MERGE INTO dim_customer t USING staging s ON t.customer_id = s.customer_id AND t.is_current = 1 WHEN MATCHED AND t.hash != s.hash THEN UPDATE SET t.is_current = 0, t.end_date = CURRENT_DATE INSERT (customer_id, attr1, attr2, effective_date, end_date, is_current) VALUES...
This easy-level SQL question appears frequently in data engineering interviews at companies like EY. While less common, it tests deeper understanding that distinguishes strong candidates. Mastering the underlying concepts (snowflake, sql) will help you answer variations of this question confidently.
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.
MERGE INTO dim_customer t USING staging s ON t.customer_id = s.customer_id AND t.is_current = 1 WHEN MATCHED AND t.hash != s.hash THEN UPDATE SET t.is_current = 0, t.end_date = CURRENT_DATE INSERT (customer_id, attr1, attr2, effective_date, end_date, is_current) VALUES (s.customer_id, s.attr1, s.attr2, CURRENT_DATE, '9999-12-31', 1) WHEN NOT MATCHED THEN INSERT ... Why hash: Efficient change detection; avoids comparing every column. Logic: Match on business key + is_current; if attributes changed, expire current row, insert new. Snowflake/SQL Server support MERGE; syntax varies.
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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.