在Oracle SQL中使用AVG（）

我有一个名为Student的表，如下所示：

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

我试图获得所有年龄大于平均年龄的学生的记录。这就是我所尝试的：

SELECT *
FROM student
WHERE age > AVG(age)

及

---

两种方法都不行

如果要使用没有group by的聚合，则不能引用其他字段。（您与*）

但是，您可以创建一个子查询

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

---

这很容易写和理解。但是，如果使用分析函数，则可以获得更好的性能，因为子查询方法是传统方法。然而，这不太可能是最有效的方法，因为它需要Oracle两次点击表格——一次是计算平均年龄，一次是提取工资高于平均水平的行。如果您使用分析函数，您可以完成同样的事情，同时只需点击表一次并执行（大约）一半的逻辑I/O操作

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

传统的方法需要18个一致的get，并且必须对表进行两次完整扫描（注意，我运行了两次测试，以获得最低值，从而排除延迟块清理之类的事情）

然而，分析函数方法在一个表扫描中只使用7个一致的get执行相同的操作

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

---

正如康拉德指出的那样，解析函数法需要排序，因此它应该比传统方法使用更多的PGA。您将以减少的I/O换取增加的RAM。正常情况下，这是一个理想的折衷方案，但您应该注意这一点。

+1非常好的替代方案，尽管值得注意，正如您的数据所示，您正在用CPU/IO交换RAM。我有点开悟。非常感谢。

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

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