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SQL Problem - multiple table joins and window functions

SQLmedium0.5 min readPremium

Multiple tables + window: join fact to dims, then apply window (PARTITION BY dim, ORDER BY date). Example: SELECT f.*, d.name, SUM(f.amount) OVER (PARTITION BY f.customer_id ORDER BY f.date) running_total FROM fact f JOIN dim_customer d ON f.customer_id = d.id. Use CTEs: with...

🤖 Analyze Your Answer
Frequency
Low
Asked at 1 company
Category
487
questions in SQL
Difficulty Split
130E|271M|86H
in this category
Total Bank
1,863
across 7 categories
Asked at these companies
Lumiq
Key Concepts Tested
joinpartitionsqlwindow

Why This Question Matters

This medium-level SQL question appears frequently in data engineering interviews at companies like Lumiq. While less common, it tests deeper understanding that distinguishes strong candidates. Mastering the underlying concepts (join, partition, sql) will help you answer variations of this question confidently.

How to Approach This

Break this problem into components. Identify the core trade-offs involved, then walk the interviewer through your reasoning step by step. Demonstrate awareness of edge cases and production considerations - this is what separates good answers from great ones.

Expert Answer
94 words

Multiple tables + window: join fact to dims, then apply window (PARTITION BY dim, ORDER BY date). Example: SELECT f., d.name, SUM(f.amount) OVER (PARTITION BY f.customer_id ORDER BY f.date) running_total FROM fact f JOIN dim_customer d ON f.customer_id = d.id. Use CTEs: with joined as (select ... joins), windowed as (select , row_number() over (...) from joined) select * from windowed where ... Why it matters: Design choices compound at scale—wrong approach can cause 100× overhead. Scalability trade-offs: Profile before optimizing; validate on sample then full. Cost implications: Suboptimal choices multiply at billion-row scale.

The complete answer continues with detailed implementation patterns, architectural trade-offs, and production-grade considerations covering performance optimization and real-world examples.

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According to DataEngPrep.tech, this is one of the most frequently asked SQL interview questions, reported at 1 company. DataEngPrep.tech maintains a curated database of 1,863+ real data engineering interview questions across 7 categories, verified by industry professionals.

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