MBRWithin(g1,g2)

Returns 1 or 0 to indicate whether the Minimum Bounding Rectangle of g1
is within the Minimum Bounding Rectangle of g2. This tests the opposite
relationship as MBRContains().

URL: http://dev.mysql.com/doc/refman/5.0/en/functions-for-testing-spatial-relations-between-geometric-objects.html#relations-on-geometry-mbr

