Calculating Distance Between 2 Cities

// Cities are points x0,y0 and x1,y1 in kilometers or miles or Smoots[1]
dx = x1 - x0;
dy = y1 - y0;
dist = sqrt(dx*dx + dy*y);

dist = sqrt((x1-x0)*(x1-x0) + (y1-y0)*(y1-y0));

SELECT geography::Point(lat1, lon1, 4326).STDistance(geography::Point(lat2, lon2, 4326))
-- computes distance in meters using eliptical model, accurate to the mm

dist = sqrt(dx*dx + dy*dy);