Expression with aggregate

Hi!

This is actually a follow-up question on my posting "[NOVICE] Join
with aggregate" a couple of days ago; thanks again to all of you who
tried to help me, and actually did help me a lot. This time, my
problem is in using in a join the aggregate result in conjunction with
a non-aggregate field in a mathematical expression. More clearly (and
again I am stripping down the example to the simplest form; actually I
am thinking of recordings on video tapes, and I also take into account
the distinction between SP/LP recordings in the summation over playing
times):

I'm using Postgres 6.5.1, and I have tables like the following:

asdf=> create table a (id int4, name text, d int2);
CREATE
asdf=> create table b (a_id int4, c int2);
CREATE

with sample data:

asdf=> insert into a values (1, 'Number one', 800);
INSERT 418805 1
asdf=> insert into b values (1, 100);
INSERT 418806 1
asdf=> insert into b values (1, 200);
INSERT 418807 1

>From this I want to produce a table having

id | name |diff
---+------------+----
1 | Number one | 500

, where diff = 800 - sum(100, 200) = 500. I know that I can achieve
this with a temporary table, or with a view, using:

asdf=> create view c as select id, name, d, sum(b.c) from a, b where
id = a_id group by id, name, d;
CREATE
asdf=> select id, name, d-sum as diff from c;
id|name |diff
--+----------+----
1|Number one| 500
(1 row)

But what I really want to do is something like:

asdf=> select id, name, d-sum(b.c) as diff from a, b where id = a_id
group by id, name, diff;
ERROR: Aggregates not allowed in GROUP BY clause

or:

asdf=> select id, name, d-sum(b.c) as diff from a, b where id = a_id
group by id, name;
ERROR: Illegal use of aggregates or non-group column in target list

Is there a way to do this without the detour via the view? Or are
there views just for this reason?

By the way, when I use \d to list the tables, why is a view always
shown with a '?' like in

asdf=> \d
Database = asdf
+------------------+----------------------------------+----------+
| Owner | Relation | Type |
+------------------+----------------------------------+----------+
| albert | a | table |
| albert | b | table |
| albert | c | view? |
+------------------+----------------------------------+----------+

? Why doesn't it just say '| albert | c | view |'?

I'd really appreciate any hints with this, even though I know how to
do it (as demonstrated above) with the use of the view.

Thanks in advance for your help,

Albert.

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