Getting pk of the most recent row, in a group by

I've got a table of "coupons" which have an expiration date. For each
type of coupon, I'd like to get the primary key of the coupon which will
expire first.

# create table coupon
(
coupon_id serial primary key,
type varchar(255),
expires date
);
insert into coupon values(DEFAULT,'free','2007-01-01');
insert into coupon values(DEFAULT,'free','2007-01-01');
insert into coupon values(DEFAULT,'free','2007-06-01');
insert into coupon values(DEFAULT,'free','2007-06-01');
insert into coupon values(DEFAULT,'50%','2008-06-01');
insert into coupon values(DEFAULT,'50%','2008-06-02');
insert into coupon values(DEFAULT,'50%','2008-06-03');

The desired query would look like:

# select coupon_id,type,expires from coupon where type='free' order by
expires limit 1;
coupon_id | type | expires
-----------+------+------------
1 | free | 2007-01-01

But be grouped by type:

# select type,min(expires),count(*) from coupon group by type;
type | min | count
------+------------+-------
free | 2007-01-01 | 4 ; pk=1
50% | 2008-06-01 | 3 ; pk=5

In the second example, is it possible to get the primary key of the row
with the minimum expires time?

--
----
Visit http://www.obviously.com/