SQL: get Nth item in each group

Question

Hi,

I have a user table like this

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

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

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

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

Cheers, Don

Answer 1

With an inner select:

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

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.

Answer 2

Is this what you mean?

SELECT registration_date
FROM user
ORDER BY registration_date
LIMIT n

Where n is the user you are concerned about.