LEFT JOIN左连接

练习内容

-- 查询所有用户及其订单（包括没有订单的用户）\nSELECT u.id, u.name, u.email, COUNT(o.order_id) AS order_count\nFROM users u\nLEFT JOIN orders o ON u.id = o.user_id\nGROUP BY u.id, u.name, u.email\nORDER BY order_count DES