Normalization, briefly

One fact, one place

Why not keep everything in one giant books table with the author's name and country repeated on every row? Because repetition causes anomalies: change an author's country and you must update every one of their books; an author with no book yet can't be recorded at all.

Normalization is the cure — split data so each fact lives in exactly one place, linked by keys:

- 1NF — no repeating groups; one value per cell (that's why order_items is its own table, not a comma-list in orders). - 2NF — every non-key column depends on the *whole* key (unit_price belongs with the order_item, not the order). - 3NF — non-key columns depend on *nothing but* the key (author *country* lives in authors, keyed by author, not copied into books).

A many-to-many relationship (orders ↔ books) is always resolved with a junction table — that's exactly what order_items is. The query below shows the join working — count how many distinct books each author has.