"What database should we use?"

This question appears in many system design interviews, usually after you have defined the core entities and access patterns. Weak answers sound like "PostgreSQL is reliable" or "MongoDB scales well."

Database selection is not about picking your favorite technology. It is about matching data requirements to database capabilities.

The right database for a messaging app is not the right database for an analytics pipeline. The right database for a payment system is not the right database for a social feed.

Interviewers ask about databases because your choice reveals how well you understand data requirements, trade-offs, and system constraints. A candidate who picks the right database and explains why demonstrates real engineering judgment.

In this chapter, you will learn: