One-to-many relationship
A one-to-many relationship exists when each row in one table has one or many related rows in a second table. Figure 2–3 shows some examples: one customer can place many orders, or a sales representative can have many customer accounts.
However, the business rule might be that for one customer there can be zero-or-many orders, one student can take zero-or-many courses, and a sales representative can have zero-or-many customer accounts. This relationship is described as a zero-or-many relationship.