Context

At companies like Stripe or Airbnb, analysts often need to answer questions that depend on intermediate results, such as finding customers above average spend or products with the highest sales. Subqueries are one of the core SQL tools for solving these problems.

Core question

Explain what a subquery is in SQL and how it works. In your answer, cover:

1. What a subquery is and where it can appear in a query
2. The difference between scalar, multi-row, and correlated subqueries
3. When a subquery is useful compared with joins or common table expressions (CTEs)
4. Common mistakes, especially around performance and incorrect filtering

Scope guidance

Keep the explanation at an interview-ready level. The interviewer is not looking for database internals, but they do expect clear definitions, practical examples, and an understanding of when subqueries are the right tool versus when another approach may be better.