How to normalize group by count results?

Question

How can the results of a "group by" count be normalized by the count's sum?

For example, given:

User     Rating (1-5)
----------------------
1        3
1        4
1        2
3        5
4        3
3        2
2        3

The result will be:

User    Count   Percentage
---------------------------
1       3      .42         (=3/7)
2       1      .14         (=1/7)
3       2      .28         (...)
4       1      .14

So for each user the number of ratings they provided is given as the percentage of the total ratings provided by everyone.

Answer 1

SELECT DISTINCT ON (user) user, count(*) OVER (PARTITION BY user) AS cnt,
       count(*) OVER (PARTITION BY user) / count(*) OVER () AS percentage;

The count(*) OVER (PARTITION BY user) is a so-called window function. Window functions let you perform some operation over a "window" created by some "partition" which is here made over the user id. In plain and simple English: the partitioned count(*) is calculated for each distinct user value, so in effect it counts the number of rows for each user value.