使用 GROUP BY 查询计算百分比

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问题描述

我有一个包含 3 列的表格，如下所示:

I have a table with 3 columns which looks like this:

File    User     Rating (1-5)
------------------------------
00001    1        3
00002    1        4
00003    2        2
00004    3        5
00005    4        3
00005    3        2
00006    2        3
Etc.

我想生成一个查询，输出以下内容(对于每个用户和评级，显示文件数量以及文件百分比):

I want to generate a query that outputs the following (for each user and rating, display the number of files as well as percentage of files):

User    Rating   Count   Percentage
-----------------------------------
1       1         3      .18
1       2         6      .35
1       3         8      .47
2       5         12     .75
2       3         4      .25

使用 Postgresql，我知道如何使用以下查询创建包含前 3 列的查询，但我不知道如何计算 GROUP BY 中的百分比:

With Postgresql, I know how to create a query that includes the first 3 columns using the following query, but I can't figure out how to calculate percentage within the GROUP BY:

SELECT
    User,
    Rating,
    Count(*)
FROM
    Results
GROUP BY
    User, Rating
ORDER BY
    User, Rating

这里我希望百分比计算适用于每个用户/评级组.

Here I want the percentage calculation to apply to each user/rating group.

推荐答案

WITH t1 AS 
 (SELECT User, Rating, Count(*) AS n 
  FROM your_table
  GROUP BY User, Rating)
SELECT User, Rating, n, 
       (0.0+n)/(COUNT(*) OVER (PARTITION BY User)) -- no integer divide!
FROM t1;

或

SELECT User, Rating, Count(*) OVER w_user_rating AS n, 
        (0.0+Count(*) OVER w_user_rating)/(Count(*) OVER (PARTITION BY User)) AS pct
FROM your_table
WINDOW w_user_rating AS (PARTITION BY User, Rating);

我想看看这些中的一个或另一个是否会使用适合您的 RDBMS 的工具产生更好的查询计划.

I would see if one of these or the other yields a better query plan with the appropriate tool for your RDBMS.

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