使用 GROUP BY 选择多个(非聚合函数)列 [英] Select multiple (non-aggregate function) columns with GROUP BY

问题描述

我试图从一列中选择最大值，同时按另一个具有多个重复值的非唯一 id 列进行分组.原始数据库看起来像:

I am trying to select the max value from one column, while grouping by another non-unique id column which has multiple duplicate values. The original database looks something like:

mukey    | comppct_r | name | type
65789    | 20        | a    | 7n
65789    | 15        | b    | 8m
65789    | 1         | c    | 1o
65790    | 10        | a    | 7n
65790    | 26        | b    | 8m
65790    | 5         | c    | 1o
...

这很好用:

SELECT c.mukey, Max(c.comppct_r) AS ComponentPercent
FROM c
GROUP BY c.mukey;

返回一个表，如:

mukey    | ComponentPercent
65789    | 20
65790    | 26
65791    | 50
65792    | 90

我希望能够在不影响 GROUP BY 函数的情况下添加其他列，将名称和类型等列包含到输出表中，例如:

I want to be able to add other columns in without affecting the GROUP BY function, to include columns like name and type into the output table like:

mukey    | comppct_r | name | type
65789    | 20        | a    | 7n
65790    | 26        | b    | 8m
65791    | 50        | c    | 7n
65792    | 90        | d    | 7n

但它总是输出一个错误，说我需要使用带有 select 语句的聚合函数.我应该怎么做?

but it always outputs an error saying I need to use an aggregate function with select statement. How should I go about doing this?

推荐答案

你有自己的 greatest-n-per-group 问题.这是可能的解决方案之一:

You have yourself a greatest-n-per-group problem. This is one of the possible solutions:

select c.mukey, c.comppct_r, c.name, c.type
from c yt
inner join(
    select c.mukey, max(c.comppct_r) comppct_r
    from c
    group by c.mukey
) ss on c.mukey = ss.mukey and c.comppct_r= ss.comppct_r

另一种可能的方法，相同的输出:

Another possible approach, same output:

select c1.*
from c c1
left outer join c c2
on (c1.mukey = c2.mukey and c1.comppct_r < c2.comppct_r)
where c2.mukey is null;

这里有一个关于这个主题的全面和解释性的答案:SQL 仅选择列上具有最大值的行

There's a comprehensive and explanatory answer on the topic here: SQL Select only rows with Max Value on a Column

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