数据库每日一题：
orders表有三个字段：id（订单编号）、customer_id（客户编号）和order_time（下单日期），在表orders中找到订单数最多客户对应的customer_id，具体如下图所示：

image.png

参考答案（有更优SQL留言共同学习）：

select o.customer_id 
from 
(select customer_id, count(customer_id) as total from orders group by customer_id) 
as o
where o.total=
(select max(o.total) 
from 
(select count(customer_id) as total from orders group by customer_id)
as o)