力扣白嫖日记(sql)

前言

练习sql语句，所有题目来自于力扣（https://leetcode.cn/problemset/database/）的免费数据库练习题。

今日题目：

586.订单最多的客户
表：Orders

列名	类型
order_number	int
customer_number	int

查找下了最多订单的客户的 customer_number 。测试用例生成后，恰好有一个客户比任何其他客户下了更多的订单。

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我那不值一提的想法：

首先分析表，题目给了一个订单表，记录了订单的编号，以及顾客的编号，现在需要找到最多订单的客户，我认为肯定要计算出现的次数，所以得先group by customer_number，然后记录次数，然后通过order by函数去排序，由于order by 默认排序是升序，所以还得加desc变成降序，最后使用limit 1取第一个数，得到下了最多订单的客户。

select customer_number
from Orders
group by customer_number
having count(customer_number)
order by count(customer_number) desc
limit 1

看了题解，我注意到了一个我做题的时候没有注意到的问题，因为题目中强调了恰好有一个客户比任何其他客户都下了更多订单，所以结果只有一个，那么如果有多么客户并列第一呢。这里就需要用到子查询，筛选条件count等于最大的customer_number。

select customer_number
from Orders
group by customer_number
having count(customer_number) = 
(
    select count(customer_number)
    from Orders
    group by customer_number
    having count(customer_number)
    order by count(customer_number) desc
    limit 1
)

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结果：

总结：

能运行就行。

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