Oracle嵌套分组

Question

问题是：对于每一天，列出已阅读最多消息数的用户标识。

user_id msgID read_date 
1   1  10 
1   2  10 
2   2  10 
2   2  23 
3   2  23 

我相信，日期是外组和user_id是内部组，但怎么做组嵌套在SQL？或以某种方式避免这一点？

Answer 1

这是一个窗函数任务：

select * 
from 
(
    select user_id, read_date, count(*) as cnt, 
     rank() 
     over (partition by read_date   -- each day 
      order by count(*) desc) as rnk -- maximum number 
    from tab 
    group by user_id, read_date 
) dt 
where rnk = 1 

这可能返回多个用户提供一个具有相同的最大数，如果你只想要一个（随机）切换到ROW_NUMBER

Answer 2

select user_id 
from 
(
    select user_id,count(msgID) 
    from table 
    group by read_date 
) 
where rownum <= 1;