ST_Area({poly|mpoly})

Returns a double-precision number indicating the area of the Polygon or
MultiPolygon argument, as measured in its spatial reference system. For
arguments of dimension 0 or 1, the result is 0. If the argument is an
empty geometry the return value is 0. If the argument is NULL the
return value is NULL.

The result is the sum of the area values of all components for a
geometry collection. If a geometry collection is empty, its area is
returned as 0.

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

