经常写sql语句的人应该知道group by语句的主要用法是进行分类汇总，下面是一种它最常见的用法（根据部门、职位分别统计业绩）：
select a.dname,b.job,sum(b.sal) sum_sal from dept a,emp b where a.deptno = b.deptno group by a.dname,b.job; dname job sum_sal -------------- --------- ---------- sales manager 2850 sales clerk 950 sales salesman 5600 accounting manager 2450 accounting president 5000 accounting clerk 1300 research manager 2975 research analyst 6000 research clerk 1900这时候，如果有人跑过来跟你说：我除了以上数据之外，还要每个部门总的业绩以及所有部门加起来的业绩，这时候你很可能会想到如下的笨方法（union all）：
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select * from (
select a.dname,b.job,sum(b.sal) sum_sal
from dept a,emp b
where a.deptno = b.deptno
group by a.dname,b.job
union all
--实现了部门的小计
select a.dname,null, sum(b.sal) sum_sal
from dept a,emp b
where a.deptno = b.deptno
group by a.dname
union all
--实现了所有部门总的合计
select null,null, sum(b.sal) sum_sal
from dept a,emp b
where a.deptno = b.deptno)
order by dname;
dname job sum_sal
-------------- --------- ----------
accounting clerk 1300
accounting manager 2450
accounting president 5000
accounting 8750
research clerk 1900
research manager 2975
research analyst 6000
research 10875
sales clerk 950
sales manager 2850
sales salesman 5600
sales 9400
29025
union all 合并笨办法产生的执行计划
-------------------------------------------------------------------------------
plan hash value: 2979078843
-------------------------------------------------------------------------------
| id | operation | name | rows | bytes | cost (%cpu)| time |
-------------------------------------------------------------------------------
| 0 | select statement | | 29 | 812 | 23 (22)| 00:00:01 |
| 1 | sort order by | | 29 | 812 | 23 (22)| 00:00:01 |
| 2 | view | | 29 | 812 | 22 (19)| 00:00:01 |
| 3 | union-all | | | | | |
| 4 | hash group by | | 14 | 756 | 8 (25)| 00:00:01 |
|* 5 | hash join | | 14 | 756 | 7 (15)| 00:00:01 |
| 6 | table access full| dept | 4 | 88 | 3 (0)| 00:00:01 |
| 7 | table access full| emp | 14 | 448 | 3 (0)| 00:00:01 |
| 8 | hash group by | | 14 | 672 | 8 (25)| 00:00:01 |
|* 9 | hash join | | 14 | 672 | 7 (15)| 00:00:01 |
| 10 | table access full| dept | 4 | 88 | 3 (0)| 00:00:01 |
| 11 | table access full| emp | 14 | 364 | 3 (0)| 00:00:01 |
| 12 | sort aggregate | | 1 | 39 | | |
|* 13 | hash join | | 14 | 546 | 7 (15)| 00:00:01 |
| 14 | table access full| dept | 4 | 52 | 3 (0)| 00:00:01 |
| 15 | table access full| emp | 14 | 364 | 3 (0)| 00:00:01 |
-------------------------------------------------------------------------------
其实，如果你知道group by的rollup扩展的话，这种需求只是小case：
select a.dname,b.job, sum(b.sal) sum_sal
from dept a,emp b
where a.deptno = b.deptno
group by rollup(a.dname,b.job);
dname job sum_sal
-------------- --------- ----------
sales clerk 950
sales manager 2850
sales salesman 5600
sales 9400
research clerk 1900
research analyst 6000
research manager 2975
research 10875
accounting clerk 1300
accounting manager 2450
accounting president 5000
accounting 8750
29025
rollup写法产生的执行计划
-----------------------------------------------------------------------------
plan hash value: 1037965942
-----------------------------------------------------------------------------
| id | operation | name | rows | bytes | cost (%cpu)| time |
-----------------------------------------------------------------------------
| 0 | select statement | | 14 | 756 | 8 (25)| 00:00:01 |
| 1 | sort group by rollup| | 14 | 756 | 8 (25)| 00:00:01 |
|* 2 | hash join | | 14 | 756 | 7 (15)| 00:00:01 |
| 3 | table access full | dept | 4 | 88 | 3 (0)| 00:00:01 |
| 4 | table access full | emp | 14 | 448 | 3 (0)| 00:00:01 |
-----------------------------------------------------------------------------
可以发现，这种方法不但sql书写方便，性能也能得到提高。
这时候，如果又有人跑过来说：除了以上数据，他还需要每个职位总的业绩，你只要把rollup换成cube就可以了，如下所示：
-- cube分组
select a.dname,b.job, sum(b.sal) sum_sal
from dept a,emp b
where a.deptno = b.deptno
group by cube(a.dname,b.job);
dname job sum_sal
-------------- --------- ----------
29025
clerk 4150
analyst 6000
manager 8275
salesman 5600
president 5000
sales 9400
sales clerk 950
sales manager 2850
sales salesman 5600
research 10875
research clerk 1900
research analyst 6000
research manager 2975
accounting 8750
accounting clerk 1300
accounting manager 2450
accounting president 5000
从上面可以看出：cube比rollup的展现的粒度更细一些。这时候，如果又有人跑过来说：他不需要那么细的数据，只需要汇总的数据，可以使用grouping sets：
---grouping sets分组
select to_char(b.hiredate,'yyyy') hire_year,a.dname,b.job, sum(sal) sum_sal
from dept a,emp b
where a.deptno = b.deptno
group by grouping sets(to_char(b.hiredate,'yyyy'),a.dname,b.job);
hire dname job sum_sal
---- -------------- --------- ----------
1987 4100
1980 800
1982 1300
1981 22825
accounting 8750
research 10875
sales 9400
clerk 4150
salesman 5600
president 5000
manager 8275
analyst 6000
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