MyBatis动态SQL

数据导入：

Create table If Not Exists RequestAccepted (requester_id int not null, accepter_id int null, accept_date date null);
Truncate table RequestAccepted;
insert into RequestAccepted (requester_id, accepter_id, accept_date) values ('1', '2', '2016/06/03');
insert into RequestAccepted (requester_id, accepter_id, accept_date) values ('1', '3', '2016/06/08');
insert into RequestAccepted (requester_id, accepter_id, accept_date) values ('2', '3', '2016/06/08');
insert into RequestAccepted (requester_id, accepter_id, accept_date) values ('3', '4', '2016/06/09');

在 Facebook 或者 Twitter 这样的社交应用中，人们经常会发好友申请也会收到其他人的好友申请。

RequestAccepted 表：

写一个查询语句，找出拥有最多的好友的人和他拥有的好友数目。

生成的测试用例保证拥有最多好友数目的只有 1 个人。

查询结果格式如下例所示。

示例：

解题思路：

with cte as
(select
    requester_id as id
from RequestAccepted 
union all
select
    accepter_id  
from RequestAccepted)

# 子查询筛选（group by having count >= all）
select 
    id,
    count(*) as num
from cte
group by id
having count(*) >= all(select count(*) from cte group by id)

# order by limit 1
select 
    id,
    count(*) as num
from cte
group by id
order by num desc
limit 1

# 窗口函数
select
    id,num
from
(select 
    id,
    count(*) as num,
    dense_rank() over(order by count(*) desc) as rnk
from cte
group by id) t
where rnk=1
;

来源：力扣（LeetCode）
链接：https://leetcode.cn/problems/friend-requests-ii-who-has-the-most-friends
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