example query for postgresql

Is there a place that gives examples of all the kind of queries
postgresql support?

Sometimes I think if there is a way to obtain a result in just one
statement without writing a function and it would be nice to have a
panoramic of all the strategies I could try for single statements.

The SQL reference
http://www.postgresql.org/docs/8.1/interactive/sql-commands.html
isn't inspirational every time

Even if it is not exactly what I'm looking for a sort of "one liner"
as the one you find for perl, sed, awk...

As an example of things the kind of things I'm looking for:

update system
set weight=subselect._avg
from (
select round(avg(weight)) as _avg
from system where type='module'
) as subselect
where name='users_commerce';

thx

--
Ivan Sergio Borgonovo
http://www.webthatworks.it