Reviewed by Aditya Kumar · Last reviewed 2026-03-24
Semi-join returns rows from the left table that have at least one match in the right table, but doesn't duplicate left rows. Implementation: SELECT * FROM a WHERE EXISTS (SELECT 1 FROM b WHERE b.id = a.id) or IN. Unlike inner join, semi-join returns one row per left match even...
This medium-level SQL question appears frequently in data engineering interviews at companies like Altimetrik. While less common, it tests deeper understanding that distinguishes strong candidates. Mastering the underlying concepts (join) 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.
Semi-join returns rows from the left table that have at least one match in the right table, but doesn't duplicate left rows. Implementation: SELECT * FROM a WHERE EXISTS (SELECT 1 FROM b WHERE b.id = a.id) or IN. Unlike inner join, semi-join returns one row per left match even if multiple right matches exist. Use cases: Filter to customers who have orders; exclude/include based on existence. Best practice: EXISTS often optimizes better than IN for subqueries; use when you don't need columns from the right table. 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.
Red Flag: Generic textbook answers. Pro-Move: 'At scale we measured X, implemented Y, achieved Z%—validated and iterated.'
Some links below are affiliate links. If you buy through them we may earn a small commission at no extra cost to you — it helps keep DataEngPrep free.
According to DataEngPrep.tech, this is one of the most frequently asked SQL interview questions, reported at 1 company. DataEngPrep.tech maintains an editor-reviewed database of 1,863 data engineering interview questions across 7 categories.