在Oracle SQL中使用AVG()

Question

我有一个名为Student的表格如下:

CREATE TABLE  "STUDENT" 
(   "ID" NUMBER(*,0), 
    "NAME" VARCHAR2(20), 
    "AGE" NUMBER(*,0), 
    "CITY" VARCHAR2(20), 
    PRIMARY KEY ("ID") ENABLE
)

我想把所有年龄都大于平均年龄的学生记录下来.这是我试过的:

SELECT *
FROM student
WHERE age > AVG(age)

和

SELECT * 
FROM student
HAVING age > AVG(age)

两种方式都不起作用!

Answer 1

如果您要使用没有组的聚合,则无法引用其他字段.(你跟*)

但是,您可以创建一个子查询.

SELECT *
FROM student
WHERE age > (SELECT AVG(age) FROM STUDENT)

这很容易编写和理解.但是,如果使用分析函数,Justin Cave在他的回答中解释说,你可以获得更好的表现

Answer 2

Conrad Fix建议的子查询方法是传统方法.然而,它不太可能是最有效的方法,因为它需要Oracle两次打击 - 一次计算平均年龄,一次取回工资高于平均水平的行.如果使用分析函数,只需按一次表并执行(大约)一半的逻辑I/O操作,即可完成相同的操作.

select *
  from (select s.*, avg(age) over () avg_age
          from student s)
 where age > avg_age

传统方法需要18个一致的获取并且必须对表进行两次完整扫描(请注意,我运行了两次测试以获得最低值以排除延迟块清除之类的事情)

SQL> ed
Wrote file afiedt.buf

  1  select *
  2    from hr.employees
  3   where salary > (select avg(salary)
  4*                    from hr.employees)
SQL> /

51 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 1945967906

---------------------------------------------------------------------------------
| Id  | Operation           | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |           |     5 |   345 |     6   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL  | EMPLOYEES |     5 |   345 |     3   (0)| 00:00:01 |
|   2 |   SORT AGGREGATE    |           |     1 |     4 |            |          |
|   3 |    TABLE ACCESS FULL| EMPLOYEES |   107 |   428 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("SALARY"> (SELECT AVG("SALARY") FROM "HR"."EMPLOYEES"
              "EMPLOYEES"))


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         18  consistent gets
          0  physical reads
          0  redo size
       5532  bytes sent via SQL*Net to client
        557  bytes received via SQL*Net from client
          5  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         51  rows processed

然而,分析函数方法在一次表扫描中执行相同的操作,只有7个一致的获取

SQL> select *
  2    from (select e.*, avg(salary) over () avg_salary
  3            from hr.employees e)
  4   where salary > avg_salary
  5  /

51 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 48081388

---------------------------------------------------------------------------------
| Id  | Operation           | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |           |   107 | 15622 |     3   (0)| 00:00:01 |
|*  1 |  VIEW               |           |   107 | 15622 |     3   (0)| 00:00:01 |
|   2 |   WINDOW BUFFER     |           |   107 |  7383 |     3   (0)| 00:00:01 |
|   3 |    TABLE ACCESS FULL| EMPLOYEES |   107 |  7383 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("SALARY">"AVG_SALARY")


Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          7  consistent gets
          0  physical reads
          0  redo size
       5220  bytes sent via SQL*Net to client
        557  bytes received via SQL*Net from client
          5  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
         51  rows processed

然而,正如康拉德指出的那样,分析函数方法需要一种排序,因此它应该比传统方法使用更多的PGA.你将减少I/O以增加RAM.通常这是一个理想的权衡,但你应该知道这一点.