[postgis-users] Collecting Polygons

[Kevin Neufeld]

It merges all geometries you supply it.  If you supply geometries from 
different tables, then yes.

SELECT ST_Union(geom)
FROM (
   SELECT geom FROM mytable1 WHERE ...
   UNION ALL
   SELECT geom FROM mytable2 WHERE ...
) AS foo;

-- Kevin

Lee Hachadoorian wrote:
> Thanks, Kevin, that worked.  I saw ST_Union, but I thought it was for
> joining polygons from different spatial tables.  Does it do that also?
> 
> --Lee
> 
> On Fri, Jun 27, 2008 at 2:19 PM, Kevin Neufeld <kneufeld at refractions.net> wrote:
>> Hi Lee,
>>
>> If you want to merge several polygons together, dissolving them into a
>> single polygon, then you'll want to use ST_Union() instead of ST_Collect().
>>
>> ST_Union() - returns the point set union of all input geometries,
>> ST_Collect() - is a simple aggregate function that concatenates geometries
>> together, ie a bunch of polygons to a multipolygon.
>>
>> -- Kevin
>>
>> Lee Hachadoorian wrote:
>>> Upgraded to 1.3.3 to get rid of previous problem with ST_Collect, but
>>> I'm still not getting the output I want.  What I want to do is take
>>> several polygons and mash them together into one polygon.  The SQL I
>>> am using is:
>>>
>>> SELECT ST_Collect(the_geom)
>>> FROM proj_mortgage.geo_hh_race
>>> WHERE pct_hispanic > 50;
>>>
>>> But ST_Collect returns a GEOMETRYCOLLECTION, and I want a polygon or
>>> multipolygon.  Do I need to do something with the GEOMETRYCOLLECTION
>>> returned by ST_Collect, or is ST_Collect the wrong function to use in
>>> the first place?
>>>
>>> Thanks,
>>> Lee Hachadoorian
>>> PhD Student, Geography
>>> Program in Earth & Environmental Sciences
>>> CUNY Graduate Center
>>> _______________________________________________
>>> postgis-users mailing list
>>> postgis-users at postgis.refractions.net
>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
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> 
> 
>