Oracle中一列的前n个不同值 [英] Top n distinct values of one column in Oracle

问题描述

我正在使用查询，查询的一部分获得某一列的前3名.

I'm using a query where a part of it gets the top 3 of a certain column.

它会创建该列的唯一子查询，并限制3行，然后将这些行过滤到主查询中以进行前3个查询.

It creates a distinct subquery of the column, limited by 3 number of rows, and then filters those rows to the main query to do the top 3.

WITH subquery AS (
  SELECT col FROM (
    SELECT DISTINCT col
    FROM tbl
  ) WHERE ROWNUM <= 3
)

SELECT col
FROM tbl
WHERE tbl.col = subquery.col

原始表格如下:

 col
-----
 a
 a
 a
 b
 b
 b
 c
 d
 d
 e
 f
 f
 f
 f

查询返回该列的前3名(而不是仅前a的前3行):

And the query returns the top 3 of the column (not the top 3 rows which would only be a):

 col
-----
 a
 a
 a
 b
 b
 b
 c

我正在尝试了解是否存在更正确的方法，因为真正的查询很大，并且使用看起来几乎相同的子查询来复制它的大小，只是为了获得前三名很难并且难以理解/modify.

I'm trying to learn if there is a more correct way of doing this as the real query is big and duplicating its size with a subquery that looks almost the same just to get the top 3 is hard to work with and understand/modify.

在Oracle中是否有更好的方法来处理一列的前3个不同值?

Is there a better way to do the top first 3 distinct values of one column in Oracle?

推荐答案

是的，您可以使用dense_rank并避免重复的代码:

Yes, you can use dense_rank and avoid duplicated code:

select col 
  from (select col, dense_rank() over (order by col) rnk from tbl)
  where rnk <= 3

^演示

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