PostGIS

ST_Collect
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ST_Collect

7.3. Geometry Constructors

Name

ST_Collect — Creates a GeometryCollection or Multi* geometry from a set of geometries.

Synopsis

+geometry +ST_Collect(+`geometry `+g1, geometry g2`)`;

+geometry +ST_Collect(+`geometry[] `+g1_array`)`;

+geometry +ST_Collect(+`geometry set `+g1field`)`;

Description

Collects geometries into a geometry collection. The result is either a Multi* or a GeometryCollection, depending on whether the input geometries have the same or different types (homogeneous or heterogeneous). The input geometries are left unchanged within the collection.

Variant 1: accepts two input geometries

Variant 2: accepts an array of geometries

Variant 3: aggregate function accepting a rowset of geometries.

If any of the input geometries are collections (Multi* or GeometryCollection) ST_Collect returns a GeometryCollection (since that is the only type which can contain nested collections). To prevent this, use ST_Dump in a subquery to expand the input collections to their atomic elements (see example below).

ST_Collect and ST_Union appear similar, but in fact operate quite differently. ST_Collect aggregates geometries into a collection without changing them in any way. ST_Union geometrically merges geometries where they overlap, and splits linestrings at intersections. It may return single geometries when it dissolves boundaries.

Availability: 1.4.0 - ST_Collect(geomarray) was introduced. ST_Collect was enhanced to handle more geometries faster.

check This function supports 3d and will not drop the z-index.

check This method supports Circular Strings and Curves.

Examples - Two-input variant

Collect 2D points.

SELECT ST_AsText( ST_Collect( ST_GeomFromText('POINT(1 2)'),
    ST_GeomFromText('POINT(-2 3)') ));

st_astext
----------
MULTIPOINT((1 2),(-2 3))

Collect 3D points.

SELECT ST_AsEWKT( ST_Collect( ST_GeomFromEWKT('POINT(1 2 3)'),
        ST_GeomFromEWKT('POINT(1 2 4)') ) );

        st_asewkt
-------------------------
 MULTIPOINT(1 2 3,1 2 4)

Collect curves.

SELECT ST_AsText( ST_Collect( 'CIRCULARSTRING(220268 150415,220227 150505,220227 150406)',
        'CIRCULARSTRING(220227 150406,2220227 150407,220227 150406)'));

        st_astext
------------------------------------------------------------------------------------
MULTICURVE(CIRCULARSTRING(220268 150415,220227 150505,220227 150406),
 CIRCULARSTRING(220227 150406,2220227 150407,220227 150406))

Examples - Array variant

Using an array constructor for a subquery.

SELECT ST_Collect( ARRAY( SELECT geom FROM sometable ) );

Using an array constructor for values.

SELECT ST_AsText(  ST_Collect(
        ARRAY[ ST_GeomFromText('LINESTRING(1 2, 3 4)'),
            ST_GeomFromText('LINESTRING(3 4, 4 5)') ] )) As wktcollect;

--wkt collect --
MULTILINESTRING((1 2,3 4),(3 4,4 5))

Examples - Aggregate variant

Creating multiple collections by grouping geometries in a table.

SELECT stusps, ST_Collect(f.geom) as geom
     FROM (SELECT stusps, (ST_Dump(geom)).geom As geom
                FROM
                somestatetable ) As f
    GROUP BY stusps