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ST_GeometryN

Signature

GEOMETRY ST_GeometryN(GEOMETRY geom, integer n);

Description

Returns the nth Geometry of geom if geom is a GEOMETRYCOLLECTION, MULTIPOINT, MULTILINESTRING or MULTIPOLYGON. Returns NULL if geom is a single Geometry.

Do I start counting from 0 or 1?

The index runs from 1 to N.

Implements the OpenGIS Simple Features Implementation Specification for SQL version 1.2.1.

Examples

SELECT ST_GeometryN('MULTIPOLYGON(((0 0, 3 -1, 1.5 2, 0 0)),
                                  ((1 2, 4 2, 4 6, 1 6, 1 2)))', 1);
-- Answer: POLYGON((0 0, 3 -1, 1.5 2, 0 0))

SELECT ST_GeometryN('MULTILINESTRING((1 1, 1 6, 2 2, -1 2),
                                     (1 2, 4 2, 4 6))', 2);
-- Answer: LINESTRING(1 2, 4 2, 4 6)

SELECT ST_GeometryN('MULTIPOINT((0 0), (1 6), (2 2), (1 2))', 2);
-- Answer: POINT(1 6)

SELECT ST_GeometryN('GEOMETRYCOLLECTION(
                       MULTIPOINT((4 4), (1 1), (1 0), (0 3)),
                       LINESTRING(2 6, 6 2),
                       POINT(4 4),
                       POLYGON((1 2, 4 2, 4 6, 1 6, 1 2)))', 3);
-- Answer: POINT(4 4)

-- Select Geometry 4 of the first Geometry in this GEOMETRYCOLLECTION.
SELECT ST_GeometryN(
           ST_GeometryN('GEOMETRYCOLLECTION(
                           MULTIPOINT((4 4), (1 1), (1 0), (0 3)),
                           LINESTRING(2 6, 6 2))', 1), 4);
-- Answer: POINT(0 3)

-- Returns NULL for single Geometries.
SELECT ST_GeometryN('LINESTRING(1 1, 1 6, 2 2, -1 2)', 1);
-- Answer: NULL

SELECT ST_GeometryN('MULTIPOINT((0 0), (1 6), (2 2), (1 2))', 0);
-- Answer: GEOMETRY index out of range. Must be between 1 and
-- ST_NumGeometries.
See also