Count for null and not null columns

count.sql

select count(a.id), sum(case when b.id is null then 0 else 1 end) as b_exists,
       sum(case when b.id is null then 1 else 0 end) as b_does_not_exist
from a left join b on a.b_id = b.id;

-- Or even better
select count(a.id),
       count(b.id) filter b.id is not null as b_exists,
       count(b.id) filter b.id is null  as b_does_not_exist
from a left join b on a.b_id = b.id;