金融机构希望找到每位客户的最大交易金额，表结构如下： Cus

SELECT customer_name, MAX(amount) FROM Customers JOIN Transactions ON Customers.customer_id = Transactions.customer_id GROUP BY customer_name;

SELECT customer_id, SUM(amount) FROM Transactions GROUP BY customer_id;

SELECT customer_name, amount FROM Customers JOIN Transactions ON Customers.customer_id = Transactions.customer_id WHERE amount IS NOT NULL;

SELECT customer_name FROM Customers WHERE customer_id IN   (SELECT customer_id FROM Transactions    WHERE amount > 0);