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What are primary keys and foreign keys? Why are they important?

SQLmedium0.4 min read

**Architectural Logic**: PK: Unique, NOT NULL identifier for entity integrity; often clustered index. FK: Reference to PK in another table; enforces referential integrity (no orphans). **Why**: PKs enable uniqueness checks, join performance, replication. FKs prevent orphaned...

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Frequency
Low
Asked at 2 companies
Category
487
questions in SQL
Difficulty Split
130E|271M|86H
in this category
Total Bank
1,863
across 7 categories
Asked at these companies
PresidioSwiggy
Key Concepts Tested
join

Why This Question Matters

This medium-level SQL question appears frequently in data engineering interviews at companies like Presidio, Swiggy. 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.

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
88 words

Architectural Logic: PK: Unique, NOT NULL identifier for entity integrity; often clustered index. FK: Reference to PK in another table; enforces referential integrity (no orphans). Why: PKs enable uniqueness checks, join performance, replication. FKs prevent orphaned child rows and encode relationships. In warehouses, PKs/FKs are often logical (not enforced) for load performance. Scalability: Enforced FKs add validation cost on INSERT/UPDATE; in high-throughput ingestion, logical FKs + batch validation are common. Cost: PK index is usually clustered (one per table); FK indexes speed up joins but add write overhead.

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 2 companies. 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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