Create a function to calculate the distance between two points on the earth. This function assumes the input to be decimal degrees and the earth to be a sphere with a radius of 6371 km

distance.sql

CREATE FUNCTION distance (lat1 REAL, lng1 REAL, lat2 REAL, lng2 REAL)	
RETURNS REAL DETERMINISTIC
RETURN 2 * 6371 * ASIN(SQRT(
	SIN(RADIANS(lat1 - lat2)/2) * SIN(RADIANS(lat1 - lat2)/2) +
	COS(RADIANS(lat1)) * COS(RADIANS(lat2)) * 
	SIN(RADIANS(lng1 - lng2)/2) * SIN(RADIANS(lng1 - lng2)/2)
));