CPMagical CP Library

Problem solution · SQL

Find Longest Calls

Find Longest Calls: 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
26 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 Find Longest Calls, 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

  • 26 lines of SQL from the credited upstream file 3124.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 codeFind Longest Calls · SQLSQL
Use this to learn the idea, then write your own version.
WITH  RankedCalls AS (    SELECT      Contacts.first_name,      Calls.type,      Calls.duration,      RANK() OVER(        PARTITION BY type        ORDER BY duration DESC      ) AS `rank`    FROM Calls    INNER JOIN Contacts      ON (Calls.contact_id = Contacts.id)  )SELECT  first_name,  type,  CONCAT(    LPAD(FLOOR(duration / 3600), 2, '0'), ':',    LPAD(FLOOR((duration % 3600) / 60), 2, '0'), ':',    LPAD(FLOOR(duration % 60), 2, '0')  ) AS duration_formattedFROM RankedCallsWHERE `rank` <= 3ORDER BY type DESC, duration DESC, first_name DESC;

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 ↗