使用GROUP BY查询计算百分比

Question

我有一个有3列的表，如下所示：

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，显示文件数量和文件百分比）：

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，但我不知道如何在GROUP BY中计算百分比：

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

在这里，我希望百分比计算适用于每个用户/评级组。

Answer 1

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);

我想看看其中一个或另一个是否能为您的RDBMS提供合适的工具来生成更好的查询计划。

Answer 2

或者，您可以采用老派的方式——可以说更容易地进行查找：

select usr.User                   as User   ,
       usr.Rating                 as Rating ,
       usr.N                      as N      ,
       (100.0 * usr.N) / total.N as Pct
from ( select User, Rating , count(*) as N
       from Results
       group by User , Rating
     ) usr
join ( select User , count(*) as N
       from Results
       group by User
     ) total on total.User = usr.User
order by usr.User, usr.Rating

干杯！

Answer 3

最好的方法是使用窗口函数。

Answer 4

在TSQL中，这应该可以工作

SELECT
    User,
    Rating,
    Count(*), SUM(COUNT(*)) OVER (PARTITION BY User, Rating ORDER BY User, Rating) AS Total,
Count(*)/(SUM(COUNT(*)) OVER (PARTITION BY User, Rating ORDER BY User, Rating)) AS Percentage
FROM
    Results
GROUP BY
    User, Rating
ORDER BY
    User, Rating

Answer 5

WITH data AS 
 (SELECT User, Rating, Count(*) AS Count 
  FROM Results
  GROUP BY User, Rating)
SELECT User, Rating, Count, 
       (0.0+n)/(SUM(Count) OVER (PARTITION BY User))
FROM data;