no output display in dropdown list

Code:

<select name="staff" id="staff" required>
<option value="">Select Staff</option>

<?php
while($row_subject = mysql_fetch_array($query_subject))
{
    $sel = '';
    if($this->input->post('staff') == $row_subject['deo_code']) {
        $sel = 'selected';
    }
    echo '<option value="'.$row_subject['deo_code'].'" '.$sel.'>'.$row_subject['deo_code']."-".$row_subject['deo_name'].'</option>';
}
?>
</select>

1) If you dont know how to use a ? b : c use if first
2) Why you use four times <?php when you can use one?