ST_Distance_Sphere(g1, g2 [, radius])

Returns the mimimum spherical distance between two points and/or
multipoints on a sphere, in meters, or NULL if any geometry argument is
NULL or empty.

Calculations use a spherical earth and a configurable radius. The
optional radius argument should be given in meters. If omitted, the
default radius is 6,370,986 meters. An ER_WRONG_ARGUMENTS error occurs
if the radius argument is present but not positive.

The geometry arguments should consist of points that specify
(longitude, latitude) coordinate values:

o Longitude and latitude are the first and second coordinates of the
  point, respectively.

o Both coordinates are in degrees.

o Longitude values must be in the range (-180, 180]. Positive values
  are east of the prime meridian.

o Latitude values must be in the range [-90, 90]. Positive values are
  north of the equator.

Supported argument combinations are (Point, Point), (Point,
MultiPoint), and (MultiPoint, Point). An ER_GIS_UNSUPPORTED_ARGUMENT
error occurs for other combinations.

An ER_GIS_INVALID_DATA error occurs if any geometry argument is not a
valid geometry byte string.

URL: http://dev.mysql.com/doc/refman/5.7/en/spatial-convenience-functions.html

