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Difference Between Truncate/Delete and Union/Union All – Performance and Usage

SQLhard0.5 min readPremium

**Architectural Logic**: TRUNCATE vs DELETE and UNION vs UNION ALL have semantic and performance implications. **TRUNCATE vs DELETE**: TRUNCATE = metadata-only, fast, no WHERE, no rollback. DELETE = row-level, supports WHERE, rollback, slower. Use TRUNCATE for full refresh;...

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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
Presidio
Key Concepts Tested
etlspark

Why This Question Matters

This hard-level SQL question appears frequently in data engineering interviews at companies like Presidio. While less common, it tests deeper understanding that distinguishes strong candidates. Mastering the underlying concepts (etl, spark) 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
100 words

Architectural Logic: TRUNCATE vs DELETE and UNION vs UNION ALL have semantic and performance implications. TRUNCATE vs DELETE: TRUNCATE = metadata-only, fast, no WHERE, no rollback. DELETE = row-level, supports WHERE, rollback, slower. Use TRUNCATE for full refresh; DELETE for selective removal. UNION vs UNION ALL: UNION = deduplicates (implicit sort/distinct); slower. UNION ALL = keeps all rows; faster. Use UNION when dedup required; UNION ALL when impossible or acceptable. Scalability: TRUNCATE O(1); DELETE O(n). UNION ALL avoids shuffle for dedup. Cost: TRUNCATE for staging; UNION ALL for ETL appends. Distributed: UNION ALL preferred in Spark to avoid expensive dedup.

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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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