WITH ranked AS (SELECT CustomerName, sale_date, ROW_NUMBER() OVER (PARTITION BY CustomerName ORDER BY sale_date) AS rn FROM sales WHERE CustomerName = 'Alice Johnson') SELECT DATEDIFF((SELECT sale_date FROM ranked WHERE rn = 3), (SELECT sale_date FROM ranked WHERE rn = 1)) AS...
This medium-level SQL question appears frequently in data engineering interviews at companies like Grover. 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.
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.
WITH ranked AS (SELECT CustomerName, sale_date, ROW_NUMBER() OVER (PARTITION BY CustomerName ORDER BY sale_date) AS rn FROM sales WHERE CustomerName = 'Alice Johnson') SELECT DATEDIFF((SELECT sale_date FROM ranked WHERE rn = 3), (SELECT sale_date FROM ranked WHERE rn = 1)) AS day_diff FROM ranked LIMIT 1. Or LAG/LEAD. Why: Extract 1st and 3rd sale dates; compute difference. Edge case: Fewer than 3 sales—returns NULL.
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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.