Envelope(g)

Returns the Minimum Bounding Rectangle (MBR) for the geometry value g.
The result is returned as a Polygon value.

The polygon 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/5.1/en/geometry-property-functions.html#general-geometry-property-functions

