从订单表中找出购买次数最多的顾客

# 下单最多的顾客的id
order_number customer_number
1 1
2 2
3 3
4 3
select customer_number
from orders
group by customer_number

# 对顾客进行分组
order by count(customer_number) desc

# 降序排每个顾客出现的次数
limit 1

#仅显示排序结果的第一条;