sql优化 用sum取代count

原SQL:select count(*) as total from jpy t where t.jb='987'

修改后:
select
  sum(case t.jb when '987' then 1 else 0 end) as total
from
  jpy t

SQL优化例子:

select
   sum(case t.jb when 987 then 1 else 0 end) as count1,
   sum(case t.jb when 988 then 1 else 0 end) as count2,
   sum(case t.jb when 989 then 1 else 0 end) as count3,
   sum(case t.jb when 1104 then 1 else 0 end) as count4,
   count(t.jb) as counTotal 
  from jpy t