SQL in a Nutshell, 2nd Edition

Ordering

You specify the order of the rows on which an analytic function operates using the ordering clause. However, this analytic ordering clause does not define the result set ordering. To define the overall result set ordering, you must use the query’s ORDER BY clause. The following use of Oracle’s FIRST_VALUE function illustrates the effect of different orderings of the partitions:

SELECT NUM, 
       SUM(NUM) OVER (PARTITION BY ODD) S,
       FIRST_VALUE(NUM) OVER (PARTITION BY ODD ORDER BY NUM ASC) first_asc,
       FIRST_VALUE(NUM) OVER (PARTITION BY ODD ORDER BY NUM DESC) first_desc
FROM ODD_NUMS;

       NUM          S  FIRST_ASC FIRST_DESC
---------- ---------- ---------- ----------
         0          2          0          2
         2          2          0          2
         1          4          1          3
         3          4          1          3

As you can see, the ORDER BY clauses in the window function invocations affect the ordering of the rows in the respective partitions when those functions are evaluated. Thus, ORDER BY NUM ASC orders partitions in ascending order, resulting in 0 for the first value in the even-number partition and 1 for the first value in the odd-number partition. ORDER BY NUM DESC has the opposite effect.

Tip

The preceding query also illustrates an important point: using window functions, you can summarize and order many different ways in the same query.