gistfile1.txt

select 
  id, 
  sum(st_area((gval).geom)
    / 10000) as total_area_ha,
  round( min((gval).val)) as min_slope,
  round( sum((gval).val * st_area((gval).geom))
    / st_area(st_collect((gval).geom))
    ) as avg_slope_sqm,
  round( max((gval).val)) as max_slope
from ( 
  select st_intersection(rast,1,db.the_geom) as gval,
    db.the_geom,
    db.gid as id
  from slope2020
  inner join
  (select gid,the_geom 
    from (select the_geom from development bounds as ga, the_geom from rivers_roads7 as gb  st_difference()as la
    --where target_use = 'Golf Area', 'Village C'
  ) as db
  on
  st_intersects(rast,db.the_geom)
) as foo
-- Here's the new line that lets you constrain the results to below a specified slope:
where (gval).val < 16
-- End of new line.
group by id, foo.the_geom;