**Approaches**: IS NULL / IS NOT NULL for filtering. **COALESCE(val1, val2, ...)**: First non-NULL value; useful for defaults. **NULLIF(val1, val2)**: Returns NULL if equal; e.g., NULLIF(divisor, 0) to avoid divide-by-zero. **Why it matters**: NULL propagates in expressions;...
Red Flag: Joining on nullable columns without considering NULL semantics. Pro-Move: Say you use COALESCE for display defaults but avoid NULLs in business keys; use sentinel values or separate NULL handling.
This medium-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 (join, sql) will help you answer variations of this question confidently.
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.
Approaches: IS NULL / IS NOT NULL for filtering. COALESCE(val1, val2, ...): First non-NULL value; useful for defaults. NULLIF(val1, val2): Returns NULL if equal; e.g., NULLIF(divisor, 0) to avoid divide-by-zero. Why it matters: NULL propagates in expressions; aggregate functions ignore NULL (except COUNT(*)). JOIN on NULL yields no match (NULL ≠ NULL). Scalability: COALESCE in SELECT is cheap; in WHERE or JOIN it can prevent index use. Cost: Explicit handling avoids silent wrong results; NULLs in join keys can cause unexpected row loss.
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Analyze My Answer — FreeAccording to DataEngPrep.tech, this is one of the most frequently asked SQL interview questions, reported at 4 companies. DataEngPrep.tech maintains a curated database of 1,863+ real data engineering interview questions across 7 categories, verified by industry professionals.