select
    sum(case when type='credit' then amount else -amount end)
from report;

select
    sum(case when type='credit' then amount
             else -amount end)
from report
where type in ('credit', 'debit');

select (
  select sum(case when type='credit' then amount else -amount end)
  from report
  ) - (select sum(amount) from total);

select sum(case type when 'credit' then amount when 'debit' then -amount else 0 end)
from report