INNER JOIN with NULL: NULL keys don't match (NULL = NULL is false), so they're excluded. Count: SELECT COUNT(*) FROM a INNER JOIN b ON a.key = b.key. If key has nulls, inner join count is less than min(|a|,|b|). To include null matches: ON (a.key = b.key OR (a.key IS NULL AND...
This medium-level SQL question appears frequently in data engineering interviews at companies like Paytm. 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.
INNER JOIN with NULL: NULL keys don't match (NULL = NULL is false), so they're excluded. Count: SELECT COUNT(*) FROM a INNER JOIN b ON a.key = b.key. If key has nulls, inner join count is less than min(|a|,|b|). To include null matches: ON (a.key = b.key OR (a.key IS NULL AND b.key IS NULL)). Count depends on null handling. Why it matters: Design choices compound at scale—wrong approach can cause 100× overhead. Scalability trade-offs: Profile before optimizing; validate on sample then full. Cost implications: Suboptimal choices multiply at billion-row scale.
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Analyze My Answer — FreeAccording 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.