为什么mysqli_num_rows（）需要参数1

Question

    <?php 

    include 'header.php'; // database connection

 ?>

<form action="search.php" method="POST">
    <input type="text" name="search" placeholder="Search">
    <button type="submit" name="submit-search">search</button>
</form>

<h1>FRONT PAGE</h1>
<h2>All Books:</h2>

<div class="book-container">
    <?php 

        $sql = "SELECT * FROM book";
        $result = mysqli_query($conn, $sql)or die("Error");
        $queryResults = mysqli_num_rows($result);

        if ($queryResults > 0) {
            while ($row = mysqli_fetch_assoc($result)) {
                echo "<div class='book-box'>
                    <h3>".$row['title']."</h3>
                    <p>".$row['bookCode']."</p>
                    <p>".$row['publisherCode']."</p>
                    <p>".$row['type']."</p>
                </div>";
            }
        }
     ?>

</div>

</body>
</html>

  

第一次在这里问一个问题，但是我运行了这段代码，并能够从数据库中列出称为书的表，但是由于某种原因，搜索无法正常工作，不断出现此错误Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, bool given in C:\xampp\htdocs\tuts\search.php on line 18