Social Network Connections and Paths

This challenge focuses on designing SQL queries to navigate and extract information from a relational database structured to represent a social network. You will need to think about how to model relationships and then write queries to find connections between users, such as direct friends and friends of friends. This is a fundamental task in understanding graph-like data within a traditional SQL environment.

Problem Description

Your goal is to design SQL queries to find specific relationships and paths within a social network. You are given two tables: Users and Friendships. The Users table contains information about individuals, and the Friendships table represents bidirectional friendships between users.

You need to write queries to answer the following questions:

Find direct friends of a given user.
Find friends of friends of a given user (excluding the user themselves and their direct friends).
Find the shortest path (in terms of friendship hops) between two users. If no path exists, indicate that.

Key Requirements:

All queries should be written in standard SQL.
The solution should be efficient and handle a reasonable number of users and friendships.
Results should be presented clearly and unambiguously.

Expected Behavior:

Queries should return the requested data in a structured format (e.g., a list of user IDs, a path represented as a sequence of user IDs).
Edge cases like users with no friends, users who are friends with themselves (though not expected by schema), or users who are not connected should be handled gracefully.

Edge Cases to Consider:

A user with no friends.
A user who is the target of a path-finding query but is not connected to the starting user.
A user being asked for their own friends.
Cycles in friendships (a user being a friend of a friend of a friend, etc.).

Examples

Let's assume the following table schemas:

Users Table:

Column Name	Data Type	Description
`user_id`	INTEGER	Unique identifier for a user.
`name`	VARCHAR	The name of the user.

Friendships Table:

Column Name	Data Type	Description
`user1_id`	INTEGER	ID of the first user in a friendship.
`user2_id`	INTEGER	ID of the second user in a friendship.
(Note: Friendships are bidirectional, meaning if user A is friends with user B, then user B is also friends with user A. This implies that an entry (A, B) also means (B, A) conceptually, though both might not be explicitly present in the table to avoid redundancy, or they might be. For this challenge, assume if (A, B) is present, it represents a bidirectional link. If your database model requires explicit entries for both directions, adjust accordingly.)

Example 1: Finding Direct Friends

Input: Users table:

`user_id`	`name`
1	Alice
2	Bob
3	Charlie
4	David

Friendships table:

`user1_id`	`user2_id`
1	2
1	3
2	4

Query to find direct friends of user_id = 1.

Output:

`friend_id`	`friend_name`
2	Bob
3	Charlie

Explanation: User 1 (Alice) is directly friends with users 2 (Bob) and 3 (Charlie) as indicated by the Friendships table.

Example 2: Finding Friends of Friends

Input: Same Users and Friendships tables as Example 1.

Query to find friends of friends of user_id = 1, excluding user 1 and their direct friends.

Output:

`fof_id`	`fof_name`
4	David

Explanation:

User 1's direct friends are User 2 and User 3.
User 2's friends are User 1 and User 4.
User 3's friends are User 1.
The friends of User 1's friends are User 1, User 2, User 3, and User 4.
Excluding User 1 (the original user) and their direct friends (User 2, User 3), the remaining friend of a friend is User 4 (David).

Example 3: Finding the Shortest Path

Input: Users table:

`user_id`	`name`
1	Alice
2	Bob
3	Charlie
4	David
5	Eve
6	Frank

Friendships table:

`user1_id`	`user2_id`
1	2
2	3
3	4
4	5
1	6

Query to find the shortest path between user_id = 1 and user_id = 5.

Output:

`path_order`	`user_id`	`name`
1	1	Alice
2	2	Bob
3	3	Charlie
4	4	David
5	5	Eve

Explanation: The shortest path from User 1 to User 5 is: Alice -> Bob -> Charlie -> David -> Eve. This involves 4 friendship hops. User 1 is also friends with User 6, but this path (1 -> 6) does not lead to User 5 efficiently.

Example 4: No Path Exists

Input: Users table:

`user_id`	`name`
1	Alice
2	Bob
3	Charlie
4	David

Friendships table:

`user1_id`	`user2_id`
1	2
3	4

Query to find the shortest path between user_id = 1 and user_id = 4.

Output: (No rows returned, or a specific indicator for "no path found" if the query design supports it).

Explanation: User 1 is connected to User 2, and User 3 is connected to User 4. However, there is no chain of friendships linking User 1 to User 4.

Constraints

The user_id will be an integer between 1 and 1,000,000.
The name will be a string with a maximum length of 100 characters.
The Friendships table can contain up to 5,000,000 entries.
Queries should ideally complete within a few seconds on a moderately sized dataset (e.g., 100,000 users, 500,000 friendships).
Assume all user_ids in Friendships table exist in the Users table.

Notes

For finding friends of friends, you'll likely need to join the Friendships table with itself or use subqueries. Be mindful of excluding the original user and their direct friends from the final result.
Finding the shortest path in a graph using SQL is typically achieved using Recursive Common Table Expressions (RCTEs). This is a powerful SQL feature that allows you to query hierarchical data.
Consider how your SQL dialect handles bidirectional relationships. If the Friendships table only stores one direction (e.g., user1_id < user2_id), you will need to account for that in your queries to treat friendships as bidirectional. For this challenge, assume the table explicitly represents all unique friendships, and you might need to query both user1_id and user2_id columns to find connections. If an entry (A, B) is present, it means A and B are friends. You might need to construct an "adjacency list" or similar view within your query to easily find all friends of a user.
For pathfinding, think about how to track the path as it grows and how to stop the recursion once the target is found or a maximum path length is reached.

