Potentially annoying question about date ranges

Hi,

   I'm going to assume that this question has been asked a gazillion
times, and is in every SQL book known to man and aliens. And I also
assume it is in the FAQ. But I'm going to ask anyway.

   I have a database in which I store my purchases -- mainly for
entertainment (you have no idea how dull my life is).

   In an effort to get some variation in my life, I thought I'd find out
how much of my valuable money I waste each day - on average.

   "select avg(sum) ... group by date" would produce a result, which
isn't correct -- it would show me how much I spend in average those days
I actually do spend anything, not counting those wonderful days when I
don't spend any money at all (and since I'm such a cheap skate, that
happens a lot).

   The table I use basically looks like this (well, the relevant parts,
anyway):

table transaction (
   id serial,
   numeric(8,2)
   dt date,
   description text
)

   Now, I get the problem. In my query, I group by date, and then take
the average of the sums. The problem is that all dates which I haven't
wasted any money should be listed with the sum 0 (such a wonderful
number, when it comes to expenses). But it obviously isn't.

   Is there a painfully obvious way (that I'm missing) to get all dates
included in a query, using my current table design, so that my average
will tell me how much I spend per day on average - including those days
I don't spend anything. (I want to look at that number every morning,
and if by the end of the day I haven't managed to keep my expenses under
that number, I whip myself ten times and cry myself to sleep).

   I assume that I have to somehow select a date-range, and somehow
merge it with the transaction table ... some way. Can it be done? (I'm
not asking for a working solution, just some hints or tips).

   Do I use unions?

--
Kind Regards,
Jan Danielsson
Te audire non possum. Musa sapientum fixa est in aure.