%OfUsersAttendedContest.sql

-- Table: Users

-- +-------------+---------+
-- | Column Name | Type    |
-- +-------------+---------+
-- | user_id     | int     |
-- | user_name   | varchar |
-- +-------------+---------+
-- user_id is the primary key (column with unique values) for this table.
-- Each row of this table contains the name and the id of a user.
 
-- Table: Register

-- +-------------+---------+
-- | Column Name | Type    |
-- +-------------+---------+
-- | contest_id  | int     |
-- | user_id     | int     |
-- +-------------+---------+
-- (contest_id, user_id) is the primary key (combination of columns with unique values) for this table.
-- Each row of this table contains the id of a user and the contest they registered into.
 

-- Write a solution to find the percentage of the users registered in each contest rounded to two decimals.

-- Return the result table ordered by percentage in descending order. In case of a tie, order it by contest_id in ascending order.

-- The result format is in the following example.

-- Example 1:

-- Input: 
-- Users table:
-- +---------+-----------+
-- | user_id | user_name |
-- +---------+-----------+
-- | 6       | Alice     |
-- | 2       | Bob       |
-- | 7       | Alex      |
-- +---------+-----------+
-- Register table:
-- +------------+---------+
-- | contest_id | user_id |
-- +------------+---------+
-- | 215        | 6       |
-- | 209        | 2       |
-- | 208        | 2       |
-- | 210        | 6       |
-- | 208        | 6       |
-- | 209        | 7       |
-- | 209        | 6       |
-- | 215        | 7       |
-- | 208        | 7       |
-- | 210        | 2       |
-- | 207        | 2       |
-- | 210        | 7       |
-- +------------+---------+
-- Output: 
-- +------------+------------+
-- | contest_id | percentage |
-- +------------+------------+
-- | 208        | 100.0      |
-- | 209        | 100.0      |
-- | 210        | 100.0      |
-- | 215        | 66.67      |
-- | 207        | 33.33      |
-- +------------+------------+
-- Explanation: 
-- All the users registered in contests 208, 209, and 210. The percentage is 100% and we sort them in the answer table by 
-- contest_id in ascending order.
-- Alice and Alex registered in contest 215 and the percentage is ((2/3) * 100) = 66.67%
-- Bob registered in contest 207 and the percentage is ((1/3) * 100) = 33.33%

-- Write your PostgreSQL query statement below
-- Solution
with total_users as 
(
    select count(user_id) as total_users from Users 
)
select contest_id,
       round(round(users_contest/(select total_users from total_users)::numeric * 100, 4)::numeric, 2) as percentage
from (
    select Register.contest_id,
        count(Users.user_id) as users_contest
    from Register left join Users 
    on Register.user_id = Users.user_id 
    group by Register.contest_id
)
order by round(round(users_contest/(select total_users from total_users)::numeric * 100, 4)::numeric, 2) desc,
         contest_id asc