Sql query to extract average grouped by a column - mysql

Question

I'm trying to generate a SQL query to extract an average montly powerusage (of a year) for an ID.
+----+------------+------------+
| 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 |
+----+------------+------------+
So if ID = 1 I want (avg november + avg december) / 2 = (1750/2 + 1650/3) / 2 = 712.5
select AVG(powerusage) as avgMontlyPowerUsage
from usagetable
where id = 1 and YEAR(date) = 2011
But this will give me 680.
How do I do a average on a group?
Many thanks for all the answers! But I see my question is incorrect. See updated question

Answer 1

Something like
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
For Edited question
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

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 3

This should give you monthly averages for every year and user. Some of the syntax may be MS SQL specific, but the logic should be good.
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

Answer 4

Try adding a group by on the id
GROUP BY id
Or the date, whichever suits.

Answer 5

SELECT SUM(powerusage) / (MONTH(MAX(`date`)) - MONTH(MIN(`date`)) + 1)
AS avgMontlyPowerUsage
FROM usagetable
WHERE id = 1
AND YEAR(`date`) = 2011
or (depending on what you need when data is sparse):
SELECT SUM(powerusage) / COUNT( DISTINCT MONTH(`date`) )
AS avgMontlyPowerUsage
FROM usagetable
WHERE id = 1
AND YEAR(`date`) = 2011
Warning: Neither of the above is optimized for performance.