ST_ExteriorRing
ST_ExteriorRing returns a closed linestring that represents the exterior ring of an input polygon. The dimension of the returned geometry is the same as that of the input geometry.
Syntax
ST_ExteriorRing(geom)
Arguments
- geom
-
A value of data type
GEOMETRY
or an expression that evaluates to aGEOMETRY
type.
Return type
GEOMETRY
of subtype LINESTRING
.
The spatial reference system identifier (SRID) value of the returned geometry is the SRID value of the input geometry.
If geom is null, then null is returned.
If geom is not a polygon, then null is returned.
If geom is empty, then an empty polygon is returned.
Examples
The following SQL returns the exterior ring of a polygon as a closed linestring.
SELECT ST_AsEWKT(ST_ExteriorRing(ST_GeomFromText('POLYGON((7 9,8 7,11 6,15 8,16 6,17 7,17 10,18 12,17 14,15 15,11 15,10 13,9 12,7 9),(9 9,10 10,11 11,11 10,10 8,9 9),(12 14,15 14,13 11,12 14))')));
st_asewkt
-----------
LINESTRING(7 9,8 7,11 6,15 8,16 6,17 7,17 10,18 12,17 14,15 15,11 15,10 13,9 12,7 9)