Handling MySQL errors with PHP

Hi,

Is there a way to handle MySQL errors gracefully such as a duplicate entry
into a UNIQUE field, the method I've been using seems a little laborious...

//see if user exists
$query = "SELECT *
FROM users
WHERE Username = '".$_POST[username]."'";
$result = @mysql_query($query);
$num = @mysql_num_rows($result);
if($num > 0){
$error = "Username already exists, please choose a another one";
header("Location: add_user.php?error=$error");
exit;
}

Thanks for your help