Question

1633. Percentage of Users Attended a Contest

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Description

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%

Solutions

Solution 1: Grouping and Subquery

We can group the Register table by contest_id and count the number of registrations for each contest. The registration rate of each contest is the number of registrations divided by the total number of registrations.

# Write your MySQL query statement below
SELECT
  contest_id,
  ROUND(COUNT(1) * 100 / (SELECT COUNT(1) FROM Users), 2) AS percentage
FROM Register
GROUP BY 1
ORDER BY 2 DESC, 1;