Percentage of Users Attended a Contest

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Medium

Description

You have two tables: Users and Register. The Register table records which users signed up for which contests. Write a SQL query to find the percentage of users registered in each contest, rounded to 2 decimal places. Return the result ordered by percentage descending. If there is a tie, order by contest_id ascending. (contest_id, user_id) is the primary key.

Database Schema

Users

Column Name	Type	Description
user_id	INT	Primary key
user_name	VARCHAR	User name

Register

Column Name	Type	Description
contest_id	INT	Contest identifier
user_id	INT	FK to Users

Example

Users

user_id	user_name
6	Alice
2	Bob
7	Alex

Register

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
209	100
210	100
215	66.67
207	33.33

Explanation:

Contests 208, 209, 210 had all 3 users (100%). Contest 215 had 2/3 users (66.67%). Contest 207 had 1/3 users (33.33%).

Approach hint

Start with a simple approach, explain the trade-off, then move toward a cleaner or more scalable solution.

Common mistake

Skipping assumptions, edge cases, or trade-offs can make an otherwise good answer feel incomplete.