ST_Centroid(mpoly)

Returns the mathematical centroid for the MultiPolygon value mpoly as a
Point. The result is not guaranteed to be on the MultiPolygon.

As of MySQL 5.7.5, 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.
If the argument is an empty geometry collection, the return value is
NULL.

URL: http://dev.mysql.com/doc/refman/5.7/en/gis-polygon-property-functions.html

