Help Needed in Date Logic

Question

Hi I am working in sqlserver 2008 R2 and below is my sample research query
i am trying to get previous 6 months data.
WITH CutomMonths
AS (
SELECT UPPER(convert(VARCHAR(3), datename(month, DATEADD(MM, DATEDIFF(MM, 0, GETDATE()) - N, 0)))) Month
,DATEADD(MM, DATEDIFF(MM, 0, GETDATE()) - N, 0) startdate
,DATEADD(MM, DATEDIFF(MM, 0, GETDATE()) - N + 1, 0) enddate
FROM (
VALUES (1)
,(2)
,(3)
,(4)
,(5)
,(6)
) x(N)
WHERE N <= 6
SELECT month
,SUM(isnull(perks.amount,0)) AS PerkAmount
FROM CutomMonths
LEFT JOIN (
select 10.00 as amount,'2014-04-03' as StartDate,'2015-04-03' as EndDate union all
select 10.00 as amount,'2014-04-03' as StartDate,'2015-04-03' as EndDate
) perks ON
CutomMonths.startdate >= perks.StartDate
AND CutomMonths.startdate < perks.EndDate
GROUP BY CutomMonths.Month
,CutomMonths.startdate
ORDER BY CutomMonths.startdate ASC
current output what i am getting:
Expected Output:
I found why the April month i din't get the $20 because the startdate of my perks CTE '2014-04-03'. If it is '2014-04-01' then i will get the expected output.
But i should not change the the date on perks. How to neglect this date issue and consider the month instead to get the expected output. please help me on this as i am struggling on my logic 
loving dotnet

Answer 1

I'm just going to focus on the JOIN criteria in this reply since my answer above and your 2nd response are essentially the same except for the JOIN.
What your are describing and what you are doing in code is conflicting. 
You are saying this:
"I just need to check whether the perks start date falls in between month start date and month end date"
..., which translated directly to code would be this:
 ON perks.StartDate >= CustomMonths.StartDate
AND perks.StartDate <= CustomMonths.EndDate
What I believe you are getting after is this:
"I just need to check whether the dates within the perks start and end date range fall in between month start date and month end date"
..., which translated directly to code would be this, which is also my answer proposed above:
ON CustomMonths.StartDate >= DATEADD(DAY, -(DAY(perks.StartDate) - 1), perks.StartDate)
AND CustomMonths.StartDate <= perks.EndDate
However, if you really want to use the code solution you proposed, then you would actually be saying this:
"I just need to check whether the dates within the perks start and end date range fall in between month start date and month end date, but if perk end date happens to be the first day of the month, then ignore it and use the previous day."
..., in which case then your code, as follows, would be the solution:
 ON CutomMonths.startdate >= DATEADD(mm, DATEDIFF(mm, 0, perks.StartDate), 0)AND CutomMonths.startdate < DATE
ADD(mm, DATEDIFF(mm, 0, perks.EndDate), 0)
NOTE: The alternate JOIN I had commented out in my proposed answer above will do the exact same thing as your latest proposed solution
 ON CustomMonths.StartDate >= DATEADD(DAY, -(DAY(perks.StartDate) - 1), perks.StartDate)
AND CustomMonths.StartDate < perks.EndDate
BTW, I see how you are getting to the first day of the month by subtracting all the months from the given date to 01/01/1901, and then turning around and adding them all back to 01/01/1901.  While that is one way to get to the first day of the month, it
seems excessive from a calculation stand point, although I haven't performed any performance tests to know for certain.
SELECT DATEADD(mm, DATEDIFF(mm, 0, '2014-04-03'), 0)
I prefer simply subtracting one less than the current day number from the given date to get to the same first day of the month value.
SELECT DATEADD(DAY, -(DAY('2014-04-03') - 1), '2014-04-03')