CPMagical CP Library

Problem solution · SQL

All the Pairs With the Maximum Number of Common Followers

All the Pairs With the Maximum Number of Common Followers: a SQL solution using relational aggregation. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Relational aggregation
Source
walkccc LeetCode Solutions
Length
16 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Relational aggregation

For All the Pairs With the Maximum Number of Common Followers, the query transforms and combines relational rows, then filters or aggregates them into the requested result.

  1. Identify the source rows and join keys.
  2. Apply filters before aggregation when possible.
  3. Group, rank, or project the final columns required by the result.

Code notes

  • 16 lines of SQL from the credited upstream file 1951.sql.
  • The implementation keeps its working state in language-native values and containers.
  • No explicit loop blocks detected.

Complexity

Review join cardinality, grouping keys, and available indexes when estimating query cost.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeAll the Pairs With the Maximum Number of Common Followers · SQLSQL
Use this to learn the idea, then write your own version.
WITH  RankedRelations AS (    SELECT      User1.user_id AS user1_id,      User2.user_id AS user2_id,      RANK() OVER(ORDER BY COUNT(User1.follower_id) DESC) AS `rank`    FROM Relations AS User1    INNER JOIN Relations AS User2      USING (follower_id)    WHERE User1.user_id < User2.user_id    GROUP BY 1, 2  )SELECT user1_id, user2_idFROM RankedRelationsWHERE `rank` = 1;

Keep practising

Did this explanation save you time? I'm a Grade 11 student building this free library to make difficult algorithms easier to understand.

Buy me a coffee ↗