How to get all the data in a circle?

Question

I have a table that has column name, latitude, longtitude. I want to find all the data inside a circle, ordered by closest distance to the centre point. To get the circle, I have a centre point (latitude/longtitude). say the latitude and longtitude are -6.12579106 and 106.65699005, then the data that will appear is data that has the names A and B. Below is my table (accident table)

name, latitude, longtitude
A, -6.13116749, 106.66625977
B, -6.10377274, 106.66703224
C, 3.5896654,   98.6738261

I want find data by closest distance in WHERE CLUASE. how to do that? Thanks for the help :)

SELECT * FROM accident
LEFT JOIN polres ON polres.id = accident.polres_id
LEFT JOIN polda ON polda.id = polres.polda_id
WHERE --(condition for find data by closest distance) 

Answer 1

You can use the following function to calculate the distance:

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;

For units parameter, you can use the following values:

'M' is statute miles (default)                     
'K' is kilometers
'N' is nautical miles

And then use the above function in the following way:

SELECT * from test where 
calculate_distance(latitude, longtitude, -6.12579106, 106.65699005, 'K') <3;