Percentage of Users Attended a Contest - Practice Coding Problems

Percentage of Users Attended a Contest - Problem

Database Easy

You are given two tables: Users and Register.

The Users table contains information about users with their user_id (primary key) and user_name.

The Register table contains contest registration data with contest_id and user_id as a composite primary key.

Task: Calculate the percentage of users who registered for each contest, rounded to 2 decimal places.

Output requirements:

Order results by percentage in descending order
For ties, order by contest_id in ascending order
Return contest_id and percentage columns

Table Schema

Users

Column Name	Type	Description
`user_id` PK	int	Unique identifier for each user
`user_name`	varchar	Name of the user

Primary Key: user_id

Column Name	Type	Description
`contest_id` PK	int	Identifier for the contest
`user_id` PK	int	User who registered for the contest

Primary Key: (contest_id, user_id)

Input & Output

Example 1 — Basic Contest Registration

Input Tables:

Users

user_id	user_name
1	Alice
2	Bob
3	Charlie
4	David

contest_id	user_id
208	1
208	2
210	2

Output:

contest_id	percentage
208	50
210	25

💡 Note:

Contest 208 has 2 registrations out of 4 total users: (2/4) × 100 = 50.00%. Contest 210 has 1 registration out of 4 total users: (1/4) × 100 = 25.00%. Results are ordered by percentage descending.

Example 2 — Equal Percentages

Input Tables:

Users

user_id	user_name
1	Alice
2	Bob

contest_id	user_id
215	1
220	2

Output:

contest_id	percentage
215	50
220	50

💡 Note:

Both contests have the same percentage (50.00%), so they are ordered by contest_id in ascending order: 215 comes before 220.

Constraints

1 ≤ user_id ≤ 1000
1 ≤ contest_id ≤ 1000
Each user can register for multiple contests
Each (contest_id, user_id) pair is unique in Register table

Visualization

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Understanding the Visualization

Input

Users table and Register table with registrations

Group & Count

Group by contest_id and count registrations

Calculate %

Divide by total users, multiply by 100, and round

Key Takeaway

🎯 Key Insight: Use GROUP BY to aggregate data and calculate percentages with proper rounding and ordering

Asked in

A Amazon 12 M Microsoft 8 F Facebook 6

Use GROUP BY with COUNT() to count registrations per contest, then calculate percentage by dividing by total users and multiplying by 100. Use ROUND() for 2 decimal places.

Table Schema

Users

Column Name	Type	Description
`user_id` PK	int	Unique identifier for each user
`user_name`	varchar	Name of the user

Primary Key: user_id

Column Name	Type	Description
`contest_id` PK	int	Identifier for the contest
`user_id` PK	int	User who registered for the contest

Primary Key: (contest_id, user_id)

Common Approaches

Approach	Time	Space	Notes
✓ GROUP BY with Subquery for Total Users	O(n + m)	O(k)	Count registrations per contest and divide by total user count

GROUP BY with Subquery for Total Users — Algorithm Steps

Step 1: Count registrations for each contest using GROUP BY
Step 2: Get total user count with subquery
Step 3: Calculate percentage and round to 2 decimal places
Step 4: Order by percentage DESC, contest_id ASC

Visualization

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Step-by-Step Walkthrough

Group

Group registrations by contest_id

Count

Count users per contest

Calculate

Divide by total users and multiply by 100

Code -

solution.c — C

Time & Space Complexity

Time Complexity

⏱️

O(n + m)

Single pass through Register table (n) plus Users count (m)

✓ Linear Growth

Space Complexity

O(k)

Storage for k distinct contests in result

✓ Linear Space

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Medium Frequency

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Smart Actions

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Table Schema

Input & Output

Constraints

Visualization

Related Problems

Table Schema

Common Approaches

GROUP BY with Subquery for Total Users — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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