sql - Calculating percentages with GROUP BY query

Question

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.

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

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

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

question from:https://stackoverflow.com/questions/6207224/calculating-percentages-with-group-by-query

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;

Or

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);

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