Description:
Suppose that a website contains two tables, the Customers table and the Orders table. Write a SQL query to find all customers who never order anything
一种比较高效的方法是左连接，左连接之后任然使用被连接的两个表或多个表来定位连接之后的表的数据。
# Write your MySQL query statement below
select Name
from (Customers c left join Orders o on c.Id=o.CustomerId)
where o.Id is null;