autosubmit将值下拉到数据库中

i'm trying to auto submit the value of drop down menu using onchange=this.form.submit() into database but i couldn't make it work. can anyone please help me. any suggestions will be highly appreciated.

here is the code:

<? 
require_once ('database.php');
?>

<form action="" method=post>
<select name="assignee"  onchange="this.form.submit()">
<option value="0">Unassigned</option>   
        <?
        $find_selected = mysql_query("select assign_to from orders where id = $id");
        $asignee =mysql_fetch_row($find_selected);
        $list=mysql_query("SELECT id, full_name from user where username <> 'root' and nature = 3");
        while($row_list=mysql_fetch_assoc($list)){
        ?>
        <option value="<? echo $row_list['id']; ?>"<? if($row_list['id']== $asignee['0']){ echo "selected"; } ?>><? echo $row_list['full_name'] ?></option>
        <?
        }
        mysql_free_result($list);
        mysql_free_result($find_selected);
        ?>
</select>
</form>
<?php 

if(isset($_POST['submit'])){

$sql = "UPDATE `orders` SET `assign_to` =  '{$_POST['assignee']}' WHERE `id` = '$id' ";
mysql_query($sql) or die(mysql_error());    
}


?>

<?php 

if(isset($_POST['assignee'])){

$sql = "UPDATE `orders` SET `assign_to` =  '{$_POST['assignee']}' WHERE id=$id";    
mysql_query($sql);
}

?>

You just defined your query but didn't executed it