Question

所以说我有一个名为Key_Values的表，看起来像

--keys--|--values--
  A     |    1     
  A     |    1     
  A     |    2     
  B     |    1     
  B     |    1     
  C     |    3   
  C     |    3     
  C     |    4

我需要编写一个select语句，该语句将获取所有不同的键以及它们各自的值的模式。它将返回此

--keys--|--values--
  A     |    1     
  B     |    1     
  C     |    3

我在找出正确的方法时遇到了一些麻烦。我知道我可以执行DISTINCT（keys），但是我不确定如何从中获取值。

Answer 1

如果数据库支持，则可以使用窗口函数：

select key, value as mode
from (select key, value, count(*) as cnt,
             row_number() over (partition by key order by count(*) desc) as seqnum
      from keyvalue
      group by key, value
     ) kv
where seqnum = 1;

在MS Access中，这要痛苦得多：

select key, value, count(*) as cnt
from keyvalue kv
group by key, value
having kv.value = (select top (1) kv2.value
                   from keyvalue kv2
                   where kv2.key = kv.key
                   group by kv2.value
                   order by count(*) desc
                  );

嵌套SQL选择？从一列中区分出来，并在另一列中获得每个值的对应模式？

1 个答案: