Polygon(ls1,ls2,...)

Constructs a Polygon value from a number of LineString or WKB
LineString arguments. If any argument does not represent a LinearRing
(that is, not a closed and simple LineString), the return value is
NULL.

URL: http://dev.mysql.com/doc/refman/5.5/en/creating-spatial-values.html#gis-mysql-specific-functions

