Select every first/last record of a partition?

Hi,

suppose a table that has records with some ID and a timestamp.

id,    ts
3,    2012/01/03
5,    2012/01/05
7,    2012/01/07
3,    2012/02/03
3,    2012/01/05
5,    2012/03/01
7,    2012/04/04

to fetch every last row of those IDs I do:

select   id, ts
from   (   select   id, ts, row_number() over ( partition by id   order 
by ts desc ) as nr from mytab ) as x
where nr = 1


Is there a another way without a subselect?
There might be more columns so the window-functions first/last won't help.