Cant display topics in a category in my php/SQL forum

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SQL

PHP

Hi
Ive been having a very tedious problem lately where Im not able to display the topics in a forum category. I have been able to display all categories in a forum-style table but when I click on one of the categories I am getting the following error but I dont understand where in my code it is going wrong:

Notice: Undefined index: topic_cat in .../category.php on line 44

I want to get the topics that are linked with a specific category (topic_cat links to cat_id). So for cat_id=2, I want all topics that are assigned topic_cat=2.

Below is my code, can anyone see any problems? Note: this is the code that is currently being processed:

else
        {
            if(mysql_num_rows($topicResult) == 0)
            {
                echo 'There are no topics in this category yet.';
            }

And this is my code:

  else
    {
        //display category data
        while($row = mysql_fetch_assoc($result))
        {
            echo '<h2>Topics in ′' . $row['cat_name'] . '′ category</h2>';
        }
     
        //do a query for the topics
        $topicSql = "SELECT 
                    topic_id,
                    topic_subject,
                    topic_date,
                    topic_cat
                FROM
                    topics
                WHERE
 44-->      topic_cat = '" . mysql_real_escape_string($_GET['topic_cat']) . "'";
         
        $topicResult = mysql_query($topicSql);
         
        if(!$topicResult)
        {
            echo 'The topics could not be displayed, please try again later.';
        }
        else
        {
            if(mysql_num_rows($topicResult) == 0)
            {
                echo 'There are no topics in this category yet.';
            }
            else
            {
                //prepare the table
                echo '<table border="1">
                      <tr>
                        <th>Topic</th>
                        <th>Created at</th>
                      </tr>';
                     
                while($row = mysql_fetch_assoc($topicResult))
                {              
                    echo '<tr>';
                        echo '<td class="leftpart">';
                            echo '<h3><a href="topic.php?topic_id=' . $row['topic_id'] . '">' . $row['topic_subject'] . '</a><h3>';
                        echo '</td>';
                        echo '<td class="rightpart">';
                            echo date('d-m-Y', strtotime($row['topic_date']));
                        echo '</td>';
                    echo '</tr>';
                }
            }
        }
    }
}