[postgis-users] Re: Aggregate?

[Paul Ramsey]

Stephen Crawford wrote:

> I don't quite understand this part:
> "Use collect(geometryn(the_geom, 1)) id your data is specified as singular
> multigeometries."

If you've loaded your data from shape files, then your POLYGON features 
will all be MULTIPOLYGON by default, even if they all have only one 
part. So when you Collect() them, you'll get a GEOMETRYCOLLECTION 
instead of a MULTIPOLYGON.

GeometryN(the_geom,1) turns one-part MULTIPOLYGONS into simple POLYGONS, 
so that when you Collect() the result is a MULTIPOLYGON.

> Also, if I want to keep a few of the record for based on another another
> value, should this work?:
> 
> SELECT name, party, collect(the_geom)) AS the_geom,
>     FROM constituencies
>    GROUP BY constituencies.name, constituencies.party;

This will return one collection for each distinct combination of name 
and party.

Paul

-- 

   Paul Ramsey
   Refractions Research
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   pramsey at refractions.net
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