我怎样才能避免这个问题 - mysqli_fetch_assoc（）期望参数1为mysqli_result，给出布尔值

Question

我想了解为什么会发生这种错误，有时我会以相同的方式呈现我的$ result变量，并且不会发生错误。我试图谷歌搜索原因，而不是运气。所以我有示例代码，其中发生以下错误。

  

mysqli_fetch_assoc（）期望参数1为mysqli_result，第26行给出布尔值

有人可以帮我纠正一下，并提供有效的解释吗？

<?php
include 'connect.php';
include 'header.php';

//first select the category based on $_GET['cat_id']
if($_SERVER['REQUEST_METHOD'] != 'get')
{
$sql = "SELECT
        cat_id,
        cat_name,
        cat_description
    FROM
        categories
    WHERE
        cat_id = " . mysqli_real_escape_string($link, $_GET['cat_id']);

$result = mysqli_query($link, $sql);

if(!isset($result))
{
echo 'The category could not be displayed, please try again later.';
}
else
{
    //display category data
    while($row = mysqli_fetch_assoc($result))
    {
        echo '<h2>Topics in ′' . $row['cat_name'] . '′ category</h2>';
    }
    //do a query for the topics
    $sql = "SELECT
                topic_id,
                topic_subject,
                topic_date,
                topic_cat
            FROM
                topics
            WHERE
                topic_cat = " . mysqli_real_escape_string($link,$_GET['topic_cat']);

    $result = mysqli_query($link, $sql);
    if(!isset($result))
    {
        echo 'The topics could not be displayed, please try again later.';
    }
    else
    {
        if(mysqli_num_rows($result)== 0)
        {
            echo 'There are no topics in this category yet.';
        }
        else
        {
            //prepare the table
            echo '<table border="1">
                  <tr>
                    <th>Topic</th>
                    <th>Created at</th>
                  </tr>'; 

            while($row = mysqli_fetch_assoc($result))
            {               
                echo '<tr>';
                    echo '<td class="leftpart">';
                        echo '<h3><a href="topic.php?id=' . $row['topic_id'] . '">' . $row['topic_subject'] . '</a><h3>';
                    echo '</td>';
                    echo '<td class="rightpart">';
                        echo date('d-m-Y', strtotime($row['topic_date']));
                    echo '</td>';
                echo '</tr>';
            }
        }
    }
 }
}
include 'footer.php';
?>