需求:查询在指定年龄段的人数
when age>25 and age <27 then 2
else 0 end as stage,count(*) as population from student
group by case when age>1 and age <26 then 1
when age>25 and age <27 then 2
else 0 end
结果
stage population
1 1 2
2 2 5
可通过stage的值区分该条记录是属于哪个年龄段