ACID, indexing, normalization, transactions, and SQL vs NoSQL.
What each letter guarantees, how databases deliver it, and where the guarantees bend.
How indexes make queries fast, what they cost, and why your index isn't being used.
The anomalies each level permits, MVCC, and optimistic vs pessimistic concurrency.
1NF through BCNF without the pedantry, and when to denormalize on purpose.
The four NoSQL families, what you trade for scale, and how to choose in an interview.
The join types, how the engine actually executes them (nested loop, hash, merge), and reading EXPLAIN.
Entities, relationships, cardinality, and every kind of key — plus natural vs surrogate, the eternal debate.