MySQL function to calculate the distance between two coordinates using the Haversine formula. Leaving it here for future reference.

distance.sql

DELIMITER $$
CREATE FUNCTION `haversine` (lat1 DECIMAL(8,6), lng1 DECIMAL(8,6), lat2 DECIMAL(8,6), lng2 DECIMAL(8,6)) RETURNS DECIMAL(8,6)
BEGIN
    DECLARE R INT;
    DECLARE dLat DECIMAL(30,15);
    DECLARE dLng DECIMAL(30,15);
    DECLARE a1 DECIMAL(30,15);
    DECLARE a2 DECIMAL(30,15);
    DECLARE a DECIMAL(30,15);
    DECLARE c DECIMAL(30,15);
    DECLARE d DECIMAL(30,15);

    SET R = 3959; -- Earth's radius in miles
    SET dLat = RADIANS( lat2 ) - RADIANS( lat1 );
    SET dLng = RADIANS( lng2 ) - RADIANS( lng1 );
    SET a1 = SIN( dLat / 2 ) * SIN( dLat / 2 );
    SET a2 = SIN( dLng / 2 ) * SIN( dLng / 2 ) * COS( RADIANS( lng1 )) * COS( RADIANS( lat2 ) );
    SET a = a1 + a2;
    SET c = 2 * ATAN2( SQRT( a ), SQRT( 1 - a ) );
    SET d = R * c;
    RETURN d;
END$$

DELIMITER ;

R=6371; -- Earth's radius in Km
R=6371000; -- Earth's radius in metres