oracle case when 用法

select * from biz_person_impl;       --人员表
select * from dm_professional_title; --专家技术职称
select * from dm_title_level;        --职称级别
实现统计人员各个职称级别数

select case when dtl.value in('初级一','初级二') then '初级' else dtl.value end title,nvl(count(1),0) num
from biz_person_impl p inner join dm_professional_title dpt
on p.titlelevel = dpt.srms_id
inner join dm_title_level dtl on dpt.level_id = dtl.id 
group by case when dtl.value in('初级一','初级二') then '初级' else dtl.value end