Buffer(g,d)

Returns a geometry that represents all points whose distance from the
geometry value g is less than or equal to a distance of d.

Buffer() supports negative distances for polygons, multipolygons, and
geometry collections containing polygons or multipolygons. For point,
multipoint, linestring, multilinestring, and geometry collections not
containing any polygons or multipolygons, Buffer() with a negative
distance returns NULL.

Buffer() was added in MySQL 5.6.1.

URL: http://dev.mysql.com/doc/refman/5.6/en/spatial-operator-functions.html

