分组操作group by 和分组的强化（rollup）
分组操作和分组函数的使用，对于编写SQL语句的人来说，是最基本的概念。
我们来看下面的例子：
在这里我们使用员工表EMP
scott@DB01> select * from emp;
EMPNO
ENAME
JOB
MGR
HIREDATE
SAL
COMM
DEPTNO
---------- ---------- --------- ---------- -------------------
---------- ---------- ----------
7369
SMITH
CLERK
7902 1980-12-17
00:00:00
800
20
7499
ALLEN
SALESMAN
7698 1981-02-20
00:00:00
1600
300
30
7521
WARD
SALESMAN
7698 1981-02-22
00:00:00
1250
500
30
7566
JONES
MANAGER
7839 1981-04-02
00:00:00
2975
20
7654
MARTIN
SALESMAN
7698 1981-09-28
00:00:00
1250
1400
30
7698
BLAKE
MANAGER
7839 1981-05-01
00:00:00
2850
30
7782
CLARK
MANAGER
7839 1981-06-09
00:00:00
2450
10
7788
SCOTT
ANALYST
7566 1987-04-19
00:00:00
3000
20
7839
KING
PRESIDENT
1981-11-17
00:00:00
5000
10
7844
TURNER
SALESMAN
7698 1981-09-08
00:00:00
1500
0
30
7876
ADAMS
CLERK
7788 1987-05-23
00:00:00
1100
20
7900
JAMES
CLERK
7698 1981-12-03
00:00:00
950
30
7902
FORD
ANALYST
7566 1981-12-03
00:00:00
3000
20
7934
MILLER
CLERK
7782 1982-01-23
00:00:00
1300
10
14 rows selected.
在员工表中有14条记录，即14个员工，我们可以看到，这14个员工分别属于3个部门（10,20,30),我们可以提出求EMP表中，每个部门的员工薪水总和
scott@DB01> select deptno,sum(sal) tsal
2
from emp
3
group by deptno;
DEPTNO
TSAL
---------- ----------
30
9400
20
10875
10
8750
在这里稍微需要注意的是：select 列表里如果出现列的话，那在group
by语句中同样需要列名，并且只能是列名本身，不能是列的别名。group
by语句可以说是oracle语句里最严格的语句，后面只能跟列的真名，别名、位置号、函数、表达式、子查询
都不被允许。当然如果只考虑实现这里已经做到了，如果我们深入了解一点的话，分组对于数据库来说是要消耗资源的，比如cpu、内存
在oracle9i之前
，分组操作内部主要通过排序来实现，10刚开始，采用hash的算法实现，我们看一下10g下，让面语句的执行计划
scott@DB01> set autotrace trace exp
scott@DB01> /
Execution Plan
----------------------------------------------------------
Plan hash value: 4067220884
---------------------------------------------------------------------------
| Id |
Operation
| Name | Rows | Bytes | Cost (%CPU)|
Time
|
---------------------------------------------------------------------------
| 0 | SELECT
STATEMENT
|
| 14
| 364
|
4 (25)| 00:00:01 |
| 1 | HASH
GROUP
BY
|
| 14
| 364
|
4 (25)| 00:00:01 |
| 2
| TABLE ACCESS FULL|
EMP
| 14
| 364
|
3 (0)| 00:00:01 |
---------------------------------------------------------------------------
Note
-----
- dynamic sampling used for
this statement
其实在有些情况下，我们可以避免hash或是sort的发生，也可以实现分组查询的效果，比如说通过索引，当然这需要你有适当的索引存在。
我们来看下面的演示：
scott@DB01> set autotrace off
scott@DB01> create table s_test(id
number,name varchar2(10),sal number);
Table created.
scott@DB01> begin
2
for i in 1..20000 loop
3
insert into s_test values(i,i||'name',i*10);
4
end loop;
5
commit;
6
end;
7
/
PL/SQL procedure successfully completed.
scott@DB01> /
PL/SQL procedure successfully completed.
scott@DB01> /
PL/SQL procedure successfully completed.
scott@DB01> select count(*) from s_test;
COUNT(*)
----------
60000
我在这里建了一张表s_test,分3次往表里插入数据1-20000，现在我的需求是，找到表里100-120的记录，以及他们出现的次数
scott@DB01> select id,name,count(*) from s_test
where id>=100 and id<=120 group by
id,name;
ID
NAME
COUNT(*)
---------- ---------- ----------
115
115name
3
101
101name
3
103
103name
3
106
106name
3
109
109name
3
118
118name
3
105
105name
3
114
114name
3
102
102name
3
104
104name
3
112
112name
3
116
116name
3
100
100name
3
110
110name
3
113
113name
3
117
117name
3
119
119name
3
107
107name
3
108
108name
3
111
111name
3
120
120name
3
21 rows selected.
