MYSQL左连接3个表(第一个表返回空值)

SELECT 
    users.user_id, 
    users.username, 
    orders.order_id, 
    products.product_id, 
    products.product_name
FROM 
    users
LEFT JOIN 
    orders ON users.user_id = orders.user_id
LEFT JOIN 
    products ON orders.product_id = products.product_id;

-- 检查users表是否有数据
SELECT COUNT(*) FROM users;

-- 确认连接字段是否存在
DESCRIBE users;
DESCRIBE orders;
DESCRIBE products;

SELECT 
    users.user_id, 
    COALESCE(users.username, 'N/A') AS username, 
    orders.order_id, 
    products.product_id, 
    COALESCE(products.product_name, 'N/A') AS product_name
FROM 
    users
LEFT JOIN 
    orders ON users.user_id = orders.user_id
LEFT JOIN 
    products ON orders.product_id = products.product_id;