知识点：
用mysql,按年龄段查询一张居民的数据（各年龄段居民的个数）
1.如：查询resident(居民表)，按照各年龄段，统计人数
2.mysql语句如下：
select ageproportion as '年龄段',count(*) as '人数' from
(
SELECT
CASE
when age>0 and age<=10 then '0-10岁'
when age>10 and age<=20 then '10-20岁'
when age>20 and age<=30 then '20-30岁'
when age>30 and age<=40 then '30-40岁'
when age>40 and age<=50 then '40-50岁'
when age>50 and age<=60 then '50-60岁'
else '60岁以上'
END
as ageproportion from resident
)a GROUP BY ageproportion
3.查询结果：
