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

