使用 GROUP BY 查询计算百分比 [英] Calculating percentages with GROUP BY query
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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