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)

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.

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