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)
