PostGIS generate graphic cut

rsql='select r.code,r.name,st_intersection(r.geom, city.geom) as geom ' \
     'from (SELECT * from gis_osm_ road ) as r,(select * from gis_ county  where  City =\' tongling \' and name=\'%s\') as city   ' \
     'where  st_intersects(r.geom, city.geom)'%(town)

among st_intersection Cut for intersecting figures
st_intersects Filter for intersecting shapes , Used to improve query speed