使用GROUP BY查询计算百分比

Question

我有一个包含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.

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

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,但我无法弄清楚如何计算GROUP BY中的百分比:

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;

要么

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);

我会看到其中一个或另一个是否为您的RDBMS提供了更好的查询计划.

Answer 2

或者,您可以采用老派的方式 - 可以说更容易理解:

select usr.User                   as User   ,
       usr.Rating                 as Rating ,
       usr.N                      as N      ,
       (100.0 * item.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