leetcode# 从不订购的客户
SQL:
select a.Name as "Customers"
from Customers a
where a.Id NOT IN (select CustomerId from Orders)
知识点：
1. 刚开始解答错误，出现语法错误：
select a.Name as "Customers"
from Customers a, Orders b
where a.Id NOT IN b.CustomerId
是因为没有留意到，NOT IN或者IN后面，要带的是一个集合，而不是一个字段！