问题描述
如何编写一个查询，其中只返回具有最高或最低列值的选定行数.
How do you write a query where only a select number of rows are returned with either the highest or lowest column value.
即一份薪水最高的 5 名员工的报告?
i.e. A report with the 5 highest salaried employees?
推荐答案
最好的方法是使用解析函数，RANK() 或 DENSE_RANK() ...
The best way to do this is with analytic functions, RANK() or DENSE_RANK() ...
SQL> select * from (
2 select empno
3 , sal
4 , rank() over (order by sal desc) as rnk
5 from emp)
6 where rnk <= 5
7 /
EMPNO SAL RNK
---------- ---------- ----------
7839 5000 1
7788 3000 2
7902 3000 2
7566 2975 4
8083 2850 5
7698 2850 5
6 rows selected.
SQL>
DENSE_RANK() 在平局时压缩间隙:
DENSE_RANK() compresses the gaps when there is a tie:
SQL> select * from (
2 select empno
3 , sal
4 , dense_rank() over (order by sal desc) as rnk
5 from emp)
6 where rnk <= 5
7 /
EMPNO SAL RNK
---------- ---------- ----------
7839 5000 1
7788 3000 2
7902 3000 2
7566 2975 3
8083 2850 4
7698 2850 4
8070 2500 5
7 rows selected.
SQL>
您喜欢哪种行为取决于您的业务需求.
Which behaviour you prefer depends upon your business requirements.
还有 ROW_NUMBER() 分析函数，我们可以使用它来返回精确的行数.但是，我们应该避免使用基于行号的解决方案，除非业务逻辑乐于在出现平局时任意截断结果集.要求五个最高值和按高值排序的前五个记录
There is also the ROW_NUMBER() analytic function which we can use to return a precise number of rows. However, we should avoid using solutions based on row number unless the business logic is happy to arbitrarily truncate the result set in the event of a tie. There is a difference between asking for the five highest values and the first five records sorted by high values
还有一个使用 ROWNUM 伪列的非解析解.这很笨拙，因为 ROWNUM 在 ORDER BY 子句之前应用，这可能会导致意外结果.很少有理由使用 ROWNUM 而不是 ROW_NUMBER() 或排名函数之一.
There is also a non-analytic solution using the ROWNUM pseudo-column. This is clunky because ROWNUM is applied before the ORDER BY clause, which can lead to unexpected results. There is rarely any reason to use ROWNUM instead of ROW_NUMBER() or one of the ranking functions.
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