如何在SQL和PHP中正确使用WHERE和JOIN

SELECT * FROM customers WHERE age > 18 AND city = 'Beijing';

SELECT orders.order_id, customers.customer_name
FROM orders
JOIN customers ON orders.customer_id = customers.customer_id;

<?php
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "database";

// 创建连接
$conn = new mysqli($servername, $username, $password, $dbname);

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

// 使用WHERE子句查询特定条件的数据
$sql = "SELECT * FROM customers WHERE age > 18 AND city = 'Beijing'";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    // 输出数据
    while($row = $result->fetch_assoc()) {
        echo "姓名: " . $row["name"]. " 年龄: " . $row["age"]. " 城市: " . $row["city"]. "<br>";
    }
} else {
    echo "0 结果";
}

// 使用JOIN操作查询关联数据
$sql = "SELECT orders.order_id, customers.customer_name
        FROM orders
        JOIN customers ON orders.customer_id = customers.customer_id";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    // 输出数据
    while($row = $result->fetch_assoc()) {
        echo "订单号: " . $row["order_id"]. " 顾客姓名: " . $row["customer_name"]. "<br>";
    }
} else {
    echo "0 结果";
}

// 关闭连接
$conn->close();
?>