Geometry Property Functions • ST_NumInteriorRing( poly ) ,  ST_NumInteriorRings( poly ) Returns the number of interior rings in the  Polygon  value  poly . ST_NumInteriorRing()  and  ST_NuminteriorRings()  handle their arguments as described in the introduction to this section. These functions return properties of  GeometryCollection  values. Unless otherwise specified, functions in this section handle their geometry arguments as follows: 12.17.7.5 GeometryCollection Property Functions 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_NumInteriorRings(ST_GeomFromText(@poly)); +---------------------------------------------+ | ST_NumInteriorRings(ST_GeomFromText(@poly)) | +---------------------------------------------+ | 1 | +---------------------------------------------+ •If any argument is  NULL  or any geometry argument is an empty geometry, the return value is  NULL . •If any geometry argument is not a syntactically well-formed geometry, an  ER_GIS_INVALID_DATA  error occurs. •If any geometry argument is a syntactically well-formed geometry in an undefined spatial reference system (SRS), an  ER_SRS_NOT_FOUND  error occurs. •Otherwise, the return value is non- NULL . • ST_GeometryN( gc ,  N ) These functions are available for obtaining geometry collection properties: Returns the  N -th geometry in the  GeometryCollection  value  gc . Geometries are numbered beginning with 1. ST_GeometryN()  handles its arguments as described in the introduction to this section. Returns the number of geometries in the  GeometryCollection  value  gc . ST_NumGeometries()  handles its arguments as described in the introduction to this section. mysql>  SET @gc = ’GeometryCollection(Point(1 1),LineString(2 2, 3 3))’; mysql>  SELECT ST_AsText(ST_GeometryN(ST_GeomFromText(@gc),1)); +-------------------------------------------------+ | ST_AsText(ST_GeometryN(ST_GeomFromText(@gc),1)) | +-------------------------------------------------+ | POINT(1 1) | +-------------------------------------------------+ mysql>  SET @gc = ’GeometryCollection(Point(1 1),LineString(2 2, 3 3))’; mysql>  SELECT ST_NumGeometries(ST_GeomFromText(@gc)); +----------------------------------------+ | ST_NumGeometries(ST_GeomFromText(@gc)) | +----------------------------------------+ | 2 | +----------------------------------------+ • ST_NumGeometries( gc ) 2316