COUNT a number of database rows affiliated with an id

Question

I'm creating a forum and right now I am trying to figure out how to count the number of replies in a topic. I am wanting to count the number of rows per topic_id #. So if there is a topic id of 5 and there are 10 rows in my database of topic_id #5, I want the it to count and output 10.

I tried to structure my query like this

 $query2 = mysqli_query($con,"SELECT t.*, COUNT(p.topic_id) AS tid2 FROM forum_topics AS t, forum_posts AS p ORDER BY topic_reply_date DESC")

All this did however was mess up my original query which was this...

$query2 = mysqli_query($con,"SELECT * FROM forum_topics WHERE `category_id` ='".$cid."' ORDER BY topic_reply_date DESC")

And it is now only showing 1 topic rather than the 15 I have and it it outputting a very large Count figure of 406.

How can I get the following code to only count the topic_id associated with the topic that is being outputted and still allow for all of my topics to be outputted?

$query2 = mysqli_query($con,"SELECT t.*, COUNT(p.topic_id) AS tid2 FROM forum_topics AS t, forum_posts AS p ORDER BY topic_reply_date DESC")
or die ("Query2 failed: %s\n".($query2->error));
$numrows2 = mysqli_num_rows($query2);
//if ( false===$query2 ) {
    // die(' Query2 failed: ' . htmlspecialchars($query2->error));
//}
if($numrows2 > 0){
    $topics .= "<table width='100%' style='border-collapse: collapse;'>";
    //Change link once discussion page is made
    $topics .= "<tr><td colspan='3'><a href='discussions.php'>Return to Discussion Index</a>".$logged."<hr /></td></tr>";
    $topics .= "<tr style='background-color: #dddddd;'><td>Topic Title</td><td width='65' align='center'>Replies</td><td width='65' 
    align='center'>Views</td></tr>";
    $topics .= "<tr><td colspan='3'><hr /></td></tr>";
    while($row = mysqli_fetch_assoc($query2)){
        $tid = $row['id'];
        $title = $row['topic_title'];
        $views = $row['topic_views'];
        $replies = $row['tid2'];
        $date = $row['topic_date'];
        $creator = $row['topic_creator'];
        $topics .= "<tr><td><a href='forum_view_topic.php?cid=".$cid."&tid=".$tid."'>".$title."</a><br /><span class='post_info'>Posted 
        by: ".$creator." on ".$date."</span></td><td align='cener'>".$replies."</td><td align='center'>".$views."</td></tr>";
        $topics .= "<tr><td colspan='3'><hr /></td></tr>";
    }

Answer 1

Use GROUP BY clause ( tutorial ):

SELECT t.*, COUNT(p.topic_id) AS tid2 
FROM forum_topics AS t JOIN forum_posts AS p on t.id = p.topic_id 
GROUP BY t.id

Also note that OUTER JOIN is used in my example (instead of CROSS JOIN in your example).