原题链接：https://leetcode.com/problems/customers-who-never-order/description/
做完这道题目我才发现，我跟不就不懂 in, exists, not in, not exists，exists 用都不会用：
# Write your MySQL query statement below
select Name as Customers from Customers where not exists (select CustomerId from Orders where Customers.Id = Orders.CustomerId);
select Name as Customers from Customers where
Id not in (select CustomerId from Orders);