描述

傳回代表 POLYGON 外環（外殼）的 LINESTRING。如果幾何不是 Polygon，則傳回 NULL。

此方法實作了 OGC 簡單功能實作規範 SQL 1.1。 2.1.5.1

此方法實作了 SQL/MM 規範。SQL-MM 3: 8.2.3, 8.3.3

此函數支援 3D 並不會捨棄 z 索引。

範例

--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)

另請參閱

ST_InteriorRingN, ST_Boundary, ST_NumInteriorRings