Social Network Connections and Paths

Problem Description

You need to write queries to answer the following questions:

Find direct friends of a given user.
Find friends of friends of a given user (excluding the user themselves and their direct friends).
Find the shortest path (in terms of friendship hops) between two users. If no path exists, indicate that.

Key Requirements:

All queries should be written in standard SQL.
The solution should be efficient and handle a reasonable number of users and friendships.
Results should be presented clearly and unambiguously.

Expected Behavior:

Queries should return the requested data in a structured format (e.g., a list of user IDs, a path represented as a sequence of user IDs).
Edge cases like users with no friends, users who are friends with themselves (though not expected by schema), or users who are not connected should be handled gracefully.

Edge Cases to Consider:

A user with no friends.
A user who is the target of a path-finding query but is not connected to the starting user.
A user being asked for their own friends.
Cycles in friendships (a user being a friend of a friend of a friend, etc.).

Examples

Let's assume the following table schemas:

Users Table:

Column Name	Data Type	Description
`user_id`	INTEGER	Unique identifier for a user.
`name`	VARCHAR	The name of the user.

Friendships Table:

Column Name	Data Type	Description
`user1_id`	INTEGER	ID of the first user in a friendship.
`user2_id`	INTEGER	ID of the second user in a friendship.
(Note: Friendships are bidirectional, meaning if user A is friends with user B, then user B is also friends with user A. This implies that an entry (A, B) also means (B, A) conceptually, though both might not be explicitly present in the table to avoid redundancy, or they might be. For this challenge, assume if (A, B) is present, it represents a bidirectional link. If your database model requires explicit entries for both directions, adjust accordingly.)

Example 1: Finding Direct Friends

Input: Users table:

`user_id`	`name`
1	Alice
2	Bob
3	Charlie
4	David

Friendships table:

`user1_id`	`user2_id`
1	2
1	3
2	4

Query to find direct friends of user_id = 1.

Output:

`friend_id`	`friend_name`
2	Bob
3	Charlie

Explanation: User 1 (Alice) is directly friends with users 2 (Bob) and 3 (Charlie) as indicated by the Friendships table.

Example 2: Finding Friends of Friends

Input: Same Users and Friendships tables as Example 1.

Query to find friends of friends of user_id = 1, excluding user 1 and their direct friends.

Output:

`fof_id`	`fof_name`
4	David

Explanation:

User 1's direct friends are User 2 and User 3.
User 2's friends are User 1 and User 4.
User 3's friends are User 1.
The friends of User 1's friends are User 1, User 2, User 3, and User 4.
Excluding User 1 (the original user) and their direct friends (User 2, User 3), the remaining friend of a friend is User 4 (David).

Example 3: Finding the Shortest Path

Input: Users table:

`user_id`	`name`
1	Alice
2	Bob
3	Charlie
4	David
5	Eve
6	Frank

Friendships table:

`user1_id`	`user2_id`
1	2
2	3
3	4
4	5
1	6

Query to find the shortest path between user_id = 1 and user_id = 5.

Output:

`path_order`	`user_id`	`name`
1	1	Alice
2	2	Bob
3	3	Charlie
4	4	David
5	5	Eve

Example 4: No Path Exists

Input: Users table:

`user_id`	`name`
1	Alice
2	Bob
3	Charlie
4	David

Friendships table:

`user1_id`	`user2_id`
1	2
3	4

Query to find the shortest path between user_id = 1 and user_id = 4.

Output: (No rows returned, or a specific indicator for "no path found" if the query design supports it).

Explanation: User 1 is connected to User 2, and User 3 is connected to User 4. However, there is no chain of friendships linking User 1 to User 4.

Constraints

The user_id will be an integer between 1 and 1,000,000.
The name will be a string with a maximum length of 100 characters.
The Friendships table can contain up to 5,000,000 entries.
Queries should ideally complete within a few seconds on a moderately sized dataset (e.g., 100,000 users, 500,000 friendships).
Assume all user_ids in Friendships table exist in the Users table.

Notes

For finding friends of friends, you'll likely need to join the Friendships table with itself or use subqueries. Be mindful of excluding the original user and their direct friends from the final result.
Finding the shortest path in a graph using SQL is typically achieved using Recursive Common Table Expressions (RCTEs). This is a powerful SQL feature that allows you to query hierarchical data.
Consider how your SQL dialect handles bidirectional relationships. If the Friendships table only stores one direction (e.g., user1_id < user2_id), you will need to account for that in your queries to treat friendships as bidirectional. For this challenge, assume the table explicitly represents all unique friendships, and you might need to query both user1_id and user2_id columns to find connections. If an entry (A, B) is present, it means A and B are friends. You might need to construct an "adjacency list" or similar view within your query to easily find all friends of a user.
For pathfinding, think about how to track the path as it grows and how to stop the recursion once the target is found or a maximum path length is reached.