SQL:在每个组中获取第N个项目 [英] SQL: get Nth item in each group

问题描述

我有一个这样的用户表

user_id | community_id | registration_date
--------------------------------------------
1       |     1        | 2008-01-01
2       |     1        | 2008-05-01
3       |     2        | 2008-01-28
4       |     2        | 2008-07-22
5       |     3        | 2008-01-11

对于每个社区，我想获取第三位用户注册的时间.我可以使用MySql的'limit'SQL扩展轻松地对单个社区执行此操作.例如，对于ID = 2的社区

For each community, I would like to get the time that the 3rd user registered. I can easily do this for a single community using MySql's 'limit' SQL extension. For example, for community with ID=2

select registration_date
from user
order by registration_date
where community_id = 2
limit 2, 1

或者，我可以通过以下方式获取第一个用户向所有社区注册的日期:

Alternatively, I can get the date that the first user registered for all communities via:

select community_id, min(registration_date) 
from user 
group by 1

但是我不知道如何在单个SQL语句中获取 all 社区的第三位用户的注册日期.

But I can't figure out how to get the registration date of the 3rd user for all communities in a single SQL statement.

干杯， 唐

推荐答案

具有内部选择:

select 
  registration_date, community_id 
from 
  user outer 
where 
  user_id IN (
    select 
      user_id 
    from 
      user inner 
    where 
      inner.community_id = outer.community_id 
    order by 
      registration_date 
    limit 2,1
  )
order by registration_date

选择一组用户，其中每个用户都是内部选择中的limit子句返回的社区中的第3个用户.

Selects the set of users where each user is the 3rd user in their community as returned by the limit clause in the inner select.

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