ST_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.

ST_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, ST_Buffer()
with a negative distance returns NULL.

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

