Polygon(ls1,ls2,...)

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

URL: http://dev.mysql.com/doc/refman/5.1/en/creating-spatial-values.html#gis-mysql-specific-functions

