with MyTable as (
select Year (bDate) as CurrentYear, month (BDAte)as CurrentMonth, sum (Cost) as CurrentSum
from Bills
left join BillsContent
on
Bills.bID=
BillsContent.bIDgroup by Year (bDate), month (BDAte)
)
select t1.CurrentYear,t1.CurrentMonth,t1.CurrentSum, t2.CurrentYear as PrevYear,
t2.CurrentMonth as PrevMonth, t2.CurrentSum as PrevSum
From MyTable T1
left join MyTable T2
on t1.CurrentYear =T2.CurrentYear-1
and
t1.CurrentMonth = t2.CurrentMonth
order by t1.CurrentYear,t1.CurrentMonth