sql query with 3 tables

[pan ac]

I have 3 tables and i want to display
the combination of some fields of the 3 tables into the result.

p table fields (Posts table for a forum):
author_id
topic_id
forum_id
post_date

t table fields (Topic Table for a forum):
title
views
posts

f table fields (main Forum table):
id
name

My SQl statement:

SELECT p.author_id, p.topic_id, p.forum_id, p.post_date, t.title, t.views, t.posts, f.name
FROM p, f, t
WHERE f.id = t.forum_id AND p.topic_id = t.tid
ORDER BY p.post_date DESC
Limit 0, 30

the results give the last 30 postings in descending order and displayed on the website.

Problem:
i only want unique topic_id to be displayed within the latest 30 posts, now if more than one person discuss the same topic within the 30 latest post, both posts will be listed.

p.author_id p.topic_id p.forum_id p.post_date t.title t.views
323 123 general 05/08/04 mysql 45
345 123 general 04/08/04 mysql 45
346 111 advanced 04/08/04 sybase 119

In the above table, i want the result to display only first and 3rd line.
How to resolve the above problem?

Thanks.
pac