Изменение данных в базе данных путем UPDATE

<?php
require 'connect.php';
$u_email=trim($_REQUEST['u_email']);
$u_qiwi=trim($_REQUEST['u_qiwi']);
$update_sql =("UPDATE users SET email='$u_email',qiwi='$u_qiwi' WHERE login='$u_login'"); 
mysql_query($update_sql) or die("Ошибка вставки" . mysql_error());
?>
<?php 
$select_sql = "SELECT email,qiwi FROM users WHERE login='$u_login'"; 
$result = mysql_query($select_sql); 
$row = mysql_fetch_array($result); 
printf("<form action='/edit.spx' method='post' name='forma'> 
<span>Новый емайл:</span><input class='val' type='text' name='u_email' maxlength='50' style='width:200px;' value=''>
<span>Новый киви:</span><input class='val' type='text' name='u_qiwi' maxlength='50' style='width:200px;' value=''>
<input class='btn_box' id='submit' type='submit' value='Редактировать запись'>
</form>",$row['email'],$row['qiwi']); 
?>

$sql = 'UPDATE tableName SET ';
if(!empty($_POST['field1'])){
    $sql .= "field1 = '{$_POST['field1']}',";
}
if(!empty($_POST['field2'])){
    $sql .= "field2 = '{$_POST['field2']}',";
}
$sql = substr($sql, 0, -1) . " WHERE <condition>";