Calculate Contest Participation Percentage in SQL

Question

Given `users` (user_id) and `register` (contest_id, user_id), find the percentage of total users registered in each contest, rounded to 2 decimal places. Sort by percentage descending, then contest_id ascending.

Accepted Answer

Optimal Solution:
SELECT 
  contest_id,
  ROUND(100.0 * COUNT(user_id) / (SELECT COUNT(*) FROM users), 2) AS percentage
FROM register
GROUP BY contest_id
ORDER BY percentage DESC, contest_id ASC;

Technical Explanation:
A scalar subquery `(SELECT COUNT(*) FROM users)` provides the global total user denominator across all contest groups.

user_id	contest_id
6	215
6	209
7	215

contest_id	percentage
215	66.67
209	33.33