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If any argument isNULL or any geometry argument is an empty geometry, the return value isNULL.

If any geometry argument is not a syntactically well-formed geometry, anER_GIS_INVALID_DATA error occurs.

If any geometry argument is a syntactically well-formed geometry in an undefined spatial reference system (SRS), anER_SRS_NOT_FOUND error occurs.

Otherwise, the return value is non-NULL.

ST_GeometryN(gc,N)

Returns theN-th geometry in theGeometryCollection valuegc. Geometries are numbered beginning with 1.

ST_GeometryN() 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)                                      |+-------------------------------------------------+

ST_NumGeometries(gc)

Returns the number of geometries in theGeometryCollection valuegc.

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_NumGeometries(ST_GeomFromText(@gc));+----------------------------------------+| ST_NumGeometries(ST_GeomFromText(@gc)) |+----------------------------------------+|                                      2 |+----------------------------------------+