[postgis-users] Merging two features

[Brent Wood]

--- Stephen Woodbridge <woodbri at swoodbridge.com> wrote:


I think that should be geomunion, not union, in the SQL.

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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