Thread: 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.

At 02:34 PM 9/30/06, Jan Danielsson wrote:
>    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).


http://www.postgresql.org/docs/8.1/static/functions-srf.html

Jan Danielsson <jan.danielsson@gmail.com> writes:
>    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.

Not sure about pure-SQL-standard ways, but the way I'd do it in PG is to
generate all the dates in the desired range using generate_series,
say like this:

select avg(dayspend) from
(select sum(coalesce(amt,0)) as dayspend from
  (select '2006-01-01'::date + n as dt
   from generate_series(0,'2006-12-31'::date-'2006-01-01'::date) as n) as days
        left join transaction on (transaction.dt = days.dt)
 group by days.dt) as ss;

You could make it a little cleaner by creating a function:

create function generate_dates(start date, stop date) returns setof date
as $$select $1 + n from generate_series(0, $2-$1) as n$$ language sql strict;

select avg(dayspend) from
(select sum(coalesce(amt,0)) as dayspend from
  (select generate_dates('2006-01-01','2006-12-31') as dt) as days
        left join transaction on (transaction.dt = days.dt)
 group by days.dt) as ss;

            regards, tom lane

Jan Danielsson wrote:
[---]
>    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.
[---]

   Many thanks to all tho replied. I have learned much, and learned
about database functions.

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