SQL查询提取按列分组的平均值

Question

我试图生成一个SQL查询来提取一个ID的平均montly powerusage（一年）。

+----+------------+------------+ 
| id | powerusage | date | 
+----+------------+------------+ 
| 1 |  750 | 2011-12-2 | 
| 1 |  1000 | 2011-12-1 | 
| 1 |  1500 | 2011-11-15 | 
| 1 |  100 | 2011-11-13 | 
| 1 |   50 | 2011-11-10 | 
| 2 |  500 | 2011-11-15 | 
| 2 |  200 | 2011-11-13 | 
+----+------------+------------+ 

因此，如果ID = 1我想要（平均十一月+平均十二月）/ 2 =（二分之一千七百五十+三分之一千六百五十○）/ 2 = 712.5

select AVG(powerusage) as avgMontlyPowerUsage 
from usagetable 
where id = 1 and YEAR(date) = 2011 

但是，这将得到我680.

我该如何做一个群体的平均水平？

非常感谢所有的答案！但是我看到我的问题是不正确的。见更新问题

Answer 1

喜欢的东西

select AVG(montlyPowerUsage) from (

SELECT MONTH(date) as mnth,sum(powerusage) as montlyPowerUsage 
from usagetable 
where id = 1 and YEAR(date) = 2011 group by MONTH(date) 

) t1 

对于编辑的问题

select AVG(montlyPowerUsage) from (

SELECT MONTH(date) as mnth,AVG(powerusage) as montlyPowerUsage 
from usagetable 
where id = 1 and YEAR(date) = 2011 group by MONTH(date) 

) t1 

Answer 2

试穿ID

GROUP BY id 

或日期为准适合添加组通过。

Answer 3

SELECT SUM(powerusage)/(MONTH(MAX(`date`)) - MONTH(MIN(`date`)) + 1) 
      AS avgMontlyPowerUsage 
FROM usagetable 
WHERE id = 1 
    AND YEAR(`date`) = 2011 

或（视当数据稀疏你的需要）：

SELECT SUM(powerusage)/COUNT(DISTINCT MONTH(`date`)) 
      AS avgMontlyPowerUsage 
FROM usagetable 
WHERE id = 1 
    AND YEAR(`date`) = 2011 

警告：无论是上述的性能进行了优化。

Answer 4

 
mysql> select avg(powerusage) 
from 
(select monthname(date), sum(powerusage) as powerusage 
from usagetable 
where id=1 and year(date)=2011 
group by monthname(date)) as avg_usage; 
+-----------------+ 
| avg(powerusage) | 
+-----------------+ 
|  1700.0000 | 
+-----------------+ 

select avg(total_powerusage) 
from 
(select monthname(date), sum(powerusage) as total_powerusage 
from usagetable 
where id=1 and year(date)=2011 
group by monthname(date) 
) as avg_usage; 

/* the use of subquery 
    is to return total of unique occurrences, 
    and sum powerusage of each occurrence, 
    which mean, you just need to apply AVG into the subquery */ 

Answer 5

这应该给你每年和用户的月平均值。某些语法可能是MS SQL特定的，但逻辑应该是好的。

SELECT id, AVG(usage), year FROM 
(SELECT id, SUM(powerusage) as usage, YEAR(date) as Year, MONTH(date) as Month 
    FROM usagetable 
    GROUP BY id, YEAR(date), MONTH(date)) as InnerTable 
GROUP BY id, year