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Hard database problems similar to Leetcodify Similar Friends are common in data engineering and SQL-heavy interviews at large tech companies. They test your ability to reason about joins, aggregations, and query optimization.
Optimization can be achieved by indexing key columns such as user_id, song_id, and day. Filtering early and minimizing unnecessary join combinations also helps reduce query execution time.
The optimal approach uses SQL joins and aggregation. By joining the listening table with itself and filtering using the friendship table, you can count shared listening events and identify similar friend pairs efficiently.
The most important concepts are self joins, GROUP BY aggregation, and filtering with HAVING. These allow you to detect overlapping listening events and count how many times two users behave similarly.