Databases

Level 6 — SQL and NoSQL in depth: indexing, query optimization, transactions, replication and sharding. Learn how data is stored before you design systems around it.

SQL FundamentalsCore
Structured Query Language from the ground up — DDL, DML, joins, aggregates, window functions, subqueries, and interview-critical patterns.
sqldatabasesjoinsaggregateswindow-functionsquery
PostgreSQL & MySQLIntermediate
Production SQL databases — ACID guarantees, storage engines, connection pooling, EXPLAIN plans, and the differences that matter in interviews.
postgresqlmysqlacidtransactionsexplainindexingproduction
Indexing & OptimizationAdvanced
How B-tree and hash indexes work, composite index strategies, covering indexes, and a systematic approach to identifying and fixing slow queries.
indexingb-treequery-optimizationperformanceexplain
Transactions & LockingAdvanced
ACID transactions, isolation levels, deadlocks, optimistic vs pessimistic locking, and the concurrency bugs that plague real systems.
transactionsacidlockingdeadlockisolationconcurrency
MongoDB & RedisIntermediate
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.
mongodbredisnosqlcachingdocument-databasekey-value
OLAP vs OLTP & Data WarehousingAdvanced
Why analytics and transactions need different databases: row vs columnar storage, normalized vs star schema, the warehouse/lake/lakehouse stack, ETL vs ELT, and why you never run reports on your production primary.
databasesolapoltpdata-warehousecolumnar