Geometry Property Functions • ST_Centroid({ poly | mpoly }) Returns the mathematical centroid for the  Polygon  or  MultiPolygon  argument as a  Point . The result is not guaranteed to be on the  MultiPolygon . This function processes geometry collections by computing the centroid point for components of highest dimension in the collection. Such components are extracted and made into a single  MultiPolygon , MultiLineString , or  MultiPoint  for centroid computation. ST_Centroid()  handles its arguments as described in the introduction to this section, with these exceptions: Returns the exterior ring of the  Polygon  value  poly  as a  LineString . ST_ExteriorRing()  handles its arguments as described in the introduction to this section. Returns the  N -th interior ring for the  Polygon  value  poly  as a  LineString . Rings are numbered beginning with 1. ST_InteriorRingN()  handles its arguments as described in the introduction to this section. mysql>  SET @poly = ’Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))’; mysql>  SELECT ST_AsText(ST_InteriorRingN(ST_GeomFromText(@poly),1)); +-------------------------------------------------------+ | ST_AsText(ST_InteriorRingN(ST_GeomFromText(@poly),1)) | +-------------------------------------------------------+ | LINESTRING(1 1,1 2,2 2,2 1,1 1) | +-------------------------------------------------------+ mysql>  SET @poly = ’Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))’; mysql>  SELECT ST_AsText(ST_ExteriorRing(ST_GeomFromText(@poly))); +----------------------------------------------------+ | ST_AsText(ST_ExteriorRing(ST_GeomFromText(@poly))) | +----------------------------------------------------+ | LINESTRING(0 0,0 3,3 3,3 0,0 0) | +----------------------------------------------------+ •The return value is  NULL  for the additional condition that the argument is an empty geometry collection. •If the geometry has an SRID value for a geographic spatial reference system (SRS), an ER_NOT_IMPLEMENTED_FOR_GEOGRAPHIC_SRS  error occurs. mysql>  SET @poly = ST_GeomFromText(’POLYGON((0 0,10 0,10 10,0 10,0 0),(5 5,7 5,7 7,5 7,5 5))’); mysql>  SELECT ST_GeometryType(@poly),ST_AsText(ST_Centroid(@poly)); +------------------------+--------------------------------------------+ | ST_GeometryType(@poly) | ST_AsText(ST_Centroid(@poly)) | +------------------------+--------------------------------------------+ | POLYGON | POINT(4.958333333333333 4.958333333333333) | +------------------------+--------------------------------------------+ • ST_ExteriorRing( poly ) • ST_InteriorRingN( poly ,  N ) 2315