mysql左连接练习题：组合两个表+从不订购的客户

题175. 组合两个表 （左连接）

思路：左连接left join ， 右表是可能地址为空的address表。

select p.firstname,p.lastname,a.city,a.state 
from person as p
left join address as a 
on p.personid=a.personid

题183. 从不订购的客户：（左连接的过滤）

思路：通过左连接显示所有客户的订单，没有订购任何东西的orders表id为null

select name as customers 
from customers as c 
left join orders as o
on c.id=o.customerid  
where o.id is null;