Top 50 SQL Interview Questions for Data Engineers (2026)

SQL remains the #1 tested skill in data engineering interviews. Every FAANG company, every fintech startup, and every enterprise data team expects fluency in advanced SQL concepts like window functions, CTEs, recursive queries, and query optimization. In our analysis of 1,800+ real interview questions, SQL-related questions appear at the highest frequency — with window functions and joins leading the pack.

Window functions (RANK, DENSE_RANK, ROW_NUMBER, LAG, LEAD, NTILE) appear in nearly every SQL round. Companies like Amazon and Google use them to test your ability to write efficient analytical queries without self-joins. Key concepts to master: - Difference between RANK(), DENSE_RANK(), and ROW_NUMBER() - Running totals and moving averages with SUM() OVER() - PARTITION BY vs GROUP BY - Frame specifications (ROWS BETWEEN, RANGE BETWEEN)

Common Table Expressions (CTEs) are tested frequently because they reveal how you structure complex logic. Interviewers want to see clean, readable SQL — not nested subqueries five levels deep. Practice writing CTEs for: - Recursive hierarchies (org charts, category trees) - Multi-step aggregations - Data deduplication patterns

Senior-level interviews always include a query optimization question. You'll need to explain execution plans, index strategies, and when to denormalize. Common optimization topics: - Clustered vs non-clustered indexes - Covering indexes and index-only scans - Partition pruning - Statistics and cardinality estimation