Problem with aggregate functions and GROUP BY

I'm having real trouble with aggregate functions. I have a table which
tracks the value of a field in another table over time - it contains a
foreign key to the row in the other table, the value, and a timestamp to
sort by. I'm trying to write a query that returns the rows containing the
most recent values for each foreign key.

The table looks like this: (fixed width fonts required, and this is a test
table):

# SELECT * FROM test ORDER BY fk, sortby;
fk | value | sortby
----+-------+--------
1 | 2 | 1
1 | 0 | 2
1 | 4 | 3
2 | 2 | 1

Now obviously, what I'm aiming for is:

fk | value | sortby
----+-------+--------
1 | 4 | 3
2 | 2 | 1

which is the highest value of 'sortby' for each value of 'fk'.

I would think that the SQL to achieve this is:

SELECT * FROM test GROUP BY fk HAVING sortby = MAX(sortby);

but running this in psql gives:

ERROR: Attribute test.value must be GROUPed or used in an aggregate
function

No matter what I do to the query, it gives me this message for test.value
and test.sortby, unless I GROUP BY all three of them, which doesn't give me
the result I want. I'm completely stumped - can anyone help?

Thanks in advance,

Alex
--
Alex Page, IT Department, Solid State Logic
E-Mail: alex(dot)page(at)solid-state-logic(dot)com
Phone: +44 (0) 1865 842 300
Web: http://www.solid-state-logic.com

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