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

Just do this:

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

By the way, COALESCE inside of SUM is redundant, even if capacity is NULL, it won't make the summary null.

To wit:

create table objects
(
    capacity int null
);

insert into objects(capacity) values (1),(2),(NULL),(3);

select sum(capacity) from objects;

That will return a value of 6, not null.

And a coalesce inside an aggregate function is a performance killer too, as your RDBMS engine cannot just rip through all the rows, it has to evaluate each row's column if its value is null. I've seen a bit OCD query where all the aggregate queries has a coalesce inside, I think the original dev has a symptom of Cargo Cult Programming, the query is way very sloooowww. I removed the coalesce inside of SUM, then the query become fast.