// 商品列表页面
$sql = "SELECT * FROM products";
$result = mysqli_query($conn, $sql);

while ($row = mysqli_fetch_assoc($result)) {
  echo $row['name'];
  echo $row['price'];
  echo $row['description'];
  echo "<a href="product_details.php?id=".$row['id']."">详情</a>";
}

// 商品详情页面
$id = $_GET['id'];
$sql = "SELECT * FROM products WHERE id = $id";
$result = mysqli_query($conn, $sql);

$row = mysqli_fetch_assoc($result);
echo $row['name'];
echo $row['price'];
echo $row['description'];
echo "<button onclick="addToCart(".$row['id'].")">添加到购物车</button>";

// 购物车页面
$sql = "SELECT * FROM cart_items WHERE user_id = $user_id";
$result = mysqli_query($conn, $sql);

while ($row = mysqli_fetch_assoc($result)) {
  echo $row['product_name'];
  echo $row['product_price'];
  echo "<button onclick="removeFromCart(".$row['id'].")">删除</button>";
}

// 添加商品到购物车
$product_id = $_GET['id'];
$sql = "INSERT INTO cart_items (user_id, product_id, product_name, product_price) VALUES ($user_id, $product_id, '$product_name', $product_price)";
mysqli_query($conn, $sql);

// 从购物车删除商品
$item_id = $_GET['item_id'];
$sql = "DELETE FROM cart_items WHERE id = $item_id";
mysqli_query($conn, $sql);

// 创建订单
$sql = "INSERT INTO orders (user_id, product_id, product_name, product_price) SELECT user_id, product_id, product_name, product_price FROM cart_items WHERE user_id = $user_id";
mysqli_query($conn, $sql);

// 更新库存
$sql = "UPDATE products INNER JOIN cart_items ON products.id = cart_items.product_id SET products.stock = products.stock - cart_items.quantity WHERE cart_items.user_id = $user_id";
mysqli_query($conn, $sql);

// 清空购物车
$sql = "DELETE FROM cart_items WHERE user_id = $user_id";
mysqli_query($conn, $sql);