Re: Getting current and average on a single row

On Wednesday 24 November 2010 1:08:27 am Mark Morgan Lloyd wrote:
> What is best practice when extracting both current and average from a
> table? Demonstration table here contains data from a cheap weather station.
>
> I can obviously get the current reading like this:
>
> select temp_out, dewpoint
> from weather
> where datetime between (now() - '10 minutes'::interval) and now()
> order by datetime desc
> limit 1;
>
> and I can get averages like this:
>
> select avg(temp_out) as avg_temp_out, avg(dewpoint) as avg_dewpoint
> from weather
> where datetime between (now() - '45 minutes'::interval) and now();
>
> In both cases there are a dozen or so columns in total. How are these
> best merged to yield a single row? Some form of join, or window functions?

I am not seeing a dozen columns, maybe rows?

I quick and dirty solution(testing needed):

select
temp_out,dewpoint,atbl.avg_temp_out,atbl.avg_dewpoint
from
(select avg(temp_out) as avg_temp_out, avg(dewpoint) as avg_dewpoint
from weather
where datetime between (now() - '45 minutes'::interval) and now()) as atbl,
weather
order by datetime desc limit 1;

>
> --
> Mark Morgan Lloyd
> markMLl .AT. telemetry.co .DOT. uk
>
> [Opinions above are the author's, not those of his employers or colleagues]

--
Adrian Klaver
adrian(dot)klaver(at)gmail(dot)com