Grouping aggregate functions

[Richard Connamacher <rich.n1@xxxxxxxxxxxxxx>]

Hey all,

I'm new on this list, and have been playing with Postgres a lot this  
week. (Love it, by the way.)

I've got a question, if anyone can help me out. I know how to use an  
aggregate function to, say, find the lowest price ever listed for a  
product. I also know how to combine that with a SELECT ... GROUP BY  
statement to find, say, the lowest price reported for each month.  
Now, what if I want to find the *average* of all the lowest prices  
for each month? Plopping that SELECT statement inside parentheses and  
inside an "avg( )" function produces an error.

What I'd love to do, and which creates an error, would be something  
like:

SELECT avg( ( SELECT min(price) FROM weekly_supply_prices GROUP BY  
month ) )

The error I get is: "ERROR:  more than one row returned by a subquery  
used as an expression" (to state the obvious). If I don't double up  
the quotes I get a syntax error.

Anyone have any idea how to do this? Or do I have to compute the  
average in another program?

Thanks!
Richard