mysql - querying unique latest post by topic -

i'm trying create active topics section, i'm running query problem.

i have 3 tables: posts, topics, categories (smallest greatest). every post associated topic, every topic associated category.

now, problem displaying recent topic posted uniquely.

here's quick scenario: user x posts in topic a, , in topic b. user y creates topic c , posts in topic a. before user y posted on a, user z posted on topic a.

on active topics list i'd show following:

• topic - y's msg
• topic c - y's msg
• topic b - x's msg

my current sql merely grabs sequence correctly, get's same topics well:

• topic - y's msg
• topic - z's msg
• topic c - y's msg
• topic b - x's msg

• topic - x's msg

  select topic_subject, post_date, post_msg, post_by, cat_color topics, posts, categories topic_id = post_topic , topic_cat = cat_id order post_date desc

if add group topic_subject or id, original topic creators, such following:

• topic c - y's msg
• topic b - x's msg
• topic - x's msg

do need subquery this? thanks!

you can add not exists subquery :

select topic_subject, post_date, post_msg, post_by, cat_color  topics, posts p, categories  topic_id = post_topic , topic_cat = cat_id , not exists  (   select 1    posts    post_topic = p.post_topic , post_date > p.post_date )