Friend Requests II: Who Has the Most Friends

Friend Requests II: Who Has the Most Friends - Problem

Database Medium

You are given a table RequestAccepted that tracks accepted friend requests between users.

Table: RequestAccepted

Column Name	Type
requester_id	int
accepter_id	int
accept_date	date

The primary key is (requester_id, accepter_id). This table contains the ID of the user who sent the request, the ID of the user who received the request, and the date when the request was accepted.

Task: Write a SQL solution to find the person who has the most friends and return their ID along with the total number of friends.

The test cases guarantee that only one person has the maximum number of friends.

Table Schema

RequestAccepted

Column Name	Type	Description
`requester_id` PK	int	ID of user who sent the friend request
`accepter_id` PK	int	ID of user who accepted the friend request
`accept_date`	date	Date when the request was accepted

Primary Key: (requester_id, accepter_id)

Note: Each row represents an accepted friend request. When a request is accepted, both users become friends with each other.

Input & Output

Example 1 — Basic Friend Network

Input Table:

requester_id	accepter_id	accept_date
1	2	2016-06-03
1	3	2016-06-08
2	3	2016-06-08
3	4	2016-06-09

Output:

id	num
3	3

💡 Note:

User 3 has the most friends (3 total). User 3 appears as: accepter when user 1 sent request, accepter when user 2 sent request, and requester when sending to user 4. This gives 3 total friend connections.

Example 2 — Tie Broken by Single Connection

Input Table:

requester_id	accepter_id	accept_date
1	2	2016-06-03
3	4	2016-06-08

Output:

id	num
1	1

💡 Note:

All users have exactly 1 friend each. The query returns user 1 with 1 friend (user 2). Each friendship creates 2 entries in our UNION result, so each person gets counted once.

Constraints

1 ≤ requester_id, accepter_id ≤ 10000
requester_id ≠ accepter_id
All friend requests in the table are accepted
Test cases guarantee exactly one person has the maximum friends

Visualization

Tap to expand

Understanding the Visualization

Input Analysis

Each row shows one accepted friendship

UNION Strategy

Combine requesters and accepters into single user list

Count Friends

GROUP BY to count total friendships per user

Key Takeaway

🎯 Key Insight: Use UNION ALL to treat friendships as bidirectional relationships, then count occurrences

Asked in

F Facebook 25 L LinkedIn 18 T Twitter 12

Use UNION ALL to combine requester_id and accepter_id into a single user column, then GROUP BY with COUNT(*) to find who appears most frequently as having friends.

Table Schema

RequestAccepted

Column Name	Type	Description
`requester_id` PK	int	ID of user who sent the friend request
`accepter_id` PK	int	ID of user who accepted the friend request
`accept_date`	date	Date when the request was accepted

Primary Key: (requester_id, accepter_id)

Note: Each row represents an accepted friend request. When a request is accepted, both users become friends with each other.

Common Approaches

Approach	Time	Space	Notes
✓ UNION with GROUP BY	O(n log n)	O(n)	Combine requester and accepter IDs using UNION, then group by user ID to count total friends

UNION with GROUP BY — Algorithm Steps

Step 1: SELECT requester_id as user_id to count sent requests
Step 2: UNION ALL with accepter_id as user_id to count accepted requests
Step 3: GROUP BY user_id and COUNT(*) to get total friends
Step 4: ORDER BY count DESC and LIMIT 1 to get the person with most friends

Visualization

Tap to expand

Step-by-Step Walkthrough

Extract Users

Get requester_id and accepter_id separately

UNION ALL

Combine into single user_id column

GROUP BY

Count occurrences per user

Code -

solution.c — C

Time & Space Complexity

Time Complexity

⏱️

O(n log n)

UNION ALL is O(n), GROUP BY with sorting is O(n log n)

⚡ Linearithmic

Space Complexity

O(n)

Temporary storage for UNION result and grouping

⚡ Linearithmic Space

34.5K Views

High Frequency

~12 min Avg. Time

892 Likes

Ln 1, Col 1

Smart Actions

💡 Explanation

AI Ready

💡 Suggestion Tab to accept Esc to dismiss

// Output will appear here after running code

Code Editor Closed

Click the red button to reopen

Table Schema

Input & Output

Constraints

Visualization

Related Problems

Table Schema

Common Approaches

UNION with GROUP BY — Algorithm Steps

Visualization

Code -

Time & Space Complexity

Select Compiler