Question

How do I go about displaying the most recent post when I have two tables, both containing a column called creation_date

This would be simple if all I had to do was get the most recent post based on posts created_on value however if a post contains replies I need to factor this into the equation.
If a post has a more recent reply I want to get the replies created_on value but also get the posts post_id and subject.

The posts table structure:

CREATE TABLE `posts` (
  `post_id` bigint(20) unsigned NOT NULL auto_increment,
  `cat_id` bigint(20) NOT NULL,
  `user_id` bigint(20) NOT NULL,
  `subject` tinytext NOT NULL,
  `comments` text NOT NULL,
  `created_on` datetime NOT NULL,
  `status` varchar(10) NOT NULL default 'INACTIVE',
  `private_post` varchar(10) NOT NULL default 'PUBLIC',
  `db_location` varchar(10) NOT NULL,
  PRIMARY KEY  (`post_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=7 ;

The replies table structure:

CREATE TABLE `replies` (
  `reply_id` bigint(20) unsigned NOT NULL auto_increment,
  `post_id` bigint(20) NOT NULL,
  `user_id` bigint(20) NOT NULL,
  `comments` text NOT NULL,
  `created_on` datetime NOT NULL,
  `notify` varchar(5) NOT NULL default 'YES',
  `status` varchar(10) NOT NULL default 'INACTIVE',
  `db_location` varchar(10) NOT NULL,
  PRIMARY KEY  (`reply_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=5 ;

Here is my query so far. I’ve removed my attempt of extracting the dates.

$strQuery = "SELECT posts.post_id, posts.created_on, replies.created_on, posts.subject ";
$strQuery = $strQuery."FROM posts ,replies ";
$strQuery = $strQuery."WHERE posts.post_id = replies.post_id ";
$strQuery = $strQuery."AND posts.cat_id = '".$row->cat_id."'";

Answer 1

$strQuery = "
  SELECT posts.post_id, GREATEST(posts.created_on, replies.created_on)  AS latestDate, posts.subject
  FROM posts, replies
  WHERE posts.post_id = replies.post_id
  AND posts.cat_id = {$row->cat_id}
  GROUP BY posts.post_id
  ORDER BY latestDate DESC;
";

UPDATE: On second looks, the above is actually incorrect, as it will not include those posts which do not yet have any replies. The more correct way to do it is:

$strQuery = "
  SELECT posts.post_id, GREATEST(posts.created_on, replies.created_on) AS latestDate,
  FROM posts
  LEFT JOIN replies ON (posts.post_id = replies.post_id)
  WHERE posts.cat_id = {$row->cat_id}
  GROUP BY posts.post_id
  ORDER BY latestDate DESC
  LIMIT 0,1;
";