ST_ExteriorRing

Signature

LINEARRING ST_ExteriorRing(GEOMETRY geom);

Description

Returns the exterior ring of geom as a LINEARRING, or NULL if geom is not a POLYGON.

Warning

This function does not work on MULTIPOLYGONs

Note

Implements the OpenGIS Simple Features Implementation Specification for SQL version 1.2.1

Examples

SELECT ST_ExteriorRing('POLYGON((0 -1, 0 2, 3 2, 3 -1, 0 -1))');
-- Answer: LINEARRING(0 -1, 0 2, 3 2, 3 -1, 0 -1)

SELECT ST_ExteriorRing('MULTIPOLYGON(((0 0, 10 0, 5 5, 0 0)),
                                      ((10 0, 5 5, 10 10, 10 0)))');
-- Answer: NULL

SELECT ST_ExteriorRing('POINT(1 2)');
-- Answer: NULL

Comparison with `ST_InteriorRingN`

SELECT ST_ExteriorRing('POLYGON((0 0, 10 0, 10 6, 0 6, 0 0),
                                (1 1, 2 1, 2 5, 1 5, 1 1),
                                (8 5, 8 4, 9 4, 9 5, 8 5))') ER,
       ST_InteriorRingN('POLYGON((0 0, 10 0, 10 6, 0 6, 0 0),
                                (1 1, 2 1, 2 5, 1 5, 1 1),
                                (8 5, 8 4, 9 4, 9 5, 8 5))', 2) IRN;

Answer:

ER	IRN
LINEARRING(0 0, 10 0, 10 6, 0 6, 0 0)	LINEARRING(8 5, 8 4, 9 4, 9 5, 8 5)

Comparison with `ST_Boundary`

SELECT ST_ExteriorRing('POLYGON((0 0, 10 0, 10 6, 0 6, 0 0),
                                (1 1, 2 1, 2 5, 1 5, 1 1))') ER,
       ST_Boundary('POLYGON((0 0, 10 0, 10 6, 0 6, 0 0),
                             (1 1, 2 1, 2 5, 1 5, 1 1))') BDRY;

Answer:

ER	BDRY
LINEARRING(0 0, 10 0, 10 6, 0 6, 0 0)	MULTILINESTRING( (0 0, 10 0, 10 6, 0 6, 0 0), (1 1, 2 1, 2 5, 1 5, 1 1))