Avoiding double-counting in aggregates with more than one join?

Hi All,

I've noticed in the past that doing aggregates while joining to more
than one table can sometimes give you unintended results. For example,
suppose I have three tables: products, sales, and resupplies. In sales I
track what I sell, and in resupplies I track my own purchases to
increase inventory. Both have a foreign key to products. Now I want to
run a report showing the total dollars sold for each product versus the
total dollars spent for each product. I could try this:

     SELECT  p.id,
             SUM(s.price * s.qty) AS total_sold,
             SUM(r.price * r.qty) AS total_spent
     FROM    products p
     LEFT OUTER JOIN sales s
     ON      s.product_id = p.id
     LEFT OUTER JOIN resupplies r
     ON      r.product_id = p.id
     GROUP BY p.id
     ;

That seems pretty safe, but actually I get bad answers,
for example if some product has this data:

     sales
     -----
     sold 1 @ $2/ea

     resupplies
     ----------
     bought 1 @ $1/eq
     bought 2 @ $1/ea

Then pre-grouping I have this:

     p.id | s.qty | s.price | r.qty | r.price
     -----+-------+---------+-------+--------
        1 |     1 |      $2 |     1 |      $1
        1 |     1 |      $2 |     2 |      $1

You can see the problem is that I'm going to double-count my sales.
What I really want is this:

     p.id | s.qty | s.price | r.qty | r.price
     -----+-------+---------+-------+--------
        1 |     1 |      $2 |     1 |      $1
        1 |       |         |     2 |      $1

In the past I've always handled these situations by aggregating each
table separately
and only then joining things together:

     WITH
     s AS (
       SELECT  product_id,
               SUM(price * qty) AS total_sold
       FROM    sales
       GROUP BY product_id) s
     ),
     r AS (
       SELECT  product_id,
               SUM(price * qty) AS total_spent
       FROM    resupplies
       GROUP BY product_id) r
     )
     SELECT  p.id,
             COALESCE(s.total_sold, 0),
             COALESCE(r.total_spent, 0)
     FROM    products p
     LEFT OUTER JOIN s
     ON      s.product_id = p.id
     LEFT OUTER JOIN r
     ON      r.product_id = p.id
     ;

Since I've guaranteed that each CTE includes at most one row per product,
this is safe from double-counting errors.

But is there a better way? My approach feels verbose
and harder to read. Also you have to type COALESCE a lot. :-)
Is there some different way of doing things I haven't thought of yet?

Also I wonder about the performance merging all these subqueries together.
Would the final merging be any faster if I had an ORDER BY in each CTE?

It seems like this pattern comes up a lot;
what have others done about it?

Thanks,
Paul