**Architectural Logic**: UNION concatenates result sets and deduplicates (implicit DISTINCT); UNION ALL concatenates and keeps all rows. **Why**: UNION requires sort/hash to detect duplicates—O(n log n) or O(n) with hash. UNION ALL is a simple concatenation—O(n)....
Red Flag: Using UNION 'to be safe' when sources are known to be disjoint—wastes compute. Pro-Move: 'Our incremental union is always UNION ALL; we deduplicate in a separate step with a defined key.'
This easy-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.
Start by clearly defining the core concept being asked about. Interviewers want to see that you understand the fundamentals before diving into implementation details. Structure your answer with a definition, then explain the practical application with a concise example.
Architectural Logic: UNION concatenates result sets and deduplicates (implicit DISTINCT); UNION ALL concatenates and keeps all rows. Why: UNION requires sort/hash to detect duplicates—O(n log n) or O(n) with hash. UNION ALL is a simple concatenation—O(n). Scalability: UNION doubles I/O (read both, write deduped). At scale, UNION on large inputs can spill and bottleneck. Cost: UNION can 2–5x bytes processed vs UNION ALL when inputs are large. Use UNION only when business requires uniqueness; use UNION ALL by default. Both require identical column count and compatible types.
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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.