ROW_NUMBER
Syntax
ROW_NUMBER is an analytic function. It assigns a 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 theorder_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.
You cannot use ROW_NUMBER or any other analytic function for expr. That
is, you cannot nest analytic functions, but you can use other built-in function expressions for expr. Please refer to "About
SQL Expressions" for information on valid forms of expr.
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 nondeterministic function. However, employee_id is a unique key, so the results of this application of the function are
deterministic.