ST_Centroid({poly|mpoly})

Returns the mathematical centroid for the Polygon or MultiPolygon
argument as a Point. The result is not guaranteed to be on the
MultiPolygon. If the argument is NULL or an empty geometry, the return
value is NULL.

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

