ST_DistanceSphere
ST_DistanceSphere returns the distance between two point geometries lying on a sphere.
Syntax
ST_DistanceSphere(geom1, geom2)
ST_DistanceSphere(geom1, geom2, radius)
Arguments
 geom1

A point value in degrees of data type
GEOMETRY
lying on a sphere. The first coordinate of the point is the longitude value. The second coordinate of the point is the latitude value. For 3DZ, 3DM, or 4D geometries, only the first two coordinates are used.  geom2

A point value in degrees of data type
GEOMETRY
lying on a sphere. The first coordinate of the point is the longitude value. The second coordinate of the point is the latitude value. For 3DZ, 3DM, or 4D geometries, only the first two coordinates are used.  radius

The radius of a sphere of data type
DOUBLE PRECISION
. If no radius is provided, the sphere defaults to Earth and the radius is computed from the World Geodetic System (WGS) 84 representation of the ellipsoid.
Return type
DOUBLE PRECISION
in the same units as the radius. If no radius is provided, the distance is in meters.
If geom1 or geom2 is null or empty, then null is returned.
If no radius is provided, then the result is in meters along the Earth's surface.
If radius is a negative number, then an error is returned.
If geom1 and geom2 don't have the same value for the spatial reference system identifier (SRID), then an error is returned.
If geom1 or geom2 is not a point, then an error is returned.
Examples
The following example SQL computes the distance in kilometers between two points on Earth.
SELECT ROUND(ST_DistanceSphere(ST_Point(122, 47), ST_Point(122.1, 47.1))/ 1000, 0);
round

13
The following example SQL computes the distances in kilometers between three airport locations in Germany: Berlin Tegel (TXL), Munich International (MUC), and Frankfurt International (FRA).
WITH airports_raw(code,lon,lat) AS ( (SELECT 'MUC', 11.786111, 48.353889) UNION (SELECT 'FRA', 8.570556, 50.033333) UNION (SELECT 'TXL', 13.287778, 52.559722)), airports1(code,location) AS (SELECT code, ST_Point(lon, lat) FROM airports_raw), airports2(code,location) AS (SELECT * from airports1) SELECT (airports1.code  ' <> '  airports2.code) AS airports, round(ST_DistanceSphere(airports1.location, airports2.location) / 1000, 0) AS distance_in_km FROM airports1, airports2 WHERE airports1.code < airports2.code ORDER BY 1;
airports  distance_in_km
+
FRA <> MUC  299
FRA <> TXL  432
MUC <> TXL  480