How To Simplify My Endless Query. Thanks.

Hello group!

I am having a problem with simplying my query...

I would like to get customers' balance info based on how many months
since they opened their accounts. The tricky part here is accounts
starting with '28' are treated differently than other accounts, they
are given 3 months grace period. In other words, for all other
accounts, their month0 balance is the balance of their open_month, and
month1 balance is the balance after the account is opened 1 month, and
so on. But accounts starting with '28' month0 balance would be the
balance after the account is opened 3 months, and month1 balance would
be the balance after the account is opened 4 months, and so on.

My query below works, but since some customers are more than 10 years
old (more than 120 months), my query is endless! Does anyone know a
better way to do the same job? Many thanks!

create table a
(person_id int
,account int
,open_date datetime)

insert into a values(1,200001,'11/15/2004')
insert into a values(2,280001,'8/20/2004')

create table b
(account int
,balance_date datetime
,balance money)

insert into b values(200001,'11/30/2004',700)
insert into b values(200001,'12/31/2004',800)
insert into b values(200001,'1/31/2005',900)
insert into b values(200001,'2/28/2005',1000)
insert into b values(280001,'8/30/2004',7000)
insert into b values(280001,'9/30/2004',8000)
insert into b values(280001,'10/31/2004',9000)
insert into b values(280001,'11/30/2004',10000)
insert into b values(280001,'12/31/2004',15000)
insert into b values(280001,'1/31/2005',20000)
insert into b values(280001,'2/28/2005',30000)

--Ideal output--

person_idacc_nomonth0_balancemonth1_balancemonth2_balancemonth3_balance
120000017008009001000
2280000110000150002000030000

select a.person_id
,a.account
,month0_balance=case
when a.account like '2%' and a.account not like '28%'
then
sum(case datediff(mm, a.open_date, balance_date) when 0
then b.balance else 0 end)
else sum(case datediff(mm, a.open_date, balance_date)
when 3 then b.balance else 0 end)
end
,month1_balance =case
when a.account like '2%' and a.account not like '28%'
then
sum(case datediff(mm, a.open_date, balance_date) when 1
then b.balance else 0 end)
else sum(case datediff(mm, a.open_date, balance_date)
when 4 then b.balance else 0 end)
end
from a as a
join b as b
on a.account=b.account
group by a.person_id, a.account