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ST_Union

Returns the union (combination) of two geometries.

Syntax​

ST_Union(geometry_a, geometry_b)

Parameters​

ParameterTypeDescription
geometry_aGeometryFirst geometry
geometry_bGeometrySecond geometry

Returns​

TypeDescription
Binary (WKB)Union geometry as WKB with geoarrow.wkb metadata

Examples​

Union of Overlapping Polygons​

SELECT ST_Union(
    ST_GeomFromText('POLYGON((0 0, 2 0, 2 2, 0 2, 0 0))'),
    ST_GeomFromText('POLYGON((1 1, 3 1, 3 3, 1 3, 1 1))')
);
-- Returns: combined L-shaped polygon

Union of Points​

SELECT ST_Union(ST_Point(0, 0), ST_Point(1, 1));
-- Returns: MULTIPOINT(0 0, 1 1)

Merge Adjacent Parcels​

SELECT ST_Union(a.geom, b.geom) as merged
FROM parcels a, parcels b
WHERE a.id = 'A' AND b.id = 'B';

Dissolve Boundaries​

SELECT category, ST_Union(geom) as dissolved
FROM regions
GROUP BY category;

Notes​

  • Merges overlapping areas into a single geometry
  • For non-overlapping geometries, returns a Multi* type
  • Result may be a GeometryCollection if input types differ
  • Use in GROUP BY for dissolving many geometries

See Also​