last and/or first in a by group

Dear postgresql experts,

I want to know if postgresql has facilities for getting the first and or the last in a by group.

Suppose I have the following table:

resnr,dep,arr,cls,dbd meaning reservationsnumber, departure station, arrival station, the class of the reservation and the daysbeforedeparture and records like:
xxx,NYC,BRA,C,80
xxx,NYC,BRA,M,75
xxx,NYC,BRA,Q,50
yyy,WAS,LIS,T,55
zzz,NYC,LIS,Z,40
zzz,NYC,LIS,J,39

I want to select only the most recent records being:
xxx,NYC,BRA,Q,50
yyy,WAS,LIS,T,55
zzz,NYC,LIS,J,39

How would you accomplish this?

I googled and found this:
http://www.postgresonline.com/journal/index.php?/archives/68-More-Aggregate-Fun-Whos-on-First-and-Whos-on-Last.html

I hope there are alternatives because I don't have to program this myself. The other option is that I load this data in SAS and do it there but I only have SAS at work and want to do this at home.

Brgds
Dino