ST_ExteriorRing
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Name

ST_ExteriorRing — Devuelva el número de anillos interiores de una geometría poligonal.

Synopsis

geometry ST_ExteriorRing(geometry a_polygon);

Descripción

Devuelve una linestring representando el anillo exterior de una geometria tipo POLYGON. Devuelve NULL si la geometria no es un poligono. No funcionará con MULTIPOLYGON

[Note]

Esta función no admite MULTIPOLÍGONOS. Para MULTIPOLÍGONOS, utilícela junto con ST_GeometryN o ST_Dump

This method implements the OGC Simple Features Implementation Specification for SQL 1.1. 2.1.5.1

This method implements the SQL/MM specification. SQL-MM 3: 8.2.3, 8.3.3

This function supports 3d and will not drop the z-index.

Ejemplos

--If you have a table of polygons
SELECT gid, ST_ExteriorRing(geom) AS ering
FROM sometable;

--If you have a table of MULTIPOLYGONs
--and want to return a MULTILINESTRING composed of the exterior rings of each polygon
SELECT gid, ST_Collect(ST_ExteriorRing(geom)) AS erings
        FROM (SELECT gid, (ST_Dump(geom)).geom As geom
                        FROM sometable) As foo
GROUP BY gid;

--3d Example
SELECT ST_AsEWKT(
        ST_ExteriorRing(
        ST_GeomFromEWKT('POLYGON((0 0 1, 1 1 1, 1 2 1, 1 1 1, 0 0 1))')
        )
);

st_asewkt
---------
LINESTRING(0 0 1,1 1 1,1 2 1,1 1 1,0 0 1)