Getting the most recent post based on date

Question

Hi guys, 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;
";

Answer 2

SELECT posts.post_id, posts.subject, replies.post_id

FROM posts LEFT JOIN replies ON post.post_id = replies.post_id

WHERE posts.cat_id = '$row->cat_id'

ORDER BY posts.post_id DESC, replies.post_id DESC

The one that has no replies will return NULL, and you can filter out using PHP on your output.

Answer 3

Get the most recent post, the most recent reply to a post, then get the most recent one of them.

This uses order by created_on desc limit 1 to only get the last item from a table. I'd suggest adding keys on created_on columns for both tables, and for post_id column in replies table.

Update: also cat_id needs indexing.

select * from (
select * from 
(
select p.post_id, p.created_on , 'post' as post_type
from posts p
where p.cat_id = '$row->cat_id'
order by p.created_on  desc limit 1
)post
union 
select * from 
(
select p.post_id, r.created_on , 'reply'
from posts p
inner join replies r on r.post_id = p.post_id
where p.cat_id = '$row->cat_id'
order by r.created_on  desc limit 1
)reply
)big order by big.created_on limit 1;