These functions return properties of 这些函数返回GeometryCollection values.GeometryCollection值的属性。
Unless otherwise specified, functions in this section handle their geometry arguments as follows:除非另有规定，否则本节中的函数按如下方式处理其几何参数：
If any argument is 如果任何参数为NULL or any geometry argument is an empty geometry, the return value is NULL.NULL或任何几何参数为空几何体，则返回值为NULL。
If any geometry argument is not a syntactically well-formed geometry, an 如果任一个几何体参数不是语法格式良好的几何体，则会发生ER_GIS_INVALID_DATA error occurs.ER_GIS_INVALID_DATA错误。
If any geometry argument is a syntactically well-formed geometry in an undefined spatial reference system (SRS), an 如果任一个几何参数是未定义空间参考系（SRS）中语法结构良好的几何体，则会发生ER_SRS_NOT_FOUND error occurs.ER_SRS_NOT_FOUND错误。
Otherwise, the return value is non-否则，返回值为非NULL.NULL。
These functions are available for obtaining geometry collection properties:这些函数可用于获取几何图形集合属性：
Returns the 返回N-th geometry in the GeometryCollection value gc. GeometryCollection值gc中的第N个几何体。Geometries are numbered beginning with 1.几何体从1开始编号。
ST_GeometryN() handles its arguments as described in the introduction to this section.ST_GeometryN()处理其参数，如本节简介中所述。
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) | +-------------------------------------------------+
Returns the number of geometries in the 返回GeometryCollection value gc.GeometryCollection值gc中的几何体数。
ST_NumGeometries() handles its arguments as described in the introduction to this section.ST_NumGeometries()处理其参数，如本节简介中所述。
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 | +----------------------------------------+