# select Name as Customers from Customers
#  where Customers.Id not in (select CustomerId from Orders )

将两张表左连接，显示CustomerId为空的名字
# select Name as Customers from Customers
#  left join Orders on Customers.Id = Orders.CustomerId
#  where CustomerId is NULL

将两张表右连接，和上面左连接思路相反
# select Name as Customers from Customers c where c.Id not in (select Customers.Id from Customers right join Orders on Customers.Id = Orders.CustomerId)