Re: DISTINCT ON

> -----Original Message-----
> From: Michael Glaesemann [mailto:grzm(at)myrealbox(dot)com]
> Sent: Saturday, 19 November 2005 12:28 p.m.

> On Nov 19, 2005, at 7:49 , Jeremy Palmer wrote:
>
>> SELECT DISTINCT ON (vector_id, obs_type)
>> id
>> FROM observation
>> ORDER BY vector_id,
>> obs_type,
>> date DESC;
>>
>> However the documentation also states that "DISTINCT ON" is not
>> part of the
>> SQL standard and should be avoided when possible, stating that
>> aggregations
>> and sub-queries should be used instead...

> Something like:
>
> select max(date), id
> from observation
> group by vector_id, obs_type;

On Nov 19, 2005, at 11:50 , Jeremy Palmer wrote:

> Unfortunately that does not work as "id" column needs to be
> included in the
> group by statement or be used in an aggregate function. If I did
> this it
> definitely would note return the correct answer, as the "id" column
> is the
> primary key for the table.

[Please don't top post. It makes the post harder to read. I've
reordered the post for readability.]

Try something like this:

select id
from (
select max(date) as date
vector_id, obs_type
from observation
group by vector_id, obs_type
) latest_observations
join observation using (date, vector_id, obs_type)

Michael Glaesemann
grzm myrealbox com