[postgis-users] Merging two features

[Stephen Woodbridge]

Brent Wood wrote:
> --- Stephen Woodbridge <woodbri at swoodbridge.com> wrote:
> 
> 
> I think that should be geomunion, not union, in the SQL.

Right, good catch. Thanks.

> Brent Wood
> 
> 
>> Jeff Dege wrote:
>>> I have a shapefile that contains a few duplicated features.
>>>
>>> They have identical attribute data, different OGRFeature numbers, and
>>> different geometries.
>>>
>>> I've imported them into PostGIS, but having two different records for
>>> the same feature is causing some problems.
>>>
>>> The schema is simple:
>>>
>>>    CREATE TABLE "public"."counties" (gid serial PRIMARY KEY,
>>>    "state" varchar(2),
>>>    "name" varchar(18),
>>>    "wtr" int2,
>>>    "fips" int4);
>>>    SELECT
>>> AddGeometryColumn('public','counties','the_geom','4267','MULTIPOLYGON',2
>>> );
>>>
>>> The duplicates are easy to find:
>>>
>>>    select state, name, count(*)
>>>    from counties
>>>    group by state, name
>>>    having count(*) > 1
>>>    ;
>>>
>>> What I would like to do is to merge these duplicate records into a
>>> single record, containing a union of the separate records' geometries.
>>> But I'm not sure of exactly how to approach it.
>> Jeff,
>>
>> Create a temp table
>>
>> insert into temp (select state, name, wtr, fips, union(the_geom) from 
>> countries group by states, name having count(*) > 1 );
>>
>> check that it did what you expected, then delete the records by state, 
>> name and insert the temp table records into the original table.
>>
>> or you can alter you table to add a column which is used to track the 
>> unioned records when they are added so you can delete the non-unioned 
>> records that you don't want. You can alter the table to drop the column 
>> when you are done.
>>
>> -Steve
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> 
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