won't you need
$sql = sprintf("SELECT FROM comments comment,author,date WHERE postnumb=%s" , $HTTP_GET_VARS['id'] );
edit: actually, I think what you have anyway, will print exactly the same as what I have done.
try echoing the $sql and then run that at either the SQL prompt or use phpMyAdmin.........
aha...........it's just dawned on me
try
$sql = "SELECT comment,author,date from comments where postnumb=". $HTTP_GET_VARS['id'] ;
try that!