Your question is Simplify Reporting Queries with CTEs. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Complex reporting queries often become hard to read when filtering, aggregating, and reusing intermediate results are all packed into one statement. In PostgreSQL, CTEs can make that logic easier to follow and maintain.
Explain how you would use a Common Table Expression (CTE) to simplify a complex SQL query. You should describe what a CTE is, how it helps break a query into logical steps, and when it is better than writing everything as one nested query. Use a reporting-style example relevant to data engineering work, such as summarizing clinical supply shipments or batch-level manufacturing metrics.
The interviewer is looking for a practical explanation rather than formal theory. Focus on readability, maintainability, debugging, and how multiple CTEs can structure a query into stages. You can also mention tradeoffs, such as when a CTE may not improve performance.