我正在尝试使用href删除行，它没有显示错误，但也没有从表中删除行。不知道我在做什么错。

delete.php
require_once "db.php";

$id = $_GET['id'];

$sql = "DELETE FROM books WHERE isbn = ". $id;

if (mysqli_query($db,$sql)){
    echo "Success";
}else{
    echo "Error";
}

main.php

if($resultSet->num_rows > 0){
        while($rows = $resultSet->fetch_assoc()){
            $au = $rows['author'];
            $bt = $rows['booktitle'];
            $rev = $rows['reserved'];
            echo"<tr><td>$au</td><td>$bt</td><td>$rev</td><td><a href='delete.php?id=".$rows['isbn']."'>Delete</a></td></tr>\n";
        }
        echo"</table>";
    }