当在while循环中使用mysqli_fetch_array时，PHP / MySQL如何知道获取下一行？

Here is the code:

这是代码:

<?php
    $q = 'SELECT * FROM categories ORDER BY category'; 
    $r = mysqli_query($dbc,$q);
    while (list($id,$category) = mysqli_fetch_array($r,MYSQLI_NUM)) {
        echo '<li><a href="/go.html?url=category.php?id=' . $id . '" title="' . $category . '">' . $category . '</a></li>';
    }
?>
<?php
    $q = 'SEL