MySQL Function to get distance between 2 locations (lat,lon) in KM

MySQL distance

DELIMITER $$
DROP FUNCTION IF EXISTS d $$
CREATE FUNCTION d(lat1 FLOAT, lon1 FLOAT, lat2 FLOAT, lon2 FLOAT) RETURNS FLOAT
DETERMINISTIC
BEGIN 
	DECLARE R FLOAT;
	DECLARE rad FLOAT;
	DECLARE a FLOAT;
	DECLARE d FLOAT;

	SET R = 6378137;
	SET rad = 3.14159265359 / 180;
  
	SET lat1 = lat1 * rad;
	SET lat2 = lat2 * rad;
	SET a = SIN(lat1) * SIN(lat2) + COS(lat1) * COS(lat2) * COS((lon2 - lon1) * rad);
	SET d = ROUND((R * ACOS(LEAST(a, 1)))/1000);
	
	RETURN d;

END$$
DELIMITER ;

Based on Leaflet.js distance function https://github.com/Leaflet/Leaflet/blob/master/src/geo/crs/CRS.Earth.js