A calendar table is not really needed for this:

select

YearMonth = dateadd(mm,datediff(mm,0,[recorddate]),0),

MyValueSum = sum([myvalue])

from

#a

group by

dateadd(mm,datediff(mm,0,[recorddate]),0)

order by

dateadd(mm,datediff(mm,0,[recorddate]),0)

Results:

YearMonth MyValueSum

----------------------- -----------

2012-01-01 00:00:00.000 30

2012-02-01 00:00:00.000 70

2012-08-01 00:00:00.000 50

2012-12-01 00:00:00.000 130