My output for my table in HTML has several columns such as userid, name, age, dob.

The table heading is simply the title of the column name, I want this to be a link, and when clicked, the selected column is sorted in order, ASC, and then DESC (on next click). I thought this was pretty straight forward but I'm having some difficulty.

So far, I have produced this, and no output is taken, apart from the URL works by displaying 'users.php?orderby=userid'

<?php
          if(isset($_GET['orderby'])){ 
$orderby = $_GET['orderby']; 
$query_sv = "SELECT * FROM users BY ".mysql_real_escape_string($orderby)." ASC"; 
} 
//default query 
else{ 
$query_sv = "SELECT * FROM users BY user_id DESC"; 
} 
?>


              <tr>
                <th><a href="<?php echo $_SERVER['php_SELF']."?orderby=userid";?>">User ID</a></th>

Hoefully if I get this working, I can sort the users by D.O.B. next also using the same principles. Does anyone have any ideas?