如果查询为空，则退出php mysql

<?php
// 连接到MySQL数据库
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "database";

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

// 检查连接是否成功
if ($conn->connect_error) {
    die("连接失败: " . $conn->connect_error);
}

// 执行查询语句
$sql = "SELECT * FROM table_name";
$result = $conn->query($sql);

// 检查查询结果是否为空
if ($result->num_rows > 0) {
    // 查询结果不为空，处理查询结果
    while($row = $result->fetch_assoc()) {
        // 处理每一行数据
        echo "字段1: " . $row["column1"]. " - 字段2: " . $row["column2"]. "<br>";
    }
} else {
    // 查询结果为空，执行退出操作
    echo "查询结果为空，退出";
    exit;
}

// 关闭数据库连接
$conn->close();
?>