CUME_DIST in oracle

The following example calculates the salary percentile for each employee in the purchasing division. For example, 40% of clerks have salaries less than or equal to Himuro.

SELECT job_id, last_name, salary, CUME_DIST() 
   OVER (PARTITION BY job_id ORDER BY salary) AS cume_dist
   FROM employees
   WHERE job_id LIKE 'PU%';

JOB_ID     LAST_NAME                     SALARY  CUME_DIST
---------- ------------------------- ---------- ----------
PU_CLERK   Colmenares                      2500         .2
PU_CLERK   Himuro                          2600         .4
PU_CLERK   Tobias                          2800         .6
PU_CLERK   Baida                           2900         .8
PU_CLERK   Khoo                            3100          1
PU_MAN     Raphaely                       11000          1