Adding relations
Your first draft model already shows that some entities have relations to some others. Now, we specify these relations in more detail to get an even better understanding of our business data model.
We will use the following simplified visual representation, ignoring that some relations can be mandatory or optional:
Figure 6.5 – Types of relations
One-to-one
One "A" entity relates to exactly one "B" entity.
The relation between Supplier and Account is a good example of this kind of relation. Each supplier has only one account, and each account belongs to only one supplier.
One-to-many
One "A" entity relates to one or more "B" entities.
Our model contains more than one one-to-many relation. They are Supplier-Invoice, Account-Invoice, and Supplier-Order. So, one supplier can submit many invoices and we can order many different products from a single supplier.
Many-to-many
One...
