一个sql题

想得到如下结果，sql语句怎么写

[解决办法]

SELECT 1 季度,AVG(产值) '平均值'
 FROM TB 
 WHERE 月份 BETWEEN 1 AND 3
 UNION ALL 
 SELECT 2 季度,AVG(产值) '平均值'
 FROM TB 
 WHERE 月份 BETWEEN 4 AND 6
 UNION ALL 
 SELECT 3 季度,AVG(产值) '平均值'
 FROM TB 
 WHERE 月份 BETWEEN 7 AND 9
 UNION ALL 
 SELECT 4 季度,AVG(产值) '平均值'
 FROM TB 
 WHERE 月份 BETWEEN 10 AND 12

[解决办法]
select 季度,avg(产值) as 平均值
from (
select case when 月份 between 1 and 3 then 1 when 月份 between 4 and 6 then 2 when 月份 between 7 and 9 then 3 else 4 end 季度 ,产值 from tablename ) as a
group by 季度
[解决办法]
select q as 季度,avg(产值) as 平均值 from(
select (月份-1)/3+1 as q,产值 from tb
)t group by q