Create a layer with fields of near neighbors

-- Scripts for Postgres/Postgis
--
-- Create a layer with fields of near neighbors from other layer
--  .Layer 'a' = Target
--  .Layer 'b' = Source of neighbors


-- In select statement  you can choose the fields in both tables 'a' and 'b'
-- This script can use in 'Custon DB Query' plugin for QGIS
select a.id as gid, a.the_geom, b.id as gid_b, ST_Distance(a.the_geom, b.the_geom) as min_dist_a_b
from
 a,b,
(
-- ID of the near neighbors
SELECT m.id as a_id, m.nn[1] as b_id
FROM
 (SELECT a.id, a.the_geom,
     ARRAY(SELECT b.id
               FROM b
        --WHERE ST_DWithin(a.the_geom, b.the_geom, 5000) -- limit of distance
        ORDER BY ST_Distance(a.the_geom, b.the_geom)
        LIMIT 1) As nn
  FROM a  ) m
) nn
where
 nn.a_id = a.id and nn.b_id = b.id