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Sample Codes (PostgreSQL)
/*::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
/*::                                                                         :*/
/*::  This routine calculates the distance between two points (given the     :*/
/*::  latitude/longitude of those points). It is being used to calculate     :*/
/*::  the distance between two locations using GeoDataSource(TM) Products    :*/
/*::                                                                         :*/
/*::  Definitions:                                                           :*/
/*::    South latitudes are negative, east longitudes are positive           :*/
/*::                                                                         :*/
/*::  Passed to function:                                                    :*/
/*::    lat1, lon1 = Latitude and Longitude of point 1 (in decimal degrees)  :*/
/*::    lat2, lon2 = Latitude and Longitude of point 2 (in decimal degrees)  :*/
/*::    unit = the unit you desire for results                               :*/
/*::           where: 'M' is statute miles (default)                         :*/
/*::                  'K' is kilometers                                      :*/
/*::                  'N' is nautical miles                                  :*/
/*::  Worldwide cities and other features databases with latitude longitude  :*/
/*::  are available at https://www.geodatasource.com                         :*/
/*::                                                                         :*/
/*::  For enquiries, please contact sales@geodatasource.com                  :*/
/*::                                                                         :*/
/*::  Official Web site: https://www.geodatasource.com                       :*/
/*::                                                                         :*/
/*::  Thanks to Kirill Bekus for contributing the source code.               :*/
/*::                                                                         :*/
/*::         GeoDataSource.com (C) All Rights Reserved 2019                  :*/
/*::                                                                         :*/
/*::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/

CREATE OR REPLACE FUNCTION calculate_distance(lat1 float, lon1 float, lat2 float, lon2 float, units varchar)
RETURNS float AS $dist$
    DECLARE
        dist float = 0;
        radlat1 float;
        radlat2 float;
        theta float;
        radtheta float;
    BEGIN
        IF lat1 = lat2 OR lon1 = lon2
            THEN RETURN dist;
        ELSE
            radlat1 = pi() * lat1 / 180;
            radlat2 = pi() * lat2 / 180;
            theta = lon1 - lon2;
            radtheta = pi() * theta / 180;
            dist = sin(radlat1) * sin(radlat2) + cos(radlat1) * cos(radlat2) * cos(radtheta);

            IF dist > 1 THEN dist = 1; END IF;

            dist = acos(dist);
            dist = dist * 180 / pi();
            dist = dist * 60 * 1.1515;

            IF units = 'K' THEN dist = dist * 1.609344; END IF;
            IF units = 'N' THEN dist = dist * 0.8684; END IF;

            RETURN dist;
        END IF;
    END;
$dist$ LANGUAGE plpgsql;

SELECT calculate_distance(32.9697, -96.80322, 29.46786, -98.53506, 'M');
SELECT calculate_distance(32.9697, -96.80322, 29.46786, -98.53506, 'K');
SELECT calculate_distance(32.9697, -96.80322, 29.46786, -98.53506, 'N');

The sample code is licensed under LGPLv3.