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What is a cross-join?

SQLmedium0.6 min readPremium

Cross-join (Cartesian product): Combines every row of table A with every row of table B. Result size: |A| × |B|. Syntax: SELECT * FROM a CROSS JOIN b or FROM a, b. Use cases: Generate all combinations (e.g., dates × products for a grid), expanding dimensions. Danger: Large...

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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
Altimetrik
Key Concepts Tested
join

Why This Question Matters

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

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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
110 words

Cross-join (Cartesian product): Combines every row of table A with every row of table B. Result size: |A| × |B|. Syntax: SELECT * FROM a CROSS JOIN b or FROM a, b. Use cases: Generate all combinations (e.g., dates × products for a grid), expanding dimensions. Danger: Large tables produce huge results. Example: CROSS JOIN calendar to get row per day. Best practice: Use only for small tables or with filters; explicit CROSS JOIN is clearer than omitting ON. 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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