Create geoJSON from postGIS table, and other queries.

postgis.sql

 /**
 *Create  geojson
 **/
 SELECT row_to_json(fc) FROM ( 
        SELECT 'FeatureCollection' As type, array_to_json(array_agg(f)) As features  FROM 
                (SELECT 'Feature' As type,ST_AsGeoJSON(lg.the_geom, 4)::json As geometry , 
        row_to_json((SELECT l FROM (SELECT value_1,value_2,value_3) As l
      )) As properties
   FROM my_table As lg ) As f ) As fc;
                                                                                 
/**
* Check if there is duplicated points 
**/ 
SELECT a.gid as gid_1, b.gid as gid_2 FROM my_table AS a, my_table AS b WHERE ST_Intersects(a.geom, b.geom) AND a.gid <> b.gid;