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Thread: group by complications

group by complications

From
Mark Fenbers
Date:
select l.lid,l.fs,max(h.obstime) from location as l
inner join height as h on h.lid = l.lid
where l.fs > 0.0
group by l.lid,l.fs;

The above query works as expected in that is fetches the lid, fs and 
time of the latest observation in the height table (for the 
corresponding lid), but I also want to fetch (i.e., add to the select 
list) the corresponding reading (h.obsvalue) which occurs at 
max(h.obstime).  I'm having trouble formulating the correct SQL syntax 
to pull out the l.lid, l.fs, and the most recent h.obvalue (with or 
without the time that it occurred).

Logistically, I want to do something like this:

select l.lid,l.fs,most_recent(h.obsvalue) from location as l
inner join height as h on h.lid = l.lid
where l.fs > 0.0
group by l.lid,l.fs;

Can someone offer hints, please?

Mark

Re: group by complications

From
chester c young
Date:
--- Mark Fenbers <Mark.Fenbers@noaa.gov> wrote:

> select l.lid,l.fs,max(h.obstime) from location as l
> inner join height as h on h.lid = l.lid
> where l.fs > 0.0
> group by l.lid,l.fs;
> 
> The above query works as expected in that is fetches the lid, fs and 
> time of the latest observation in the height table (for the 
> corresponding lid), but I also want to fetch (i.e., add to the select
> list) the corresponding reading (h.obsvalue) which occurs at 
> max(h.obstime).  I'm having trouble formulating the correct SQL
> syntax 
> to pull out the l.lid, l.fs, and the most recent h.obvalue (with or 
> without the time that it occurred).
> 
> Logistically, I want to do something like this:
> 
> select l.lid,l.fs,most_recent(h.obsvalue) from location as l
> inner join height as h on h.lid = l.lid
> where l.fs > 0.0
> group by l.lid,l.fs;
> 

use your original query as part of the from clause, then add columns to
it through a subquery or a join.  try something like this:

select q1.*,
(select obsvalue from height where lid=q1.lid and obstime=q1.obstime) as obsvalue
from
(select l.lid,l.fs,max(h.obstime) as obstime1 from location as l
inner join height as h on h.lid = l.lid
where l.fs > 0.0
group by l.lid,l.fs ) q1;


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Re: group by complications

From
Mark Fenbers
Date:
Wow!  I didn't know you could have a (select ...) as a replacement for a 'from' table/query.  Your SQL worked as-is,
exceptI had to add a 'limit 1' to the first subquery.<br /><br /> Thanks!  I would have never figured that out on my
own!<br/><br /> Mark<br /><br /> chester c young wrote: <blockquote
cite="mid20060214022937.77875.qmail@web54305.mail.yahoo.com"type="cite"><pre wrap="">--- Mark Fenbers <a
class="moz-txt-link-rfc2396E"href="mailto:Mark.Fenbers@noaa.gov"><Mark.Fenbers@noaa.gov></a> wrote:
 
 </pre><blockquote type="cite"><pre wrap="">select l.lid,l.fs,max(h.obstime) from location as l
inner join height as h on h.lid = l.lid
where l.fs > 0.0
group by l.lid,l.fs;

The above query works as expected in that is fetches the lid, fs and 
time of the latest observation in the height table (for the 
corresponding lid), but I also want to fetch (i.e., add to the select
list) the corresponding reading (h.obsvalue) which occurs at 
max(h.obstime).  I'm having trouble formulating the correct SQL
syntax 
to pull out the l.lid, l.fs, and the most recent h.obvalue (with or 
without the time that it occurred).

Logistically, I want to do something like this:

select l.lid,l.fs,most_recent(h.obsvalue) from location as l
inner join height as h on h.lid = l.lid
where l.fs > 0.0
group by l.lid,l.fs;
   </pre></blockquote><pre wrap="">
use your original query as part of the from clause, then add columns to
it through a subquery or a join.  try something like this:

select q1.*,
(select obsvalue from height where lid=q1.lid and obstime=q1.obstime) as obsvalue
from
(select l.lid,l.fs,max(h.obstime) as obstime1 from location as l
inner join height as h on h.lid = l.lid
where l.fs > 0.0
group by l.lid,l.fs ) q1;


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 </pre></blockquote>

Re: group by complications

From
Markus Schaber
Date:
Hi, Mark,

Mark Fenbers schrieb:
> Wow!  I didn't know you could have a (select ...) as a replacement for a
> 'from' table/query.  Your SQL worked as-is, except I had to add a 'limit
> 1' to the first subquery.
> 
> Thanks!  I would have never figured that out on my own!

SQL has more power than most think, and PostgreSQL does a good job in
implementing most of it. Much more than some other well known "free"
databases.

Have a look at http://www.postgresql.org/docs/8.1/static/sql-select.html
and the other SQL commands at
http://www.postgresql.org/docs/8.1/static/sql-commands.html (or your
local copy of the manual).

HTH,
Markus