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How to merge two tables with identical structures into one?

SQLmedium0.2 min read

**UNION ALL**: SELECT * FROM t1 UNION ALL SELECT * FROM t2—concatenate; keeps duplicates. **UNION**: Removes duplicates; slower. **MERGE/UPSERT**: When conflict resolution needed—INSERT ... ON CONFLICT, MERGE. **Schema**: Ensure column order and types match. **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
Aarete
Key Concepts Tested
partition

Why This Question Matters

This medium-level SQL question appears frequently in data engineering interviews at companies like Aarete. While less common, it tests deeper understanding that distinguishes strong candidates. Mastering the underlying concepts (partition) 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
44 words

UNION ALL: SELECT FROM t1 UNION ALL SELECT FROM t2—concatenate; keeps duplicates. UNION: Removes duplicates; slower. MERGE/UPSERT: When conflict resolution needed—INSERT ... ON CONFLICT, MERGE. Schema: Ensure column order and types match. Large: Partition; parallel load. Idempotency: Add run_id; dedup if needed.

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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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