WITH ranked AS (SELECT *, ROW_NUMBER() OVER (PARTITION BY key_col1, key_col2 ORDER BY created_at ASC) AS rn FROM my_table) DELETE FROM my_table WHERE (key_col1, key_col2, created_at) IN (SELECT key_col1, key_col2, created_at FROM ranked WHERE rn > 1). **Why**: ORDER BY...
This medium-level SQL question appears frequently in data engineering interviews at companies like BCG. 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 *, ROW_NUMBER() OVER (PARTITION BY key_col1, key_col2 ORDER BY created_at ASC) AS rn FROM my_table) DELETE FROM my_table WHERE (key_col1, key_col2, created_at) IN (SELECT key_col1, key_col2, created_at FROM ranked WHERE rn > 1). Why: ORDER BY created_at ASC keeps earliest. For engines without CTE+DELETE: use temp table or MERGE. Best practice: Run in transaction; backup first; add unique constraint after dedup.
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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.