An entity-relationship (ER) model, proposed by Peter Chen in 1976, describes data before any table exists. An entity is a thing you store facts about, an attribute is one such fact, and a relationship connects entities. In the sheet, email, cust_name and country describe a customer, sku, title and authors a book, and status an order, while qty and price describe one book on one order: a relationship with attributes of its own.
| Kind | Shop example | Becomes |
|---|---|---|
| Simple | price, country | One column |
| Composite | a postal address | One column per part you query |
| Derived | an order total | Computed, or a generated column (Generated Columns) |
| Multi-valued | a book's authors | A separate table (First Normal Form) |
Something with facts of its own is an entity: a category has a name and a parent. A country code has nothing else the shop stores, so it stays an attribute of the customer.