我们来看一下语句的执行计划
scott@DB01> set autotrace trace exp
scott@DB01> /
Execution Plan
----------------------------------------------------------
Plan hash value: 752916570
-----------------------------------------------------------------------------
| Id |
Operation
| Name | Rows
| Bytes | Cost (%CPU)|
Time
|
-----------------------------------------------------------------------------
| 0 | SELECT
STATEMENT
|
| 163 | 3260
|
58 (6)| 00:00:01 |
| 1 | HASH
GROUP
BY
|
| 163 | 3260
|
58 (6)| 00:00:01 |
|* 2 | TABLE
ACCESS FULL| S_TEST | 163
| 3260
|
57 (4)| 00:00:01 |
-----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 -
filter("ID">=100 AND "ID"<=120)
Note
-----
- dynamic sampling used for
this statement
在执行计划中我们发现，成本Cost是58，还有cpu的消耗，在执行计划的第2步，我们发现为了实现分组，oracle做了hash。接下来我们建一个组合索引看看
scott@DB01> create index s_id_n_idx on
s_test(id,name);
Index created.
scott@DB01> select id,name,count(*) from s_test
where id>=100 and id<=120 group by
id,name;
Execution Plan
----------------------------------------------------------
Plan hash value: 826362002
-----------------------------------------------------------------------------------
| Id |
Operation
|
Name
| Rows | Bytes | Cost (%CPU)|
Time
|
-----------------------------------------------------------------------------------
| 0 | SELECT
STATEMENT
|
| 63
| 1260
|
2 (0)| 00:00:01 |
| 1 | SORT
GROUP BY
NOSORT|
| 63
| 1260
|
2 (0)| 00:00:01 |
|* 2 | INDEX
RANGE SCAN | S_ID_N_IDX
| 63
| 1260
|
2 (0)| 00:00:01 |
-----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 -
access("ID">=100 AND "ID"<=120)
filter("ID">=100 AND "ID"<=120)
Note
-----
- dynamic sampling used for
this statement
在第一步中，应该做的排序并没有做 SORT GROUP BY NOSORT，这样就节省了cpu。
当然在这个例子当中，我们发现了一个重要的问题，就是语句的成本急剧下降，当然，这是通过索引，改变了数据的访问方法造成的，以后有机会在讨论索引的时候，我们会展开来说。
我们接下来看这样一个需求，根据表里的deptno和job求分组，得到每个job下的薪水综合，然后在部门级别做汇总，求小计，在整张表汇总，求总计
scott@DB01> select deptno,job,empno,ename,sal from
emp order by deptno,job;
DEPTNO
JOB
EMPNO
ENAME
SAL
---------- --------- ---------- ---------- ----------
10
CLERK
7934
MILLER
1300
10
MANAGER
7782
CLARK
2450
10
PRESIDENT
7839
KING
5000
20
ANALYST
7788
SCOTT
3000
20
ANALYST
7902
FORD
3000
20
CLERK
7876
ADAMS
1100
20
CLERK
7369
SMITH
800
20
MANAGER
7566
JONES
2975
30
CLERK
7900
JAMES
950
30
MANAGER
7698
BLAKE
2850
30
SALESMAN
7654
MARTIN
1250
30
SALESMAN
7521
WARD
1250
30
SALESMAN
7499
ALLEN
1600
30
SALESMAN
7844
TURNER
1500
其实需求本身很简单，如果仅仅是为了实现的话，使用集合并运算符union就可以了，不过union的效率在这里是非常的低。
scott@DB01> select deptno,job,sum(sal) tsal from emp
group by deptno,job
2 union
3 select
deptno,to_char(null),sum(sal) from emp group by
deptno
4 union
5 select
to_number(null),to_char(null),sum(sal) from emp;
DEPTNO
JOB
TSAL
---------- --------- ----------
10
CLERK
1300
10
MANAGER
2450
10
PRESIDENT
5000
10
8750
20
ANALYST
6000
20
CLERK
1900
20
MANAGER
2975
20
10875
30
CLERK
950
30
MANAGER
2850
30
SALESMAN
5600
30
9400
29025
13 rows selected.
