[性能调优] SQL优化技巧

- 使用count case语句结合，把多次全扫描改写成一次全扫描
select
    ym,
    count(case when cnt between 1 and 50 then 1 end) count1,
    count(case when cnt between 51 and 100 then 1 end) count2
from history group by ym order by ym;