ST_Envelope(g)

Returns the minimum bounding rectangle (MBR) for the geometry value g,
or NULL if the argument is NULL. 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/5.6/en/gis-general-property-functions.html

