Thread: sql or pgsql question, accessing a created value

Hope someone's out there for this one.  Basically I'm creating a summary table of many
underlying tables in one select statement ( though that may have to change ).  My problem
can be shown in this example..

select my_function( timeofmeasurement, longitude ) as solarnoon,
        extract(epoch from ( timeofmeasurement - solarnoon ) as solardiff
        ( case when solardiff < 3600 then 'Y' else 'N' end ) as within_solar_hour
from
    my_table;

But I get an error along the lines of
ERROR:  column "solarnoon" does not exist
LINE 8:  extract(epoch from (timeofmeasurement - solarnoon) ) as sola...

It's probably a compile-time run-time sort of chicken and egg thing. ;)

So I' off onto pl/pgsql, but still not having much luck.  Full under construction
sql right now is:



create or replace function load_air_temp_summary()
returns void as $$
declare solarnoon timestamp;
         solardiff interval;
BEGIN
select count(*) from (
select
  aird.current_temp, aird.minimum_temp, aird.measured_at,
  subd.datum_id, subd.datum_type,
  subm.person_id, subm.site_id,
  loc.latitude, loc.longitude,
  select solarnoon( aird.measured_at, loc.longitude ) INTO solarnoon  <-- ** trying to save the value
  from air_temp_data aird,
      submission_data subd,
      submissions subm,
      sites sites,
      locations loc
where
  subd.datum_type = 'AirTempDatum' and
  subd.datum_id = aird.id and
  subd.submission_id = subm.id and
  subm.site_id = sites.id and
  loc.locatable_type = 'Site' and
  sites.id = loc.locatable_id
) as fred;
END $$ LANGUAGE plpgsql;

but it dislikes the third "select" stmt, or if I remove that select stmt, I get

ERROR:  syntax error at or near "("
LINE 1: ...d, subm.site_id, loc.latitude, loc.longitude,  $1 ( aird.mea...


Any tips or tricks on how I should approach this are appreciated. How do I store
and use values that are calculated on the fly.

-ds

select my_function( timeofmeasurement, longitude ) as solarnoon,
        extract(epoch from ( timeofmeasurement - solarnoon ) as solardiff
        ( case when solardiff < 3600 then 'Y' else 'N' end ) as
within_solar_hour from
    my_table;

But I get an error along the lines of
ERROR:  column "solarnoon" does not exist LINE 8:  extract(epoch from
(timeofmeasurement - solarnoon) ) as sola...

It's probably a compile-time run-time sort of chicken and egg thing. ;)

>>>>>>>>>>>>>>>>>>>>>>>>>

It is.  You need to use sub-selects.

SELECT solarnoon, solardiff, CASE... AS within_solar_hour
FROM
SELECT solarnoon, func() AS solardiff
FROM (
SELECT func() AS solarnoon
) AS sn -- close solarnoon from
) AS sd -- close solardiff from


David J.

On 11/07/2011 20:19, David Salisbury wrote:
>
> Hope someone's out there for this one. Basically I'm creating a summary
> table of many
> underlying tables in one select statement ( though that may have to
> change ). My problem
> can be shown in this example..
>
> select my_function( timeofmeasurement, longitude ) as solarnoon,
> extract(epoch from ( timeofmeasurement - solarnoon ) as solardiff
> ( case when solardiff < 3600 then 'Y' else 'N' end ) as within_solar_hour
> from
> my_table;
>
> But I get an error along the lines of
> ERROR: column "solarnoon" does not exist
> LINE 8: extract(epoch from (timeofmeasurement - solarnoon) ) as sola...
>

One (slightly messy) way to do that is create another, outer layer of
SELECT - so your on-the-fly calculations are executed in the sub-select,
and the values are then available to the outer select. You have three
levels of dependency, so you'll need two subqueries:

<not tested>

select
   x.solarnoon,
   x.solardiff,
   (case when x.solardiff < 3600 then 'Y' else 'N' end) as
   within_solar_hour
from (
   select
     extract(epoch from (y.timeofmeasurement - y.solarnoon) as solardiff,
     y.timeofmeasurement
   from (
     select
       my_function(timeofmeasurement, longitude) as solarnoon,
       timeofmeasurement
     from
       my_table
   ) y
) x;

</not tested>

I think you can also do it more elegantly with a CTE; not something I've
played with yet, but you can read about it here:

   http://www.postgresql.org/docs/9.0/static/queries-with.html


HTH,

Ray.


--
Raymond O'Donnell :: Galway :: Ireland
rod@iol.ie