SELECT name, gid
FROM geonames
ORDER BY geom <-> st_setsrid(st_makepoint(-90,40),4326)
LIMIT 10;

ST_Distance(geom::geography, ST_GeogFromText(''))

--Find the nearest hospital to each school
--that is within 3000 units of the school.
-- We do an ST_DWithin search to utilize indexes to limit our search list
-- that the non-indexable ST_Distance needs to process
--If the units of the spatial reference is meters then units would be meters
SELECT DISTINCT ON (s.gid) s.gid, s.school_name, s.the_geom, h.hospital_name
    FROM schools s
        LEFT JOIN hospitals h ON ST_DWithin(s.the_geom, h.the_geom, 3000)
    ORDER BY s.gid, ST_Distance(s.the_geom, h.the_geom);

--The schools with no close hospitals
--Find all schools with no hospital within 3000 units
--away from the school.  Units is in units of spatial ref (e.g. meters, feet, degrees)
SELECT s.gid, s.school_name
    FROM schools s
        LEFT JOIN hospitals h ON ST_DWithin(s.the_geom, h.the_geom, 3000)
    WHERE h.gid IS NULL;