为了得到比较高效的sql，我们可以借助于oracle分组里面的rollup来实现，我们可以得到同样的效果
scott@DB01> select deptno,job,sum(sal) tsal from emp
group by rollup(deptno,job);
DEPTNO
JOB
TSAL
---------- --------- ----------
10
CLERK
1300
10
MANAGER
2450
10
PRESIDENT
5000
10
8750
20
CLERK
1900
20
ANALYST
6000
20
MANAGER
2975
20
10875
30
CLERK
950
30
MANAGER
2850
30
SALESMAN
5600
30
9400
29025
13 rows selected.
第一直观的表现，使用rollup要比使用分组再union的方法语句简单很多，更重要的是，我们只对emp访问了一次。
为了进一步比较，我们来看一下语句的执行计划
scott@DB01> set autotrace trace exp
scott@DB01> select deptno,job,sum(sal) tsal from emp
group by deptno,job
2 union
3 select
deptno,to_char(null),sum(sal) from emp group by
deptno
4 union
5 select
to_number(null),to_char(null),sum(sal) from emp;
Execution Plan
----------------------------------------------------------
Plan hash value: 3412076862
-----------------------------------------------------------------------------
| Id |
Operation
| Name | Rows | Bytes | Cost (%CPU)|
Time
|
-----------------------------------------------------------------------------
| 0 | SELECT
STATEMENT
|
| 29
| 825
|
14 (79)| 00:00:01 |
| 1 | SORT
UNIQUE
|
| 29
| 825
|
14 (79)| 00:00:01 |
| 2
|
UNION-ALL
|
|
|
|
|
|
| 3
| HASH GROUP
BY
|
| 14
| 448
|
5 (40)| 00:00:01 |
| 4
|
TABLE ACCESS FULL| EMP
| 14
| 448
|
3 (0)| 00:00:01 |
| 5
| HASH GROUP
BY
|
| 14
| 364
|
5 (40)| 00:00:01 |
| 6
|
TABLE ACCESS FULL| EMP
| 14
| 364
|
3 (0)| 00:00:01 |
| 7
| SORT
AGGREGATE
|
|
1 | 13
|
4 (25)| 00:00:01 |
| 8
|
TABLE ACCESS FULL| EMP
| 14
| 182
|
3 (0)| 00:00:01 |
-----------------------------------------------------------------------------
Note
-----
- dynamic sampling used for
this statement
scott@DB01> select deptno,job,sum(sal) tsal from emp
group by rollup(deptno,job);
Execution Plan
----------------------------------------------------------
Plan hash value: 52302870
-----------------------------------------------------------------------------
| Id |
Operation
| Name | Rows | Bytes | Cost (%CPU)|
Time
|
-----------------------------------------------------------------------------
| 0 | SELECT
STATEMENT
|
| 14
| 448
|
4 (25)| 00:00:01 |
| 1 | SORT
GROUP BY
ROLLUP|
| 14
| 448
|
4 (25)| 00:00:01 |
| 2
| TABLE ACCESS
FULL | EMP
| 14
| 448
|
3 (0)| 00:00:01 |
-----------------------------------------------------------------------------
Note
-----
- dynamic sampling used for
this statement
通过比较发现，两个语句的成本cost会差出很多14vs4。所以，如果我们以后有上面类似的需求的话，可以考虑使用rollup。
注：rollup语法
select
a,b,组函数
from 表
group by
rollup(a,b);
这个语法相当于 group by a,b union group a union group by
null的sql语句的组合
以上就是Oracle SQL篇（四）group by 分组与分组的加强 rollup的详细内容，更多关于Oracle SQL篇（四）group by 分组与分组的加强 rollup的资料请关注九品源码其它相关文章！
