Php 计算结果表的行数

我有一个表，显示数据库表的结果，还有一个单元格，显示数据库中文件数的结果。看起来像这样

<?php
include ('../../include/dbConnect.php');
include ('../include/header.php');
include ('../include/checkAdmin.php');



$sql = "SELECT * from categories";
$result = mysqli_query($connection,$sql);
$count = mysqli_num_rows($result);
if($count > 0)

    ?>


<div class="text">
<h2>Categoties</h2>
    <form class="action" action="add.php" method="POST">
<button type="submit" class="btn" name="add_category">Add Category</button>
</form></div>
<br>
 <table>
        <thead>
            <tr>

                <th width= "300"><a href='?order=category'>Category</a></th>
                <th colspan="3" width= "150">Action</th>
                <th width= "150">Number of files</th>

            </tr>
        </thead>
        <tbody> 

    <?php
    while ($row = $result -> fetch_assoc()):    ?>

        <tr>

                <td><?php echo $row['category']; ?></td>
                <td>
    <form action='edit.php?id=<?php echo $row['id']; ?>' method="post">
    <button type="submit" class="btn" name="edit_btn" >Edit</button></form></td>

    <td>   
    <form action='list.php?id=<?php echo $row['id']; ?>' method="post">
    <button type="submit" class="btn" name="list_btn" >Files</button></form> </td>   
    <td>
    <form action='delete.php?id=<?php echo $row['id']; ?>' method="post">
        <input type="hidden" name="id" value="<?php echo $row['id']; ?>">
        <input type="submit" name="submit" class="btn" value="Delete">
    </form></td>
    <td><?php  $query = "SELECT * from files WHERE category_id =" . $row['category'];
                $res = mysqli_query($connection,$query);
                $r = mysql_num_rows($res); 
                **echo** 
                ?></td>

    </tr>

        <?php endwhile; ?>

        </tbody>
    </table><br>

<?php include('../include/footer.php');
?>

类别
添加类别


行动
文件数

我想回显每个类别中的文件数。如您所见，这些表与category_id相连。

用于获取行数

您可以使用下面给定的查询来解决问题-

$query = "SELECT count(*) from files WHERE category_id =" . $row['category'];

我得到mysql\u num\u rows（）期望参数1是资源，给定布尔值

<td><?php $query = "SELECT count(*) from files WHERE category_id =" . $row['category']; 
          $res = mysqli_query($connection,$query); 
          $r = mysql_num_rows($res); 
          echo $r;
?></td>