Databases

Structured Query Language from the ground up — DDL, DML, joins, aggregates, window functions, subqueries, and interview-critical patterns.

Production SQL databases — ACID guarantees, storage engines, connection pooling, EXPLAIN plans, and the differences that matter in interviews.

The two most important NoSQL databases — MongoDB's document model for flexible schemas, Redis as an in-memory powerhouse for caching, queues, and pub/sub.

How B-tree and hash indexes work, composite index strategies, covering indexes, and a systematic approach to identifying and fixing slow queries.

ACID transactions, isolation levels, deadlocks, optimistic vs pessimistic locking, and the concurrency bugs that plague real systems.