SQL questions from BCG data engineering interviews.
These sql questions are sourced from BCG data engineering interviews. Each includes an expert-level answer. This set leans toward the medium-difficulty band most real interviews actually live in (7 of 14). Recurring themes are partition, join, and sql — these patterns appear most often in real interviews and reward the deepest preparation. Many of these questions also surface at Accenture and Cognizant, so the preparation transfers across companies. Average answer is around 1 minute of reading — plan roughly 1 hour to work through the full set thoughtfully.
This collection contains 14 curated questions: 5 easy, 7 medium, and 2 hard. There's a strong foundation of fundamentals-focused questions — ideal for building confidence before tackling advanced topics.
The most frequently tested areas in this set are partition (7), join (5), sql (4), snowflake (2), optimization (2), and window (1). Focusing on these topics will give you the highest return on your preparation time.
Start with the easy questions to warm up and solidify fundamentals. Medium-difficulty questions form the bulk of real interviews — spend the most time here and practice explaining your reasoning out loud. Hard questions often appear in senior and staff-level rounds; attempt them after you're comfortable with the basics. For each question, try answering before revealing the solution. Use our AI Mock Interview to simulate real interview conditions and get instant feedback on your responses.
Write an SQL query to find the second-highest salary from an employee table.
Compare Redshift, BigQuery, and Snowflake in terms of cost, performance, and scalability.
Describe a scenario where you had to collaborate with a cross-functional team to deliver a solution.
Describe how you would implement Slowly Changing Dimensions (SCD) in an ETL workflow.
Describe strategies for optimizing a slow-running query on a massive dataset.
Discuss a situation where you had to balance technical priorities and business goals.
Explain the difference between Star and Snowflake schemas. When would you choose one over the other?
Explain the use of surrogate keys vs. natural keys in data modeling.
Given an unoptimized query execution plan, how would you diagnose and improve performance?
Kafka Partitioning: How would you ensure even load distribution across Kafka partitions in a high-volume system?
Merge two dictionaries and remove keys with null values.
What are the key design principles for a cloud-based data warehouse?
What considerations are important when designing a dimensional model for a ridesharing app?
Write a query to remove duplicate records from a table while retaining the earliest entry.
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