A big part of data modeling is deciding which entity types to model.
For those familiar with object-oriented programming concepts, an entity type is similar to a class. Each
individual entity of an entity type (think of an instance of a class) will be characterized by a set of attribute
values. Attributes are the ???adjectives??? or descriptors of the entities the database will store. For instance, extending
the example from two paragraphs above, a particular student could have the attributes ???Bill Smith,??? ???Akron, OH,???
???Fisher Dorm,??? 323, ???Computer Science,??? ???Professor Findley,??? etc.
The structure of a database is described by its ???schema.??? As we will see later, in order to convert the
data model to a relational database schema, each entity instance of an entity type must be unique. Something in
the set of attributes must make each entity different from all other entities of the same type. Returning to the
example in the previous paragraph, we expect only one Bill Smith, and if there are two or more, we will find
a way to make the different Bill Smith entities unique. We will assign a ???key??? to each entity of the student entity
type such that we can distinguish each student.
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