Relationships

ERDs are not solely about entities but are also about their relationships. Let's return to the example where we sell products to customers. Do Product and Customer have a relationship?

Figure 2.5 – Is there a relationship between Product and Customer?

At first glance, you might think Product and Customer do have a relationship. A customer buys your products. Nouns are often good indicators of entities, while verbs are indicators of relationships. "Buys" seems to suggest there is a relationship. However, we need to look closer. What if a customer buys a product and next week they come back to buy the same product again? We probably want to keep records of both events, both sales transactions, individually. They might fall in different calendar quarters, meaning there is a difference for you in terms of payments of VAT to the government.

There is a third entity: Sales transaction or Order. Every time you log in to an online...