描述

返回表示多边形外环的线串。 如果几何图形不是多边形，则返回 NULL。

此方法实现了 SQL 1.1 的 OGC 简单功能规范。 2.1.5.1

该方法实现了SQL/MM规范。 SQL-MM 3: 8.2.3, 8.3.3

该函数支持 3d 并且不会丢失 z-index。

示例

--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)

相关信息

ST_InteriorRingN, ST_Boundary, ST_NumInteriorRings