How to use sum in a where clause to calculate an avg

Question

I'm still new to SQL, I have just been using PostgreSQL for a week. I am a little stuck.

I'm using the NY dataset ( http://workshops.opengeo.org/postgis-intro/about_data.html ).

I have to find a way to list all the neighborhoods with more Asian population than the average of corresponding borough and I have to display the percentage of Asian population.

I thought about this:

select name, 100 * sum(c.popn_asian)/(select (avg(popn_asian)) from nyc_census_blocks group by boroname)
from nyc_neighborhoods n, nyc_census_blocks c
where (ST_Contains(n.geom, c.geom) = true )
and (sum(c.popn_asian)) > (select avg(popn_white) from nyc_census_blocks c1 group by c1.boroname) 

But I am not allowed to use sum after the where clause apparently... I'd like to know why and learn how to solve this problem.

Thanks

Answer 1

select name, 100 * sum(c.popn_asian)/(select (avg(popn_asian)) from nyc_census_blocks group by boroname)
from nyc_neighborhoods n, nyc_census_blocks c
where ST_Contains(n.geom, c.geom) = true 
group by name
having sum(c.popn_asian) > (select avg(popn_white) from nyc_census_blocks c1 group by c1.boroname)