ST_ExteriorRing — 返回表示多边形外环的线串。
geometry ST_ExteriorRing(
geometry a_polygon)
;
返回表示多边形的外环(壳)的LINESTRING。如果几何图形不是多边形,则返回NULL。
此函数不支持MULTIPOLYGON。对于与一起使用的MULTIPOLYGON ST_GeometryN 或 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.
--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)