average_nearest_neighbor_distance.sql

SELECT avg(mindist) AS avgNND
FROM
( SELECT aname, min(dist) AS mindist
  FROM 
    ( SELECT a.geometry, ST_Distance(a.geometry, b.geometry)*0.00062 AS dist,
             a.name AS aname, b.name AS bname
      FROM parks AS a, parks AS b
      WHERE a.name <> b.name
      ORDER BY aname, dist ASC ) AS distances

  GROUP BY aname ) AS mindistance;