使用 GROUP BY 查询计算百分比

Posted 2023-02-16

【中文标题】使用 GROUP BY 查询计算百分比

【英文标题】：Calculating percentages with GROUP BY query

【发布时间】：2011-09-06 15:00:50

【问题描述】：

我有一个包含 3 列的表格，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（对于每个用户和评分，显示文件数和文件百分比）：

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，我希望将百分比计算应用于每个用户/评级组。

【参考方案1】：

最好的方法是使用window functions。

【讨论】：

您能详细说明一下吗？

【参考方案2】：

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;

或者

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);

我会看看其中一个或另一个是否使用适合您的 RDBMS 的工具产生更好的查询计划。

【讨论】：

谢谢安德鲁！我对您的第二个查询进行了稍微修改的版本：select user, rating, cnt, cnt::float * 100/(sum(cnt) over (partition by user)) from (select user, rating, count(*) as cnt from tbl group by user, rating) a order by user,rating

在这两个例子中都需要使用 SUM(COUNT(*)) 吗？第一个示例中的 (SUM(COUNT(*)) OVER (PARTITION BY User)) 之类的？使用 SUM 我得到预期值，否则我将除以评级数而不是它们的总和。

@TylerDeWitt，我想是的，请参阅上面 DeadMonkey 的评论。

【参考方案3】：

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！

【讨论】：

谢谢！这也可以在不使用窗口函数的情况下工作。我必须在上面的查询中将“item”更改为“usr”并删除第一个 count(*)。

【参考方案4】：

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;

【参考方案5】：

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