4 lessons
ER modeling: designing tables before you build them
Entities, attributes, and relationships - how to go from a real-world idea to a set of tables, on paper, before writing any SQL.
Functional dependencies and normalization (1NF-3NF)
The precise, mathematical reason certain table designs cause update anomalies, and the normal forms that systematically fix them.
Indexes: how databases actually find things fast
The B-tree explained from scratch, why it gives O(log n) lookups, and when adding an index makes things worse, not better.
Constraints and data integrity in depth
Foreign key actions (CASCADE, RESTRICT, SET NULL), CHECK constraints revisited, and referential integrity as the schema's immune system.