Most system design interviews eventually reach the same question: "How would you store this data?"

How you answer reveals whether you understand the workload. The database layer is where many promising architectures run into hard limits: write throughput, query shape, consistency, schema evolution, and operational complexity.

The database is often the hardest component to change later and the most expensive to get wrong.

The database is often the constraint. Design it around the wrong access pattern, and application-layer optimization can only do so much.

Database design in interviews comes down to trade-offs: consistency vs availability, read performance vs write performance, simplicity vs scalability. Memorizing SQL syntax or comparing PostgreSQL against MySQL matters far less than showing you understand the consequences of each choice.

Every choice has consequences, and a strong answer shows that you understand them.

This chapter covers the concepts these questions usually probe: SQL vs NoSQL, schema design, normalization, indexing, partitioning, sharding, replication, consistency models, transactions, query optimization, and examples that tie those choices back to requirements.