各位，我有一个表，表中有人员的ID，姓名，年龄，我想写一个sql语句，得出小于20的有多少人？20--30，30--40，40-50，50-60，60-70，大于70的人数，sql语句怎么写呀？
------解决思路----------------------
with tbTemp(ID,name,age) as(
select '1001','Jimmy','20'
union all
select '1002','Tom','35'
union all
select '1003','John','85'
union all
select '1004','John1','75'
union all
select '1005','John2','25'
union all
select '1006','John3','65'
)
select case when age<20 then '小于20'
when age between 20 and 30 then '20-30'
when age between 31 and 40 then '30-40'
when age between 41 and 50 then '40-50'
when age between 51 and 60 then '50-60'
when age between 61 and 70 then '60-70'
when age>70 then '70以上' end Item
,count(1) Numbers
from tbTemp
group by case when age<20 then '小于20'
when age between 20 and 30 then '20-30'
when age between 31 and 40 then '30-40'
when age between 41 and 50 then '40-50'
when age between 51 and 60 then '50-60'
when age between 61 and 70 then '60-70'
when age>70 then '70以上' end