need help understanding a function

i had previously asked for help writing/improving a function that i need to calculate a premium based on differing values for each month. the premium is split in to 12 months and earned on a percentage for each month. so if the policy start in march and we are in jan we will have earned 10 months worth. so i need to add up the monthly earning to give us the total earned. wach company wil have differeing earnings values for each month.

my original code is Here. its ghastly and slow hence the request for help.

and i was presented with the following code. the code works but returns stupendously large figures.

begin

set @begin=datepart(month,@outdate)
set @end=datepart(month,@experiencedate)

;with a as
(
    select *,
    case calmonth
        when 'january' then 1
        when 'february' then 2
        when 'march' then 3
        when 'april' then 4
        when 'may' then 5
        when 'june' then 6
        when 'july' then 7
        when 'august' then 8
        when 'september' then 9
        when 'october' the开发者_如何学运维n 10
        when 'november' then 11
        when 'december' then 12
        end as Mnth
        from tblearningpatterns
        where clientname=@client
            and earningpattern=@pattern
    )
    ,
    b as
    (
        select
            earningvalue,
            Mnth,
            earningvalue as Ttl
        from a
        where Mnth=@begin

        union all
        select
            a.earningvalue,
            a.Mnth,
            cast(b.Ttl*a.earningvalue as decimal(15,3)) as Ttl
        from a
            inner join b
                on a.Mnth=b.Mnth+1
        where a.Mnth<=@end

    )

    select @earningvalue=
        Ttl
    from b
        inner join
        (
            select max(Mnth) as Mnth
            from b
        ) c
            on b.Mnth=c.Mnth
    option(maxrecursion 12)


    SET @earnedpremium =  @earningvalue*@premium 

end

can someone please help me out?

it is becuase the earnings are being multiplied to the earnings in the recursive CTE. b.Ttl*a.earningvalue

This query seems wrong though ... just from looking at it it will fall over when lets say you go from one year to the next. and the month goes from 12 to 1 as your example

i would creat a recursive CTE that basically anchors using your start date and then in the part after the union i'd add 1 $onth to the Datetime till it exceeds the end Date