It is possible to de ne a set of entities and the relationships among them in a number of different ways. The main diffrerence is in how we deal with attributes.

• Consider the entity set employee with attributes employee-name and phone-number.
• We could argue that the phone be treated as an entity itself, with attributes phone-number and location.
• Then we have two entity sets, and the relationship set EmpPhn de ning the association between employees and their phones.
• This new de nition allows employees to have several (or zero) phones.
• New de nition may more accurately reflect the real world.
• We cannot extend this argument easily to making employee-name an entity.

The question of what constitutes an entity and what constitutes an attribute depends mainly on the structure of the real world situation being modeled, and the semantics associated with the attribute in question.