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What is the difference between a primary key and a unique key?

SQLhard0.4 min read

**Primary Key**: Unique identifier; NOT NULL; one per table; often clustered. **Unique Key**: Enforces uniqueness; can have NULL (SQL allows one NULL per column in uniqueness); multiple per table. **Why it matters**: PK defines identity and referential integrity; unique...

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Frequency
Low
Asked at 4 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
AccentureCognizantEPAMYash Technologies
Interview Pro Tip

Red Flag: Saying 'unique key allows multiple NULLs' (depends on SQL dialect). Pro-Move: Clarify that in data lakes, PK is a logical concept—Delta Lake enforces via merge; enforcement is eventual.

Key Concepts Tested
sparksql

Why This Question Matters

This hard-level SQL question appears frequently in data engineering interviews at companies like Accenture, Cognizant, EPAM, and 1 others. While less common, it tests deeper understanding that distinguishes strong candidates. Mastering the underlying concepts (spark, sql) will help you answer variations of this question confidently.

How to Approach This

This is a senior-level question that tests architectural thinking. Lead with the high-level design, then drill into specifics. Discuss trade-offs explicitly - there is rarely one correct answer. Show awareness of scale, fault tolerance, and operational complexity.

Expert Answer
79 words

Primary Key: Unique identifier; NOT NULL; one per table; often clustered. Unique Key: Enforces uniqueness; can have NULL (SQL allows one NULL per column in uniqueness); multiple per table. Why it matters: PK defines identity and referential integrity; unique constrains alternate keys (e.g., email). Scalability: PK is often the clustering key; choice affects physical layout. Unique indexes enable lookups. Cost: Each constraint adds index overhead. Production note: In distributed systems (Spark, Delta), PK is logical; enforcement may be application-level.

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