Sub-SELECT uses un-GROUPed attribute: how to solve.

Hi all!

I want to make a report of (let's say) object sales, and i'm getting the
error: Sub-SELECT uses un-GROUPed.

I think this will be better understand with an example:
(this is a over-simplified one, but it gets the point).

(See data model and data to fill it below.)

If i have:

SALES:
-------------------
Object | Datetime
-------------------
chair | 2001-01-15
chair | 2001-01-16
table | 2001-01-17
chair | 2001-02-15
table | 2001-02-16

I want to get something as follows:

Object | Month | Sales | Total sales on that month
------------------------------------------

chair | 01 | 2 | 3
table | 01 | 1 | 3

chair | 02 | 1 | 2
table | 02 | 1 | 2

well, if i do:

select
object, to_char(sale_date, 'MM') as Month, count(*) as Sales
from sales
group by to_char(sale_date, 'MM'), object;

object | month | sales
--------+-------+-------
chair | 01 | 2
table | 01 | 1
chair | 02 | 1
table | 02 | 1

It's Ok.

If i do:

select
object, to_char(sale_date, 'MM') as Month, count(*) as Sales,
(select count(*)
from sales s2
where to_char(s2.sale_date, 'MM') = to_char(s1.sale_date, 'MM')
) as Total_Sales
from sales s1
group by to_char(sale_date, 'MM'), object;

ERROR: Sub-SELECT uses un-GROUPed attribute s1.sale_date from outer query

I get that error. I can understand it, since in the subselect i'm using
s1.sale_date that is not grouped yet.

My question is:
is there a way to get that result?

I've though of using a function, so it woud be:
select
object, to_char(sale_date, 'MM') as Month, count(*) as Sales,
total_for_month(sale_date, 'MM') as Total_Sales
from sales s1
group by to_char(sale_date, 'MM'), object;

But i'd like to do it without using functions because i would have to
define a function for each different period i want to show results.
(for example, to get year or quarter totals instead of months).

I think this is a common query to report sales. Does anybody found a
beautiful solution?

Thanks for your attention.

-- Gaizka Villate

-- Data model for the example:

create table sales (
object varchar(30),
sale_date date
);

copy sales from stdin using delimiters '|';
chair|2001-01-15
chair|2001-01-16
table|2001-01-17
chair|2001-02-15
table|2001-02-16
\.
;