Обсуждение: query help
I need some help with rewriting a query. I have a fairly complicated query (for me anyway) that dumps daily climate data, filling in missing data with monthly averages (one line per day).
I want to output monthly averages (one line per month). I am having a hard time wrapping my head around this. Particularly how to deal with the doy column (day of year). I have tried several approaches and my forehead is starting to get my keyboard bloody.
Here is the daily query:
SELECT CASE
WHEN w.station_id = site_near.station_id THEN w.obs_id
ELSE s.obs_id
END AS obs_id,
site_near.station_id,
site_near.longname,
w.year,
w.doy,
--replace missing values (-999) with the monthly average
CASE w.tmax
WHEN -999 THEN avgtmax.avg
ELSE w.tmax
END,
CASE w.tmin
WHEN -999 THEN avgtmin.avg
ELSE w.tmin
END,
CASE s.par
WHEN -999 THEN avgpar.avg
ELSE s.par
END,
CASE w.precip
WHEN -999 THEN avgprecip.avg
ELSE w.precip
END
FROM site_near
INNER JOIN solar s
ON (site_near.ref_solar_station_id = s.station_id
AND site_near.obs_year = s.year)
INNER JOIN weather w
ON (site_near.ref_weather_station_id = w.station_id
AND site_near.obs_year = w.year
AND s.date = w.date)
INNER JOIN (SELECT MONTH,
round(avg(tmax)::numeric, 2) AS avg
FROM weather
WHERE tmax != -999
GROUP BY MONTH) AS avgtmax
ON (w.month = avgtmax.month)
INNER JOIN (SELECT MONTH,
round(avg(tmin)::numeric, 2) AS avg
FROM weather
WHERE tmin != -999
GROUP BY MONTH) AS avgtmin
ON (w.month = avgtmin.month)
INNER JOIN (SELECT MONTH,
round(avg(par)::numeric, 2) AS avg
FROM solar
WHERE par != -999
GROUP BY MONTH) AS avgpar
ON (s.month = avgpar.month)
INNER JOIN (SELECT MONTH,
round(avg(precip)::numeric, 2) AS avg
FROM weather
WHERE precip != -999
GROUP BY MONTH) AS avgprecip
ON (w.month = avgprecip.month)
--select station to output climate data by id number
WHERE w.station_id = 219101
On 8/14/07, Kirk Wythers <kwythers@umn.edu> wrote:
>
> I need some help with rewriting a query. I have a fairly complicated query
> (for me anyway) that dumps daily climate data, filling in missing data with
> monthly averages (one line per day).
>
> I want to output monthly averages (one line per month). I am having a hard
> time wrapping my head around this. Particularly how to deal with the doy
> column (day of year). I have tried several approaches and my forehead is
> starting to get my keyboard bloody.
I think this came up on IRC today, so perhaps this is only for the
archives' sake, but you want to do something like this:
Assuming you have a table as follows:
CREATE TABLE climate_data (
measurement_time timestamp,
measurement_value integer);
...and you insert data into it regularly, you can get the average
measurement over a period of time with date_trunc(), which will
truncate a date or timestamp value to match whatever precision you
specify. For example, see the following:
eggyknap=# select date_trunc('month', now());
date_trunc
------------------------
2007-08-01 00:00:00-06
(1 row)
Note: the -06 at the end means I'm in mountain time.
So if you want to get the average measurement over a month's time, you
need to do something like this:
SELECT DATE_TRUNC('MONTH', measurement_time), AVG(measurement_value)
FROM climate_data GROUP BY DATE_TRUNC('MONTH', measurement_time);
This will chop all the measurement_time values down to the month the
measurement was taken in, put all measurements in groups based on the
resulting value, and take the average measurement_value from each
group.
- Josh