latitude/longitude distance function - for mysql (original source: http://derickrethans.nl/spatial-indexes-mysql.html - syntax changed to fit in 1 line for easy use in a rails migration).

distance.sql

CREATE FUNCTION distance (latA double, lonA double, latB double, LonB double)
    RETURNS double DETERMINISTIC
    RETURN 2 * ASIN(SQRT((SIN((radians(latA) - radians(latB))/2) * SIN((radians(latA) - radians(latB))/2) +
        COS(radians(latA)) * COS(radians(latB)) * SIN((radians(lonA) - radians(LonB))/2)*SIN((radians(lonA) - radians(LonB))/2)))) * 6371.01