请教（an introduction to oracle 9i 标准教程中的问题）

Question

SELECT DECODE(GROUPING(department_name), 1, 'All Departments',
   department_name) AS department,
   DECODE(GROUPING(job_id), 1, 'All Jobs', job_id) AS job,
   COUNT(*) "Total Empl", AVG(salary) * 12 "Average Sal"
   FROM employees e, departments d
   WHERE d.department_id = e.department_id
   GROUP BY ROLLUP (department_name, job_id);DEPARTMENT                     JOB        Total Empl Average Sal
------------------------------ ---------- ---------- -----------
Accounting                     AC_ACCOUNT          1       99600
Accounting                     AC_MGR              1      144000
Accounting                     All Jobs            2      121800
Administration                 AD_ASST             1       52800
Administration                 All Jobs            1       52800
Executive                      AD_PRES             1      288000
Executive                      AD_VP               2      204000
Executive                      All Jobs            3      232000
Finance                        FI_ACCOUNT          5       95040
Finance                        FI_MGR              1      144000
Finance                        All Jobs            6      103200