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ST_Union

Signatures

GEOMETRY ST_Union(GEOMETRY geomA);
GEOMETRY ST_Union(GEOMETRY geomA, GEOMETRY geomB);
GEOMETRY ST_Union(GEOMETRYCOLLECTION geom);

Description

ST_Union can be used as a scalar or an aggregate function.

Scalar function

Computes the union of one or more Geometries. Input Geometries can be (MULTI)POINTs, (MULTI)LINESTRINGs, (MULTI)POLYGONs or GEOMETRYCOLLECTIONs.

Aggregate function

Computes the union of a set of Geometries. geom is a GEOMETRYCOLLECTION resulting from an ST_Accum operation on a table.

In both cases:
  • If no input Geometriy is given, the result is NULL.
  • Output Geometries can be single or multiple.
Implements the OpenGIS Simple Features Implementation Specification for SQL version 1.2.1.

Examples

Scalar function
geomA Polygon geomB Polygon
POLYGON((1 1, 7 1, 7 6, 1 6, 1 1)) POLYGON((3 2, 8 2, 8 8, 3 8, 3 2)) 
SELECT ST_Union(geomA, geomB) FROM input_table;
-- Answer: POLYGON((7 2, 7 1, 1 1, 1 6, 3 6, 3 8, 8 8, 8 2, 7 2))

Aggregate function
CREATE TABLE input_table(geom POLYGON);
INSERT INTO input_table VALUES
     ('POLYGON((1 1, 7 1, 7 6, 1 6, 1 1))'),
     ('POLYGON((3 2, 8 2, 8 8, 3 8, 3 2))'),
     ('POLYGON((1 7, 2 7, 2 8, 1 8, 1 7))');
SELECT ST_Union(ST_Accum(geom)) FROM input_table;
-- Answer: MULTIPOLYGON(((7 2, 7 1, 1 1, 1 6, 3 6, 3 8, 8 8, 8 2, 7 2)),
--                       ((1 7, 2 7, 2 8, 1 8, 1 7)))

See also