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 ;

The short version, tested in MySql 5.7

create function GetDistance(lat1 real, lng1 real, lat2 real, lng2 real) returns real no sql
return ATAN2(SQRT(POW(COS(RADIANS(lat2)) * SIN(RADIANS(lng1 - lng2)), 2) +
POW(COS(RADIANS(lat1)) * SIN(RADIANS(lat2)) - SIN(RADIANS(lat1)) *
COS(RADIANS(lat2)) * COS(RADIANS(lng1 - lng2)), 2)),
(SIN(RADIANS(lat1)) * SIN(RADIANS(lat2)) + COS(RADIANS(lat1)) *
COS(RADIANS(lat2)) * COS(RADIANS(lng1 - lng2)))) * 6372.795;

CREATE FUNCTION GetDistance(
    lat1 REAL,
    lng1 REAL,
    lat2 REAL,
    lng2 REAL
) RETURNS REAL NO SQL RETURN ATAN2(
    SQRT(
        POW(
            COS(RADIANS(lat2)) * SIN(RADIANS(lng1 - lng2)),
            2
        ) + POW(
            COS(RADIANS(lat1)) * SIN(RADIANS(lat2)) - SIN(RADIANS(lat1)) * COS(RADIANS(lat2)) * COS(RADIANS(lng1 - lng2)),
            2
        )
    ),
    (
        SIN(RADIANS(lat1)) * SIN(RADIANS(lat2)) + COS(RADIANS(lat1)) * COS(RADIANS(lat2)) * COS(RADIANS(lng1 - lng2))
    )
) * 6372.795;

Please use datatype float instead of decimal in this. Mysql doesnt support -ve values for decimal datatype. You'll hit issues if you use points crossing hemispheres with this code