Oracle Database - SQL - ROW_NUMBER function

ROW_NUMBER is an non-deterministic analytic function. The function assigns a sequential unique number:

• to each row to which it is applied (either each row in the partition or each row returned by the query)
• in the ordered sequence of rows specified in the order_by_clause, beginning with 1.

By nesting a subquery using ROW_NUMBER inside a query that retrieves the ROW_NUMBER values for a specified range, you can find a precise subset of rows from the results of the inner query. This use of the function lets you implement:

• top-N,
• bottom-N,
• and inner-N reporting.

For consistent results, the query must ensure a deterministic sort order.

It has the following syntax:

ROW_NUMBER ( ) OVER ( [partition by myColumn] [order by myOtherColumn])

SELECT channel_desc, calendar_month_desc,
   TO_CHAR(TRUNC(SUM(amount_sold), -5), '9,999,999,999') SALES$,
   ROW_NUMBER() OVER (ORDER BY TRUNC(SUM(amount_sold), -6) DESC) AS ROW_NUMBER 
FROM sales, products, customers, times, channels
WHERE sales.prod_id=products.prod_id AND sales.cust_id=customers.cust_id
  AND sales.time_id=times.time_id AND sales.channel_id=channels.channel_id
  AND times.calendar_month_desc IN ('2001-09', '2001-10')
GROUP BY channel_desc, calendar_month_desc;

CHANNEL_DESC         CALENDAR SALES$         ROW_NUMBER
-------------------- -------- -------------- ----------
Direct Sales         2001-10       1,000,000          1
Direct Sales         2001-09       1,100,000          2
Internet             2001-09         500,000          3
Partners             2001-09         600,000          4
Partners             2001-10         600,000          5
Internet             2001-10         700,000          6

Note that there are three pairs of tie values in these results. Like NTILE, ROW_NUMBER is a non-deterministic function, so each tied value could have its row number switched. To ensure deterministic results, you must order on a unique key. Inmost cases, that will require adding a new tie breaker column to the query and using it in the ORDER BY specification.

For each department in the sample table oe.employees, the following example assigns numbers to each row in order of employee's hire date:

SELECT department_id, last_name, employee_id, ROW_NUMBER()
   OVER (PARTITION BY department_id ORDER BY employee_id) AS emp_id
   FROM employees;

DEPARTMENT_ID LAST_NAME                 EMPLOYEE_ID     EMP_ID
------------- ------------------------- ----------- ----------
           10 Whalen                            200          1
           20 Hartstein                         201          1
           20 Fay                               202          2
           30 Raphaely                          114          1
           30 Khoo                              115          2
           30 Baida                             116          3
           30 Tobias                            117          4
           30 Himuro                            118          5
           30 Colmenares                        119          6
           40 Mavris                            203          1
. . .
          100 Popp                              113          6
          110 Higgins                           205          1
          110 Gietz                             206          2

ROW_NUMBER is a non-deterministic function. However, employee_id is a unique key, so the results of this application of the function are deterministic.

• Datawarehousing guide - SQL for Analysis and Reporting
• ROW_NUMBER