How can I combine these two queries into one?

Question

I am writing a script for my custom forums that determines if new threads or replies have been posted in a certain board since the user's last visit to that board(board_marks). I have two tables: one that stores threads, another that stores replies. I can easily find if there are any new replies in a board by using a left join. I want to add a bit that finds if there are new threads in that board. How would I do this?

My Tables:

CREATE TABLE IF NOT EXISTS `forum_threads` (
  `thread_id` int(15) NOT NULL AUTO_INCREMENT,
  `board_id` int(15) NOT NULL,
  `author_id` int(15) NOT NULL,
  `updater_id` int(15) NOT NULL,
  `title` text NOT NULL,
  `content` text NOT NULL,
  `date_posted` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `date_updated` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `views` int(15) NOT NULL,
  `status` tinyint(1) NOT NULL,
  `type` tinyint(1) NOT NULL COMMENT '0 normal, 1 sticky, 2 global.',
  PRIMARY KEY (`thread_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1;

CREATE TABLE IF NOT EXISTS `forum_messages` (
  `message_id` int(15) NOT NULL AUTO_INCREMENT,
  `thread_id` int(15) NOT NULL,
  `author_id` int(15) NOT NULL,
  `modifier_id` int(15) DEFAULT NULL,
  `content` text NOT NULL,
  `date_posted` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `date_modified` timestamp NULL DEFAULT NULL,
  `status` tinyint(1) NOT NULL,
  PRIMARY KEY (`message_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1;

My Script:

$this->db->select('m.message_id, m.thread_id, m.date_posted, t.thread_id, t.board_id');
$this->db->from('forum_messages AS m');
$this->db->join('forum_threads AS t', 'm.thread_id = t.thread_id', 'left');
$this->db->where('t.board_id', $board_id);
$this->db->where('m.date_posted > "'.$last_mark['date_marked'].'"');

if($query_new_messages = $this->db->get()){

    if($query_new_messages->num_rows() > 0){

          $contains_new_posts = TRUE;

    } else {

           $contains_new_posts = FALSE;

    }

 }

The actual query:

SELECT 
     m.message_id, m.thread_id, m.date_posted, t.thread_id, t.board_id
FROM (forum_messages AS m) 
LEFT JOIN forum_threads AS t ON m.thread_id = t.thread_id
WHERE `t`.`board_id` = '10' AND `m`.`date_posted` > "2014-03-02 06:01:31"

PS: I am doing this in CodeIgniter.

Answer 1

An OR condition will get you the data you want:

SELECT 
     m.message_id, m.thread_id, m.date_posted, t.thread_id, t.board_id
FROM (forum_messages AS m) 
    LEFT JOIN forum_threads AS t ON m.thread_id = t.thread_id
WHERE `t`.`board_id` = '10' 
    AND (`m`.`date_posted` > "2014-03-02 06:01:31" 
         OR `t`.`date_posted` > "2014-03-02 06:01:31")

You should benchmark this, however. This solution is probably not very optimized. You should benchmark doing two fast queries - one for posts and one for threads - vs doing one potentially slower one.