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SQL UNION and Set Operations Interview Questions
Set operation questions test whether you can combine and compare result sets from multiple queries.
Expect UNION, UNION ALL, INTERSECT, and EXCEPT with understanding of duplicate handling and performance implications.
Interviewers look for correct usage patterns, awareness of when to use UNION vs UNION ALL, and edge case handling.
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Common SQL set operation interview patterns
- UNION vs UNION ALL for combining distinct vs all rows
- Using INTERSECT to find common rows between queries
- Using EXCEPT for anti-joins and finding differences
- Performance considerations with large result sets
- Combining multiple queries with different schemas
- Set operations with ORDER BY and LIMIT
- Nested set operations and complex query combinations
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Common mistakes with SQL set operations
- Using UNION when UNION ALL is needed (or vice versa)
- Forgetting that UNION removes duplicates (performance cost)
- Misunderstanding the position of ORDER BY with multiple queries
- Incorrect column ordering across multiple SELECT statements
- Not considering NULL handling in set operations
- Column count or type mismatches in UNION queries
How set operation questions are evaluated in interviews
Correctness and efficiency trade-offs matter significantly.
Demonstrate understanding of UNION vs UNION ALL impact on performance.
Explain when set operations are appropriate vs JOINs or subqueries.
Related SQL concepts
SQL UNION and Set Operations Interview FAQs
When should I use UNION vs UNION ALL?
Use UNION when you need to remove duplicates from combined result sets. Use UNION ALL when all rows should be included or you know duplicates won't exist. UNION ALL is faster since it skips the deduplication step.
What is the difference between UNION and INTERSECT/EXCEPT?
UNION combines rows from both queries (with or without duplicates). INTERSECT returns only rows that appear in both queries. EXCEPT returns rows from the first query that don't appear in the second query.