如何在PHP中的SELECT语句中执行子查询？

<?php
// 假设有一个名为"users"的表，包含"id"和"username"两列

// 创建数据库连接
$servername = "your_servername";
$username = "your_username";
$password = "your_password";
$dbname = "your_dbname";

$conn = new mysqli($servername, $username, $password, $dbname);
if ($conn->connect_error) {
    die("连接失败: " . $conn->connect_error);
}

// 执行查询语句，获取所有用户名以及对应的id
$sql = "SELECT id, username FROM users WHERE id IN (SELECT id FROM users WHERE username LIKE '%John%')";

$result = $conn->query($sql);

if ($result->num_rows > 0) {
    // 输出每行数据
    while($row = $result->fetch_assoc()) {
        echo "id: " . $row["id"]. " - Username: " . $row["username"]. "<br>";
    }
} else {
    echo "0 结果";
}

// 关闭数据库连接
$conn->close();
?>