Envelope(g)

Returns the minimum bounding rectangle (MBR) for the geometry value g.
The result is returned as a Polygon value that is defined by the corner
points of the bounding box:

POLYGON((MINX MINY, MAXX MINY, MAXX MAXY, MINX MAXY, MINX MINY))

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

