Avg and Std function - null values

[Pina Kelwin]

Hi,
(B
(BI have a table with the sales of each product by month (in the format 
(Byyyymm).
(BFor example: (table name: ventas)
(B
(Bsale_date     prod_id    sales
(B200301       A            20
(B200302       A            16
(B200303       A            18
(B200301       B            12
(B200302       B              3
(B200304       B            10
(B
(BI would like to have the average sales per month of each product.
(BIf I use:
(B"SELECT prod_id, avg(sales) as average
(B  FROM ventas
(B  GROUP by prod_id"
(B
(Bthen I would get the average sales from Jan 2003 to Mar 2003 for product A 
(B(18.0) and the average sales for product B (8.33). 
(BHowever, as you would have probably noticed, my table covers the sales 
(Bperiod Jan 2003 to Apr 2003. I would like to get the average sales for the 
(Bperiod Jan-Apr 2003 for each product: A (13.5) and B (6.25).
(B
(BI know that if I add the missing lines with value 0 the problem would be 
(Bsolved, but I would like to know an efficient way to add those lines 
(Bwithout having to check the table for missing values with a script (I also 
(Bneed to calculate the standard deviation).
(B
(BThanks in advance for your help.
(B
(Bkelwin
(B
(B_________________________________________________________________
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(B
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