All the Pairs With the Maximum Number of Common Followers - Problem
You are given a table Relations that represents follower relationships between users.
Task: Find all pairs of users that have the maximum number of common followers.
- A common follower is someone who follows both users in a pair
- Return pairs
(user1_id, user2_id)whereuser1_id < user2_id - Only return pairs with the maximum count of common followers
For example, if the maximum number of common followers between any two users is 3, return all pairs that have exactly 3 common followers.
Table Schema
Relations
| Column Name | Type | Description |
|---|---|---|
user_id
PK
|
int | ID of the user being followed |
follower_id
PK
|
int | ID of the user who is following |
Primary Key: (user_id, follower_id)
Note: Each row indicates that follower_id follows user_id. The combination of user_id and follower_id is unique.
Input & Output
Example 1 — Basic Common Followers
Input Table:
| user_id | follower_id |
|---|---|
| 1 | 3 |
| 2 | 3 |
| 7 | 3 |
| 1 | 4 |
| 2 | 4 |
| 7 | 4 |
| 1 | 5 |
| 6 | 5 |
Output:
| user1_id | user2_id |
|---|---|
| 1 | 2 |
| 1 | 7 |
| 2 | 7 |
💡 Note:
Users 1, 2, and 7 all have followers 3 and 4 in common. Each pair (1,2), (1,7), and (2,7) has 2 common followers, which is the maximum. User 6 only shares follower 5 with user 1, giving them 1 common follower, which is less than the maximum.
Example 2 — Single Pair Maximum
Input Table:
| user_id | follower_id |
|---|---|
| 1 | 3 |
| 2 | 3 |
| 1 | 4 |
| 2 | 4 |
| 1 | 5 |
| 2 | 5 |
| 6 | 7 |
Output:
| user1_id | user2_id |
|---|---|
| 1 | 2 |
💡 Note:
Users 1 and 2 have 3 common followers (3, 4, 5), which is the maximum. User 6 has no common followers with any other user, so only pair (1,2) is returned.
Constraints
-
1 ≤ user_id ≤ 1000 -
1 ≤ follower_id ≤ 1000 -
user_id ≠ follower_id(users cannot follow themselves)
Visualization
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Understanding the Visualization
1
Input
Relations table with user-follower pairs
2
Self-Join
Join on follower_id to find common followers
3
Output
Pairs with maximum common follower count
Key Takeaway
🎯 Key Insight: Self-join tables on common attributes to find relationships between different entities
💡
Explanation
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