Get number of posts in a topic PHP - 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

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.

Answer 3

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 4

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']