Databases (Interview Foundations)

Databases show up in almost every role: they are the “ground truth” and the bottleneck.

Concepts to master

• Data modeling: entities, relationships, normalization vs denormalization
• Indexes: what they accelerate and what they cost
• Transactions: atomicity and isolation; practical pitfalls
• Replication and read scaling
• Partitioning/sharding and hotspots

“Good enough” SQL knowledge

• join types and when they change row counts
• group by + having
• window functions basics
• explain plans (conceptually)

Common failure modes

• Index cargo cult: adding indexes everywhere

  • Cost: write amplification, storage, slower inserts/updates

• Hot partition: all traffic hits one shard

  • Fix: choose partition keys with access patterns in mind

• Ignoring isolation: phantom reads / lost updates

  • Fix: pick correct isolation/locking approach, use idempotency

Archetype emphasis

• FAANG: expects reasoning about scale + consistency.
• Enterprise: expects migrations, compatibility, and operational constraints.
• Automotive/industrial: expects reliability, traceability, sometimes stricter governance.