SELECT *
FROM   my_table
WHERE  coordinates
    && -- intersects,  gets more rows  -- CHOOSE ONLY THE
    @ -- contained by, gets fewer rows -- ONE YOU NEED!
    ST_MakeEnvelope (
        xmin, ymin, -- bounding
        xmax, ymax, -- box limits
        my_srid)

WHERE  ST_MakeEnvelope (...)
    ~ -- contains, gets same fewer rows
    coordinates

SELECT ST_Y(the_geom) AS latitude, ST_X(the_geom) as longitude
from units u where the_geom && ST_MakeEnvelope(left, bottom, right, top, 4326)