phpcms 按时间排序

<?php
// 数据库连接
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "phpcms";

$conn = new mysqli($servername, $username, $password, $dbname);

if ($conn->connect_error) {
    die("连接失败: " . $conn->connect_error);
}

// 按发布时间降序排序
$sql = "SELECT id, title, publish_time FROM news ORDER BY publish_time DESC";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    while($row = $result->fetch_assoc()) {
        echo "ID: " . $row["id"]. " - 标题: " . $row["title"]. " - 发布时间: " . $row["publish_time"]. "<br>";
    }
} else {
    echo "0 结果";
}
$conn->close();
?>

$sql = "SELECT id, title, publish_time FROM news ORDER BY publish_time DESC";

$sql = "SELECT id, title, publish_time FROM news WHERE publish_time IS NOT NULL ORDER BY publish_time DESC";