Question

我希望你能帮我完成我的作业：）

我们需要构建一个输出前N名最佳薪酬员工的查询。

我的版本完美无缺例如前三名：

SELECT name, salary
FROM staff
WHERE salary IN ( SELECT * 
                  FROM ( SELECT salary
                         FROM staff 
                         ORDER BY salary DESC ) 
                  WHERE ROWNUM <= 3 )
ORDER BY salary DESC
;

请注意，这将输出排名前3位且薪水相同的员工。

1：迈克，4080年   2：史蒂夫，2800
  2：苏珊，2800
  2：杰克，2800
  3：Chloe，1400

但现在我们的老师不允许我们使用ROWNUM 我搜索得很远，没有找到任何可用的东西。

我的第二个解决方案感谢Justin Caves的提示。

首先我试过这个：

SELECT name, salary, ( rank() OVER ( ORDER BY salary DESC ) ) as myorder
FROM staff
WHERE myorder <= 3
;

错误消息是：“myorder：invalid identifier”

感谢DCookie，现在很清楚：

“[...]分析在之后应用 where子句被评估，哪个这就是你得到错误的原因是无效的标识符。“

环绕一个SELECT解决了这个问题：

SELECT *
FROM ( SELECT name, salary, rank() OVER ( ORDER BY salary DESC ) as myorder FROM staff )
WHERE myorder <= 3
;

我的老师再次罢工，不允许这种奇特的分析功能。

来自@Justin Caves的

第三个解决方案。

“如果分析功能也是如此不允许，我能做的另一种选择想象 - 一个你永远不会，永远，实际上写在实践中，就像“

SELECT name, salary
  FROM staff s1
 WHERE (SELECT COUNT(*)
          FROM staff s2
         WHERE s1.salary < s2.salary) <= 3

Answer 1

因为这是家庭作业，所以提示而不是答案。您将需要使用分析函数。 ROW_NUMBER，RANK或DENSE_RANK可以根据您想要处理关系的方式工作。

如果也不允许使用分析函数，我可以想象的另一个选项 - 就是你永远不会，实际上在实践中写的那个，就像是

SELECT name, salary
  FROM staff s1
 WHERE (SELECT COUNT(*)
          FROM staff s2
         WHERE s1.salary < s2.salary) <= 3

关于性能，我不会依赖查询计划中的COST编号 - 这只是一个估计值，通常不可能比较不同SQL语句的计划之间的成本。您最好不要查看查询实际执行的一致获取次数，并考虑查询性能随着表中行数的增加而缩放的情况。第三个选项的效率远远低于其他两个选项，因为它需要两次扫描STAFF表。

我没有你的STAFF表，所以我将使用SCOTT模式中的EMP表

分析函数解决方案实际上与ROWNUM解决方案一样进行7次一致性获取

Wrote file afiedt.buf

  1  select ename, sal
  2    from( select ename,
  3                 sal,
  4                 rank() over (order by sal) rnk
  5            from emp )
  6*  where rnk <= 3
SQL> /

ENAME             SAL
---------- ----------
smith             800
SM0               950
ADAMS            1110


Execution Plan
----------------------------------------------------------
Plan hash value: 3291446077

--------------------------------------------------------------------------------
-
| Id  | Operation                | Name | Rows  | Bytes | Cost (%CPU)| Time
|
--------------------------------------------------------------------------------
-
|   0 | SELECT STATEMENT         |      |    14 |   672 |     4  (25)| 00:00:01
|*  1 |  VIEW                    |      |    14 |   672 |     4  (25)| 00:00:01
|*  2 |   WINDOW SORT PUSHED RANK|      |    14 |   140 |     4  (25)| 00:00:01
|   3 |    TABLE ACCESS FULL     | EMP  |    14 |   140 |     3   (0)| 00:00:01
--------------------------------------------------------------------------------
-

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

   1 - filter("RNK"<=3)
   2 - filter(RANK() OVER ( ORDER BY "SAL")<=3)


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

SQL> select ename, sal
  2    from( select ename, sal
  3            from emp
  4           order by sal )
  5   where rownum <= 3;

ENAME             SAL
---------- ----------
smith             800
SM0               950
ADAMS            1110


Execution Plan
----------------------------------------------------------
Plan hash value: 1744961472

--------------------------------------------------------------------------------
| Id  | Operation               | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT        |      |     3 |   105 |     4  (25)| 00:00:01 |
|*  1 |  COUNT STOPKEY          |      |       |       |            |          |
|   2 |   VIEW                  |      |    14 |   490 |     4  (25)| 00:00:01 |
|*  3 |    SORT ORDER BY STOPKEY|      |    14 |   140 |     4  (25)| 00:00:01 |
|   4 |     TABLE ACCESS FULL   | EMP  |    14 |   140 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------


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

   1 - filter(ROWNUM<=3)
   3 - filter(ROWNUM<=3)


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

然而，COUNT（*）解决方案实际上会执行99次一致性获取，并且必须对表执行两次完整扫描，因此效率低10倍以上。随着表中行数的增加，它的扩展性会更差

SQL> select ename, sal
  2    from emp e1
  3   where (select count(*) from emp e2 where e1.sal < e2.sal) <= 3;

ENAME             SAL
---------- ----------
JONES            2975
SCOTT            3000
KING             5000
FORD             3000
FOO


Execution Plan
----------------------------------------------------------
Plan hash value: 2649664444

----------------------------------------------------------------------------
| Id  | Operation           | Name | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      |    14 |   140 |    24   (0)| 00:00:01 |
|*  1 |  FILTER             |      |       |       |            |          |
|   2 |   TABLE ACCESS FULL | EMP  |    14 |   140 |     3   (0)| 00:00:01 |
|   3 |   SORT AGGREGATE    |      |     1 |     4 |            |          |
|*  4 |    TABLE ACCESS FULL| EMP  |     1 |     4 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------

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

   1 - filter( (SELECT COUNT(*) FROM "EMP" "E2" WHERE
              "E2"."SAL">:B1)<=3)
   4 - filter("E2"."SAL">:B1)


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

Answer 2

你必须用另一个select包装语句的原因是因为外部select语句是将结果集限制为所需行数的语句。这是一个helpful link on analytics。如果你自己运行内部选择，你会明白为什么你必须这样做。在评估where子句之后应用分析，这就是为什么你得到myorder是无效标识符的错误。

Answer 3

甲骨文？窗函数怎么样？

select * from 
(SELECT s.*, row_number over (order by salary desc ) as rn FROM staff s )
where rn <=3

Answer 4

当您使用count(distinct <exp>)时，相同排名的最高工资将被视为平局排名。

select NAME, SALARY
from  STAFF STAFF1
where 3 >= ( select count(distinct STAFF2.SALARY) RANK
               from STAFF STAFF2
              where STAFF2.SALARY >= STAFF1.SALARY)

Answer 5

您可以在Oracle 12c中解决此问题

select NAME, SALARY
from  STAFF
order by SALARY DESC
FETCH FIRST 3 ROWS ONLY

（FETCH FIRST语法是Oracle 12c的新增功能）

在没有ROWNUM的情况下选择前N行？

5 个答案: