Oracle 中一列的前 n 个不同值

Top n distinct values of one column in Oracle(Oracle 中一列的前 n 个不同值)

问题描述

我正在使用一个查询，其中一部分获取特定列的前 3 个.

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

它创建列的不同子查询，限制为 3 行，然后将这些行过滤到主查询以执行前 3 行.

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

所以原来的表是这样的:

So the original table is like this:

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

并且查询返回列的前 3 行(不是前 3 行，它只会是 a):

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有更正确的方法来执行此操作，因为实际查询很大，并且使用看起来几乎相同的子查询复制其大小只是为了获得前 3 名很难使用和理解/修改.

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.

是否有更好的方法来处理 Oracle 中一列的前 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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