1633. Percentage of Users Attended a Contest.sql

-- Question
-- +-------------+---------+
-- | 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%

-- Solution
select 
    contest_id, 
    round(count(distinct user_id)/(
        select count(distinct user_id)
        from Users
    )*100, 2) percentage
from Register
group by contest_id
order by percentage desc, contest_id;