Was: fetch first rows of grouped data

Hi there

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.

Thanks for any tips.

Claudia

I think I may have come up with a possible solution.

Create a selection that produces a unique identifier for each city
ordered by date then use array_accum to collect the unique identifiers
for each city, then match the first two elements of the array with the
identifiers.

For instance if you had a table :

CREATE TABLE crazy_talk (
ct_id bigserial primary key,
ct_city text,
ct_date date,
ct_data text
) ;

Then you could use :

SELECT
ct_id ,
ct_city ,
ct_date ,
ct_data
FROM
crazy_talk ,
(SELECT
ct_city AS city,
array_accum(ct_id) as match
FROM
crazy_talk
ORDER BY
ct_city ,
ct_date
GROUP BY
ct_city ) AS data_set
WHERE
ct_city = city AND
ct_id IN (match[0],match[1])
ORDER BY
ct_city ,
ct_date
;

I hope this helps, I did not try it, but I think it should work.

PS if you don't have array_accum here it is :

CREATE AGGREGATE array_accum (
BASETYPE = anyelement,
SFUNC = array_append,
STYPE = anyarray,
INITCOND = '{}'
);