Презентация на тему Analysis and Design of Data Systems. Enhanced ER (EER) Mode. (Lecture 11)

of the ER model
Enhanced ER (EER) Model

need to be represented explicitly because of their significance:
Subclasses

and Superclasses

superclass

Each of these subgrupings is called a subclass

subclass

Each entity that is a member of a subclass is also an entity of a corresponding superclass (secretary is also an employee)
It is not necessary that every entity in a superclass is a member of some subclass

of subclasses of an entity type
The set of subclasses

that forms a specialization is defined on the basis of some distinguishing characteristic of the entities in the superclass

Based on the job type

Based on the method of pay

subclass can also be a member of another subclass

(salaried employee who is also an engineer belongs to the two subclasses ENGINEER and SALARIED_EMPLOYEE of the EMPLOYEE entity type)

Entity that is a member of a subclass inherits all the attributes of the entity as a member of the superclass

Entity that is a member of a subclass also inherits all the relationships in which the superclass participates

More on Subclasses and Superclasses

a data model:
Certain attributes may apply to some but

not all entities of the superclass
Some relationship types may be participated in only by entities that are members of the subclass.

Specialization process allows us to do the following:

Define a set of subclasses of an entity type
Establish additional specific attributes with each subclass
Establish additional specific relationship types between each subclass and other entity types or other subclasses

some attribute of the superclass we can determine exactly

the entities that will become members of each subclass. Such subclasses are called
predicate-defined subclasses.

OTHER

Defining predicate

membership condition on the same attribute of the superclass,

the specialization itself is called an attribute-defined specialization

Defining attribute

When we do not have a condition for determining membership in a subclass, the subclass is called user-defined

a member of at most one of the subclasses

of the specialization.

A specialization that is attribute-defined implies the disjointness constraint
Disjointness also applies to user-defined subclasses of a specialization

disjoint, their sets of entities may be overlapping
that is,

the same (real-world) entity may be a member of more than one subclass of the specialization

in the superclass must be a member of at

least one subclass in the specialization

partial specialization allows an entity not to belong to any of the subclasses

Total

Partial

is called a shared subclass
Shared sublcasses inherit attributes and

relationships from multiple classes. That concept is known as multiple inheritance

same key
Superclasses can have different keys

the three superclasses ENGINEER, MANAGER, and SALARIED_EMPLOYEE, so an

entity that is a member of ENGINEERING_MANAGER must exist in all three.
This represents the constraint that an engineering manager must be an ENGINEER, a MANAGER, and a SALARIED_EMPLOYEE simultaneously;
that is, ENGINEERING_MANAGER is a subset of the intersection of the three classes (sets of entities).
Shared subclass such as ENGINEERING_MANAGER inherits all the attributes of its superclasses SALARIED_EMPLOYEE, ENGINEER, and MANAGER

Union subclass is a subset of the union of its superclasses. Hence, an entity that is a member of OWNER must exist in only one of the superclasses. This represents the constraint that an OWNER may be a COMPANY, a BANK, or a PERSON.
Union subclass such as OWNER entity inherits the attributes of a COMPANY, a PERSON, or a BANK, depending on the superclass to which the entity belongs.