Get number of posts in a topic PHP

Question

How do I get to display the number of posts on a topic like a forum. I used this... (how very noobish):

function numberofposts($n)
{
    $sql = "SELECT * FROM posts
            WHERE topic_id = '" . $n . "'";

    $result = mysql_query($sql) or die(mysql_error());          
    $count = mysql_num_rows($result);           

    echo number_format($count);
}

The while loop of listing topics:

<?php

$sql = "SELECT * FROM topics ORDER BY topic_id ASC LIMIT $start, $limit";
        $result = mysql_query($sql) or die(mysql_error());

while($row = mysql_fetch_array($result))
{
?>
<div class="topics">
    <div class="topic-name">
        <p><?php echo $row['topic_title']; ?></p>
    </div>
    <div class="topic-posts">
        <p><?php echo numberofposts($row['topic_id']); ?></p>
    </div>
</div>
<?php
}
?>

Although it is a bad method of doing this... All I need is to know what would be the best method, don't just point me out to a website, do it here, because I'm trying to learn much. Okay? :D

Thanks.

EDIT:

SQL:

Posts table:

CREATE TABLE `posts` (
  `post_id` mediumint(8) NOT NULL AUTO_INCREMENT,
  `topic_id` mediumint(8) NOT NULL,
  `forum_id` mediumint(8) NOT NULL,
  `user_id` mediumint(8) NOT NULL,
  `post_time` varchar(100) NOT NULL,
  `post_timestamp` mediumint(20) NOT NULL,
  `post_ip` varchar(20) NOT NULL,
  `post_reported` tinyint(1) NOT NULL,
  `post_reportdesc` text NOT NULL,
  PRIMARY KEY (`post_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=2 ;

--
-- Dumping data for table `posts`
--

INSERT INTO `posts` VALUES(1, 1, 0, 1, '15th Junenee', 0, '', 0, '');

Topics table:

CREATE TABLE `topics` (
  `topic_id` mediumint(8) NOT NULL AUTO_INCREMENT,
  `section_name` varchar(25) NOT NULL,
  `topic_title` varchar(120) NOT NULL,
  `topic_description` char(120) NOT NULL,
  `user_id` mediumint(8) NOT NULL,
  `topic_time` varchar(100) NOT NULL,
  `topic_views` varchar(1000) NOT NULL,
  `topic_up` mediumint(11) NOT NULL,
  `topic_down` mediumint(11) NOT NULL,
  `topic_status` tinyint(1) NOT NULL,
  PRIMARY KEY (`topic_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=1 ;

This should help you understand a bit more.

Answer 1

You can use:

"SELECT COUNT(topic_id) FROM posts WHERE topic_id = '?'"

The ? is a place-holder. If you use mysql, you should use mysql_real_escape_string:

$sql = "SELECT COUNT(topic_id)
        WHERE topic_id = '" . mysql_real_escape_string($n) . "'";

If you use mysql_fetch_array, $row[0] will be the count. You can name it but it's not necessary.

A better option is some kind of prepared statements, such as PDOStatement or mysqli_stmt. This helps prevent SQL injection.

Answer 2

You could use this SQL query instead if you only want the number of records:

SELECT COUNT(topic_id) AS 'count' WHERE topic_id = '123'

Then after you do:

$result = mysql_query($sql);
if ($result !== false)
{
    $row = mysql_fetch_assoc($result);
    if ($row !== false)
        echo('Number of rows = ' . $row['count']);
}

Answer 3

SELECT topic_title, COUNT(*) AS toipic_count
FROM topics
GROUP BY topic_id
ORDER BY topic_id ASC

Then you will not need any more mysql-querys, just use:

$row['topic_count']

Answer 4

When you're listing topics, you should include the post count in a join:

SELECT t.*, COUNT(p.post_id) AS post_count
FROM topic t LEFT JOIN posts p ON p.topic = t.topic_id 
GROUP BY t.topic_id

Didn't study your schema too much, but you get the point.