[英]max count together in an sql query 2
This is referring to the question i asked before, and got a really quick answaer ( max count together in an sql query ). 这是指我之前问过的问题,并且得到了非常快的回答( 在sql查询中最大和在一起 )。 The Problemset is similar, but the solution in the prevous question would force me to access a db in a loop which will cause performance problems.
问题集是相似的,但是上一个问题中的解决方案迫使我循环访问数据库,这将导致性能问题。 So what i have now, after some joins is:
因此,经过一些加入,我现在拥有的是:
id | description
0 | bla
0 | blub
0 | bla
1 | blablub
1 | bla
... | ...
As u can see, now the id is not a primary key anymore. 如您所见,现在id不再是主键。 What i want is to get the most used description for each id in the resultset.
我想要的是获取结果集中每个ID的最常用的描述。 It should look something like this:
它看起来应该像这样:
id | most_popular_description | times_the_desc_appeared_for_an_id
0 | bla | 2
1 | blablub | 1
... | ... | ...
This should do the trick. 这应该可以解决问题。
select id, description, COUNT(description)
from mytable
group by id, description
order by 3 desc
If you only want the most popular items, then I believe this should give you the result set you're looking for. 如果您只想要最受欢迎的商品,那么我相信这应该会为您提供想要的结果集。 There are other ways of doing this, but stats_mode is the easiest way to obtain the "most prevalent" value in a group (ie the Mode).
还有其他方法可以执行此操作,但是stats_mode是获取组中“最普遍”值(即Mode)的最简单方法。
SELECT t.id,
t.description AS most_popular_description,
COUNT(*) AS times_the_desc_appeared_for_an_id
FROM mytable t INNER JOIN (
SELECT id, stats_mode(description) AS desc FROM mytable GROUP BY id
) a ON t.id = a.id AND t.description = a.desc
GROUP BY t.id, t.description;
Note that the nested query (inline view) is necessary since you also want the count. 请注意,嵌套查询(内联视图)是必需的,因为您还需要计数。
I think you can use the dense_rank() analytic function to get the top N for each group set. 我认为您可以使用density_rank()分析函数来获取每个组的前N个。
Something like this: 像这样:
select id, description, times_the_desc_appeared_for_an_id
from
(
select id, description, count(description) times_the_desc_appeared_for_an_id
dense_rank() over (partition by id, description order by count(description) desc) position
from mytable
group by id, description
)
where
position <= 3
order by id, times_the_desc_appeared_for_an_id;
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