1. Object-based logical models:

Describe data at the conceptual and view levels.

Provide fairly exible structuring capabilities.

Allow one to specify data constraints explicitly.

Over 30 such models, including

• Entity-relationship model.

• Object-oriented model.
• Binary model.
• Semantic data model.
• Infological model.
• Functional data model.

2. At this point, we'll take a closer look at the entity-relationship (E-R) and object-oriented models.

The E-R Model

1. The entity-relationship model is based on a perception of the world as consisting of a collection of basic objects (entities) and relationships among these objects.

• An entity is a distinguishable object that exists.
• Each entity has associated with it a set of attributes describing it.
• E.g. number and balance for an account entity.
• A relationship is an association among several entities.
• e.g. A cust acct relationship associates a customer with each account he or she has.
• The set of all entities or relationships of the same type is called the entity set or relationship set.
• Another essential element of the E-R diagram is the mapping cardinalities, which express the number of entities to which another entity can be associated via a relationship set.

We'll see later how well this model works to describe real world situations.

2. The overall logical structure of a database can be expressed graphically by an E-R diagram:

• rectangles: represent entity sets.
• ellipses: represent attributes.
• diamonds: represent relationships among entity sets.
• lines: link attributes to entity sets and entity sets to relationships.

The Object-Oriented Model

1. The object-oriented model is based on a collection of objects, like the E-R model.

• An object contains values stored in instance variables within the object.
• Unlike the record-oriented models, these values are themselves objects.
• Thus objects contain objects to an arbitrarily deep level of nesting.
• An object also contains bodies of code that operate on the the object.
• These bodies of code are called methods.
• Objects that contain the same types of values and the same methods are grouped into classes.
• A class may be viewed as a type de nition for objects.
• Analogy: the programming language concept of an abstract data type.
• The only way in which one object can access the data of another object is by invoking the method of that other object.
• This is called sending a message to the object.
• Internal parts of the object, the instance variables and method code, are not visible externally.
• Result is two levels of data abstraction.

For example, consider an object representing a bank account.

• The object contains instance variables number and balance.
• The object contains a method pay-interest which adds interest to the balance.
• Under most data models, changing the interest rate entails changing code in application programs.
• In the object-oriented model, this only entails a change within the pay-interest method.

2. Unlike entities in the E-R model, each object has its own unique identity, independent of the values it
contains:

• Two objects containing the same values are distinct.
• Distinction is maintained in physical level by assigning distinct object identi ers.