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.

URL: http://dev.mysql.com/doc/refman/5.5/en/gis-polygon-property-functions.html

