PostGIS
Name
ST_CollectionExtract — Given a geometry collection, returns a multi-geometry containing only elements of a specified type.
Synopsis
geometry +`*`+ST_CollectionExtract*(`geometry
`collection`)`;
geometry +`*`+ST_CollectionExtract*(`geometry `collection,
integer type`)`;
Description
Given a geometry collection, returns a homogeneous multi-geometry.
If the `type` is not specified, returns a multi-geometry containing only geometries of the highest dimension. So polygons are preferred over lines, which are preferred over points.
If the `type` is specified, returns a multi-geometry containing only that type. If there are no sub-geometries of the right type, an EMPTY geometry is returned. Only points, lines and polygons are supported. The type numbers are:
-
1 == POINT
-
2 == LINESTRING
-
3 == POLYGON
For atomic geometry inputs, the geometry is retured unchanged if the input type matches the requested type. Otherwise, the result is an EMPTY geometry of the specified type. If required, these can be converted to multi-geometries using ST_Multi.
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Availability: 1.5.0
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Examples
Extract highest-dimension type:
SELECT ST_AsText(ST_CollectionExtract(
'GEOMETRYCOLLECTION( POINT(0 0), LINESTRING(1 1, 2 2) )'));
st_astext
---------------
MULTILINESTRING((1 1, 2 2))Extract points (type 1 == POINT):
SELECT ST_AsText(ST_CollectionExtract(
'GEOMETRYCOLLECTION(GEOMETRYCOLLECTION(POINT(0 0)))',
1 ));
st_astext
---------------
MULTIPOINT((0 0))Extract lines (type 2 == LINESTRING):
SELECT ST_AsText(ST_CollectionExtract(
'GEOMETRYCOLLECTION(GEOMETRYCOLLECTION(LINESTRING(0 0, 1 1)),LINESTRING(2 2, 3 3))',
2 ));
st_astext
---------------
MULTILINESTRING((0 0, 1 1), (2 2, 3 3))