知識點：
用mysql,按年齡段查詢一張居民的數據（各年齡段居民的個數）
1.如：查詢resident(居民表)，按照各年齡段，統計人數
2.mysql語句如下：
select ageproportion as '年齡段',count(*) as '人數' from
(
SELECT
CASE
when age>0 and age<=10 then '0-10歲'
when age>10 and age<=20 then '10-20歲'
when age>20 and age<=30 then '20-30歲'
when age>30 and age<=40 then '30-40歲'
when age>40 and age<=50 then '40-50歲'
when age>50 and age<=60 then '50-60歲'
else '60歲以上'
END
as ageproportion from resident
)a GROUP BY ageproportion
3.查詢結果：
