Averaging an Average

Question

I'm trying to average an average and I'm running into a bit of trouble. I want to determine a daily average of sales and then average the daily averages. So, if I have 3 sales for Day 1 ($5, $10 and $15, average sale being $10) and 4 sales for Day 2 ($1, $2, $3 and $4, average sale being $2.50), I want to average those together. Total Sales (Avg: $10 and $2.50).
I made some progress, by using the PARTION BY function, but the problem is that I can't run an average...
SELECT a.month, a.sales, AVG(a.sales) OVER(PARTITION BY a.day ) AS Daily_Sales
FROM schema.profits a
But now I want to be able to Average that Daily_Sales calculation and then group by the month to get my final result. However, when I try to add in average to the daily_sales, I get "ORA-30483: window functions are not allowed here". Any ideas?
Thanks,
Jeff C.

Answer 1

A slightly better version
SQL>set sqlblanklines on
SQL>set numformat 99.99
SQL>
SQL>WITH DATA AS (
  2  SELECT 5 sales,trunc(SYSDATE-4) da FROM dual UNION ALL
  3  SELECT 10 sales,trunc(SYSDATE-4) FROM dual UNION ALL
  4  SELECT 15 sales,trunc(SYSDATE-4) FROM dual UNION ALL
  5 
  6  SELECT 1 sales,trunc(SYSDATE-3) da FROM dual UNION ALL
  7  SELECT 2 sales,trunc(SYSDATE-3) FROM dual UNION ALL
  8  SELECT 3 sales,trunc(SYSDATE-3) FROM dual UNION ALL
  9  SELECT 4 sales,trunc(SYSDATE-3) FROM dual UNION ALL
10 
11  SELECT 1 sales,trunc(SYSDATE-2) da FROM dual UNION ALL
12  SELECT 2 sales,trunc(SYSDATE-2) FROM dual UNION ALL
13  SELECT 3 sales,trunc(SYSDATE-2) FROM dual UNION ALL
14  SELECT 4 sales,trunc(SYSDATE-2) FROM dual UNION ALL
15 
16  SELECT 5 sales,trunc(SYSDATE+1) da FROM dual UNION ALL
17  SELECT 10 sales,trunc(SYSDATE+1) FROM dual UNION ALL
18 
19  SELECT 15 sales,trunc(SYSDATE+2) FROM dual UNION ALL
20  SELECT 1 sales,trunc(SYSDATE+2) da FROM dual UNION ALL
21  SELECT 2 sales,trunc(SYSDATE+2) FROM dual UNION ALL
22  SELECT 3 sales,trunc(SYSDATE+2) FROM dual UNION ALL
23  SELECT 4 sales,trunc(SYSDATE+2) FROM dual
24  )
25  select trunc(da), avg(sales) daily_avg, avg(avg(sales)) over () avg_of_daily_avg
26  from data
27  group by trunc(da)
28  order by 1;
TRUNC(DA) DAILY_AVG AVG_OF_DAILY_AVG
21-MAY-10     10.00             5.50
22-MAY-10      2.50             5.50
23-MAY-10      2.50             5.50
26-MAY-10      7.50             5.50
27-MAY-10      5.00             5.50