使用GROUP BY GROUPING SETS相当于把需要GROUP的集合用UNION ALL联合起来。
当GROUPING SETS里面的分组元素越多时，使用GROUPING SETS比使用UNION ALL性能更好，这可能和使用GROUPING SETS只需要访问一次表有关。
如下两段查询的结果是相等的：
q1（GROUPING SETS）：
SELECT department_id, job_id, manager_id, AVG (salary) FROM employees
GROUP BY GROUPING SETS ((department_id, job_id), (job_id, manager_id));
DEPARTMENT_ID JOB_ID MANAGER_ID AVG(SALARY)
------------- ---------- ---------- -----------
SH_CLERK 122 3200
AC_MGR 101 12000
ST_MAN 100 7280
ST_CLERK 121 2675
SA_REP 148 8650
SH_CLERK 120 2900
SH_CLERK 124 2825
MK_MAN 100 13000
AD_PRES 24000
FI_MGR 101 12000
SA_REP 146 8500
SH_CLERK 123 3475
AD_ASST 101 4400
IT_PROG 102 9000
IT_PROG 103 4950
FI_ACCOUNT 108 7920
PU_MAN 100 11000
ST_CLERK 122 2700
SA_REP 145 8500
AC_ACCOUNT 205 8300
AD_VP 100 17000
DEPARTMENT_ID JOB_ID MANAGER_ID AVG(SALARY)
------------- ---------- ---------- -----------
ST_CLERK 120 2625
ST_CLERK 124 2925
SA_REP 147 7766.66667
SA_REP 149 8333.33333
HR_REP 101 6500
PR_REP 101 10000
ST_CLERK 123 3000
SH_CLERK 121 3675
PU_CLERK 114 2780
SA_MAN 100 12200
MK_REP 201 6000
110 AC_ACCOUNT 8300
90 AD_VP 17000
50 ST_CLERK 2785
80 SA_REP 8396.55172
50 ST_MAN 7280
80 SA_MAN 12200
110 AC_MGR 12000
90 AD_PRES 24000
60 IT_PROG 5760
100 FI_MGR 12000
DEPARTMENT_ID JOB_ID MANAGER_ID AVG(SALARY)
------------- ---------- ---------- -----------
30 PU_CLERK 2780
50 SH_CLERK 3215
20 MK_MAN 13000
100 FI_ACCOUNT 7920
SA_REP 7000
70 PR_REP 10000
30 PU_MAN 11000
10 AD_ASST 4400
20 MK_REP 6000
40 HR_REP 6500
52 rows selected.
q2（UNION ALL）：
SELECT NULL department_id, job_id, manager_id, AVG (salary) FROM employees
GROUP BY (job_id, manager_id)
UNION ALL
SELECT department_id, job_id, NULL manager_id, AVG (salary)
FROM employees
GROUP BY (department_id, job_id);
DEPARTMENT_ID JOB_ID MANAGER_ID AVG(SALARY)
------------- ---------- ---------- -----------
SH_CLERK 122 3200
AC_MGR 101 12000
ST_MAN 100 7280
ST_CLERK 121 2675
SA_REP 148 8650
SH_CLERK 120 2900
SH_CLERK 124 2825
MK_MAN 100 13000
AD_PRES 24000
FI_MGR 101 12000
SA_REP 146 8500
SH_CLERK 123 3475
AD_ASST 101 4400
IT_PROG 102 9000
IT_PROG 103 4950
FI_ACCOUNT 108 7920
PU_MAN 100 11000
ST_CLERK 122 2700
SA_REP 145 8500
AC_ACCOUNT 205 8300
AD_VP 100 17000
DEPARTMENT_ID JOB_ID MANAGER_ID AVG(SALARY)
------------- ---------- ---------- -----------
ST_CLERK 120 2625
ST_CLERK 124 2925
SA_REP 147 7766.66667
SA_REP 149 8333.33333
HR_REP 101 6500
PR_REP 101 10000
ST_CLERK 123 3000
SH_CLERK 121 3675
PU_CLERK 114 2780
SA_MAN 100 12200
MK_REP 201 6000
110 AC_ACCOUNT 8300
90 AD_VP 17000
50 ST_CLERK 2785
80 SA_REP 8396.55172
50 ST_MAN 7280
80 SA_MAN 12200
110 AC_MGR 12000
90 AD_PRES 24000
60 IT_PROG 5760
100 FI_MGR 12000
DEPARTMENT_ID JOB_ID MANAGER_ID AVG(SALARY)
------------- ---------- ---------- -----------
30 PU_CLERK 2780
50 SH_CLERK 3215
20 MK_MAN 13000
100 FI_ACCOUNT 7920
SA_REP 7000
70 PR_REP 10000
30 PU_MAN 11000
10 AD_ASST 4400
20 MK_REP 6000
40 HR_REP 6500
52 rows selected.
