解法一：（左外连接 + not null）
select A.Name as Customers
from Customers A left join Orders B
on A.Id = B.CustomerId
where B.Id is null
解法二：（not in + 子查询）
select Name as Customers from Customers c where c.Id not in(
select CustomerId from Orders
)
原文：https://www.cnblogs.com/vegetableDD/p/11641110.html