aggregate of an empty result set

Question

I would like the aggregates of an empty result set to be 0. I have tried the following:

SELECT SUM(COALESCE(capacity, 0))
  FROM objects
 WHERE null IS NOT NULL;         


        

Answer 1

The thing is, the aggregate always returns a row, even if no rows were aggregated (as is the case in your query). You summed an expression over no rows. Hence the null value you're getting.

Try this instead:

select coalesce(sum(capacity),0)
from objects
where false;