 2 31 5/1 5/31
18 2 26 6/1 6/30
19 2 43 7/1 7/31
20 2 32 8/1 8/31
21 2 18 9/1 9/30
22 2 20 10/1 10/31
23 2 47 11/1 11/30
24 2 33 12/1 12/31
I'd like to write one query that gives me estimated usage for each month (starting now until the last month that we serve an account) for all accounts being served during that month.
The results would be as follows:
Month-Year Total Est Usage
------------------------------
Oct-12 0 (none being served)
Nov-12 0 (none being served)
Dec-12 34 (only accountid 1 being served)
Jan-13 23 (accountid 1 & 2 being served)
Feb-13 24 (accountid 1 & 2 being served)
Mar-13 51 (accountid 1 & 2 being served)
...
Dec-13 33 (only accountid 2 being served)
Jan-14 0 (none being served)
Feb-14 0 (none being served)
I'm assuming I need to sum and then do a Group By...but not really sure logically how I'd lay this out.

Answer 1

Revised Answer:
I've created a Months table with columns MonthID, Month with values like (201212, 12), (201301, 1), ...
I've also reorganised the usage table to have a month column rather than the start date and end date, as it makes the idea clearer.
See http://sqlfiddle.com/#!3/f57d84/6 for details
The query is now:
Select
m.MonthID,
Sum(u.EstimatedUsage) TotalEstimatedUsage
From
Accounts a
Inner Join
Usage u
On a.AccountID = u.AccountID
Inner Join
Months m
On m.MonthID Between
Year(a.AccountStartDate) * 100 + Month(a.AccountStartDate) And
Year(a.AccountEndDate) * 100 + Month(a.AccountEndDate) And
m.Month = u.Month
Group By
m.MonthID
Order By
1
Previous answer, for reference which assumed usages ranges were full dates rather than just months.
Select
Year(u.StartDate),
Month(u.StartDate),
Sum(Case When a.AccountStartDate <= u.StartDate And a.AccountEndDate >= u.EndDate Then u.EstimatedUsage Else 0 End) TotalEstimatedUsage
From
Accounts a
Inner Join
Usage u
On a.AccountID = u.AccountID
Group By
Year(u.StartDate),
Month(u.StartDate)
Order By
1, 2