ST_DWithin — Returns true if the geometries are within the specified distance of one another
g2, double precision
Returns true if the geometries are within the specified distance of one another. The distance is specified in units defined by the spatial reference system of the geometries. For this function to make sense, the source geometries must both be of the same coorindate projection, having the same SRID.
This function call will automatically include a bounding box comparison that will make use of any indexes that are available on the geometries.
Prior to 1.3, ST_Expand was commonly used in conjunction with && and ST_Distance to achieve the same effect and in pre-1.3.4 this function was basically short-hand for that construct. From 1.3.4, ST_DWithin uses a more short-circuit distance function which should make it more efficient than prior versions for larger buffer regions.
This method implements the OpenGIS Simple Features Implementation Specification for SQL.
--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;