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Cc: "D'Arcy J.M. Cain" <darcy@druid.net>, Claudia Kosny <ckosny@gmx.net>,
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From: Michael Glaesemann <grzm@seespotcode.net>
Subject: Re: fetch first rows of grouped data
Date: Mon, 27 Aug 2007 13:12:48 -0500
To: Joel Richard <postgres@joelrichard.com>

[Please don't top post as it makes the discussion more difficult to  
follow.]

On Aug 27, 2007, at 12:34 , Joel Richard wrote:

>
> On Aug 27, 2007, at 12:51 PM, D'Arcy J.M. Cain wrote:
>
>> On Mon, 27 Aug 2007 18:36:47 +0200
>> Claudia Kosny <ckosny@gmx.net> wrote:
>>> I have a list of events that take place in a certain city at a  
>>> certain
>>> date. Now I would like to have the first two (ordered by date)  
>>> events
>>> for each city.
>>>
>>> Is there a way to do this with one query?
>>> I am using PostgreSQL 7.4.
>>
>> I believe you are looking for the LIMIT keyword.  Check the docs  
>> on the
>> web site.

> That's what I thought at first, but this is not the case. She's  
> looking for the first two dates in -each- city in the table. I  
> initially thought that this could be accomplished with GROUP BY and  
> LIMIT, but GROUP BY can only give you the first date for each city,  
> not the first two dates for each city.
>
> So far, I haven't come up with any clever ideas. I'm not sure this  
> can be done in SQL.
>
> --Joel
>

I can't think of an easy way to do it, but certainly can do it in SQL  
using correlated subqueries, something like

select city, event, event_date, (
     select count(event)
     from events i
     where i.city = o.city
         and i.event_date < o.event_date
         and event_date > current_date -- make sure they're future  
events
         ) as nearness_rank
from events o
where event_date > current_date -- make sure they're future events
having nearness_rank <= 1;

Note that this can potentially show more than 2 events if the most  
recent upcoming events "tie" (have the same event_date).

Michael Glaesemann
grzm seespotcode net