I have a table of orders. There I need to find out which 3 partner_id's have made the largest sum of amount_totals, and sort those 3 from biggest to smallest.

    testdb=# SELECT amount_total, partner_id FROM sale_order;
     amount_total | partner_id
    --------------+------------
          1244.00 |          9
          3065.90 |         12
          3600.00 |          3
          2263.00 |         25
          3000.00 |         10
          3263.00 |          3
           123.00 |         25
          5400.00 |         12
    (8 rows)

Just starting SQL, I find it confusing ...

Aggregated amounts

If you want to list aggregated amounts, it can be as simple as:

SELECT partner_id, sum(amount_total) AS amout_suptertotal
FROM   sale_order
GROUP  BY 1
ORDER  BY 2 DESC
LIMIT  3;

The 1 in GROUP BY 1 is a numerical parameter, that refers to the position in the SELECT list. Just a notational shortcut for GROUP BY partner_id in this case.

This ignores the special case where more than three partner would qualify and picks 3 arbitrarily (for lack of definition).

Individual amounts

SELECT partner_id, amount_total
FROM   sale_order
JOIN  (
   SELECT partner_id, rank() OVER (ORDER BY sum(amount) DESC) As rnk
   FROM   sale_order
   GROUP  BY 1
   ORDER  BY 2
   LIMIT  3
   ) top3 USING (partner_id)
ORDER  BY top3.rnk;

This one, on the other hand includes all peers if more than 3 partner qualify for the top 3. The window function rank() gives you that.

The technique here is to group by partner_id in the subquery top3 and have the window function rank() attach ranks after the aggregation (window functions execute after aggregate functions). ORDER BY is applied after window functions and LIMIT is applied last. All in one subquery.

Then I join the base table to this subquery, so that only the top dogs remain in the result and order by rnk.

Window functions require PostgreSQL 8.4 or later.

This is rather advanced stuff. You should start learning SQL with something simpler probably.

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