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Problem solution · SQL

Number of Comments per Post

Number of Comments per Post: 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
14 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 Number of Comments per Post, 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

  • 14 lines of SQL from the credited upstream file 1241.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 codeNumber of Comments per Post · SQLSQL
Use this to learn the idea, then write your own version.
WITH  Posts AS (    SELECT DISTINCT sub_id AS post_id    FROM Submissions    WHERE parent_id IS NULL  )SELECT  Posts.post_id,  COUNT(DISTINCT Comments.sub_id) AS number_of_commentsFROM PostsLEFT JOIN Submissions AS Comments  ON (Posts.post_id = Comments.parent_id)GROUP BY 1;

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