MySQL 8.0

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SELECT ((ST_Distance_Sphere(POINT(0, 0), POINT(180, 0))) -- half_the_circumference
       *2 -- doubling to get the full cirumference
       )/(2*pi()) -- 2*pi*r is the circumference, so the circumference/2*pi is the radius
       AS mySQL_assumed_radius;

mySQL_assumed_radius

6370986

-- better yet, without the doubling
SELECT ST_Distance_Sphere(ST_SRID(POINT(0, 0),4326), 
                          ST_SRID(POINT(180, 0),4326)) -- half_the_circumference
       /pi() -- 2*pi*r is the circumference, so half the circumference/pi is the radius
       AS mySQL_assumed_radius_4326;

mySQL_assumed_radius_4326

6371008.771415059

-- better yet, without the doubling
SELECT ST_Distance_Sphere(POINT(0, 0), POINT(180, 0)) -- half_the_circumference
       /pi() -- 2*pi*r is the circumference, so half the circumference/pi is the radius
       AS mySQL_assumed_radius;

mySQL_assumed_radius

6370986

SELECT ST_Distance_Sphere(
  POINT(0, 90), 
  POINT(0, -90)
) AS latitudinal_distance_pole_to_pole;
SELECT ST_Distance_Sphere(
  POINT(0, 0), 
  POINT(180, 0)
) AS longitudinal_around_half_the_globe;

latitudinal_distance_pole_to_pole

20015042.813723423

longitudinal_around_half_the_globe

20015042.813723423

SELECT ST_Distance_Sphere(
  POINT(-74.0060, 40.7128), -- New York
  POINT(-118.2437, 34.0522) -- Los Angeles
) AS distance;

distance

3935737.605975668

SELECT ST_Distance_Sphere(
  ST_SRID(POINT(-74.0060, 40.7128),4326),-- New York
  ST_SRID(POINT(-118.2437, 34.0522),4326) -- Los Angeles
) AS distance;

distance

3935751.6732353647

SELECT ST_Distance_Sphere(
  ST_SRID(POINT(-74.0060, 40.7128),4326),-- New York
  ST_SRID(POINT(-118.2437, 34.0522),4326), -- Los Angeles
  6378100 -- custom radius, conforming to MongoDB's config
) AS distance;

distance

3940132.3475947683