i am having trouble with my php code where i want to display all the topics from mysql database but it don't display anything and just echo's he posts could not be displayed, please try again later. could someone advise me about what i am doing wrong here thanks. session is in header.php

PHP Code:
<?php
include 'mysql.php';
include 
'header.php';
//retrieving basic data from topic
$sql "SELECT
           id,
           subject
       FROM
           topics
       WHERE
           id = "
mysql_real_escape_string($_GET['id']);
           
$result mysql_query($sql);
if(!
$result)
{
    echo 
'The topic could not be displayed, please try again later.' mysql_error();
}
else
{
    if(
mysql_num_rows($result) == 0)
    {
        echo 
'This topic dose not exist.';
    }
    else
    {
        while(
$row mysql_fetch_assoc($result))
        {
            
//display post data
            
echo '<table class="topic" border="1">
                   <tr>
                       <th colspan="2">' 
$row['subject'] . '</th>
                   </tr>'
;
        }
            
//fetch the posts from the database on post.postby = users.id tells us that these are both the same through left join.
            
$sql "SELECT
                       users.id,
                       post.content,
                       post.date,                      
                       post.topic,
                       post.postby,
                       users.username
                   FROM
                       post
                   LEFT JOIN
                       users
                   ON
                       post.postby = users.id
                   WHERE
                       post.topic = " 
mysql_real_escape_string($_GET['id']);          
            
$result mysql_query($sql);      
            if(!
$result)
            {
                echo 
'<tr><td>The posts could not be displayed, please try again later.</tr></td></table>';
            }
            else
            {
                while(
$row mysql_fetch_assoc($result))
                {
                    echo 
'<tr class="topic-post">
                           <td class="user-post">' 
$row['username'] . '<br/>' date('d-m-Y H:i'strtotime($row['date'])) . '</td>
                           <td class="post-content">' 
htmlentities(stripslashes($row['content'])) . '</td>
                         </tr>'
;
                }
            }
           
            if(!
$_SESSION['loggedIn'])
            {
                echo 
'<tr><td colspan=2>You must be <a href="signin.php">signed in</a> to reply. You can also <a href="signup.php">sign up</a> for an account.';
            }
            else
            {
                
//show reply box
                
echo '<tr><td colspan="2"><h2>Reply:</h2><br />
                   <form method="post" action="reply.php?id=' 
$row['id'] . '">
                       <textarea name="reply-content"></textarea><br /><br />
                       <input type="submit" value="Submit reply" />
                   </form></td></tr>'
;
            }
           
            
//finish the table
            
echo '</table>';
        }
    }
include 
'footer.php';
?>