ST_Area(poly)

Returns a double-precision number indicating the area of the argument,
as measured in its spatial reference system. For arguments of dimension
0 or 1, the result is 0.

Additionally, as of MySQL 5.7.5: 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: http://dev.mysql.com/doc/refman/5.7/en/gis-polygon-property-functions.html

