select the last post

hi there,

hi have a table called forum, and i've got this query:
this query select the messages and the number of replies to the
message individualy.
i'm trying to select the last post of each set of replies, like we see
in the foruns.
does anyone can help me
best regards,
etur

SELECT id, subject, to_char(post_time, 'YYYY.MM.DD @
HH24:MI:SS'::text) AS post_time,
content, login, messages.parent_id, replies.answered
FROM ( SELECT f.id, f.subject, f.post_time, f.content, u.login, f.parent_id
FROM forum f, system_users u
WHERE f.parent_id IS NULL AND f.user_id = u.id) messages
LEFT JOIN ( SELECT forum.parent_id, count(forum.id) AS answered
FROM forum
GROUP BY forum.parent_id) replies
ON messages.id = replies.parent_id;