Databases
The store of truth for your application. Postgres dominates; SQLite is increasingly production-viable; Redis is for caching and queues; specialized DBs only when justified.
Advanced databases
Beyond CRUD — how indexes and the query planner work, transactions and isolation levels, the N+1 problem, connection pooling, and safe schema migrations. Why "the database is slow" is usually a missing index.
ORMs
The layer between your code and SQL. Drizzle, Prisma, Kysely — and when raw SQL is the right answer.