Databases
SQL, joins, indexes, transactions
From SELECT to ACID. Schema design and normalization, joins, indexes, transactions and isolation levels, and when (and when NOT) to reach for NoSQL.
Why we don't just dump everything in a JSON file. Durability, concurrency, queries.
SELECT, INSERT, UPDATE, DELETE — the four CRUD operations and the grammar around them.
INNER, LEFT, RIGHT, FULL — what each does and when you actually use them.
1NF, 2NF, 3NF — and the practical guideline that covers 95% of real-world tables.
B-trees, composite indexes, the cost of indexing, and how to read EXPLAIN.
BEGIN, COMMIT, ROLLBACK — and why your bank really, really cares about isolation levels.
Document, key-value, wide-column, graph — what each is good at, and what it costs you.