ENHANCED ENTITY RELATIONSHIP (EER) MODEL

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Instructor: Faisal Anwer and Mohammad Nadeem

SOURCE:Fundamentals of Database Systems (6th Edition) by Ramez Elmasi, Shamkant B. Navathe + Database System Concepts (English) 6th Edition by Abraham Silberschatz , S. Sudarshan , Henry F. Korth + Freely Accessible Web Resources

EER MODEL

ER modelling is usually sufficient for most of the database

applications.

However, new database requirements are emerging and so,

enhancements in existing ER modelling are required.

ER model, when incorporates new concepts, is called

Enhanced ER model.

The major enhancements are:

Superclass/Subclass relationship

Specialization

Generalization

Category

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SUPERCLASS/SUBCLASS RELATIONSHIP

In many cases an entity type has numerous subgroupings or

subtypes of its entities that are meaningful.

They need to be represented explicitly because of their

significance to the database application.

Example:

EMPLOYEE could be PERMANENT or TEMPORARY.

STUDENT could be UNDERGRADUATE or

POSTGRADUATE.

ACCOUNT could be SAVING, CURRENT or DMAT.

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EXAMPLES

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CONCEPTS ABOUT SUPERCLASS/SUBCLASS

Reasons for using superclass/subclass:

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

Every instance of any subclass is also a member of its

superclass.

Relationship between a superclass and any one of its

subclasses is called class/subclass relationship (also called

IS-A relationship).

A subclass inherits all the attributes and relationships of

its superclass.

A subclass can have relationship types with other entities,

independently of the superclass.5

SPECIALIZATION

Specialization is the process of defining a set of subclasses

of an entity type; this entity type is called the superclass of

the specialization.

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SPECIALIZATION

Usually, specialization is described by a defining attribute.

Example:

Job type (SECRETARY, ENGINEER, TECHNICIAN)

Method of pay (SALARIED, HOURLY)

One entity can have many specializations.

Attributes of subclass are called specific or local

attributes.

Relationship types of subclass are called specific

relationship types.

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SUMMARY ABOUT SPECIALIZATION

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

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GENERALIZATION

Generalization process can be viewed as being functionally

the inverse of the specialization process.

We suppress the differences among several entity types,

identify their common features, and generalize them into a

single superclass of which the original entity types are

special subclasses.

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GENERALIZATION

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CONSTRAINTS ON SPECIALIZATION AND

GENERALIZATION

A specialization may result in one or more subclasses.

Predicate defined subclasses:

Subclasses are usually identified by a condition on the value

of some attribute of the superclass.

Such subclasses are called predicate-defined (or

condition-defined) subclasses.

Example:

(Job_type = ‘Secretary’)

This condition is a constraint specifying that exactly

those entities of the EMPLOYEE entity type whose

attribute value for Job_type is ‘Secretary’ belong to the

subclass.

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CONSTRAINTS ON SPECIALIZATION AND

GENERALIZATION

If all subclasses are based on the same attribute of the

superclass, the specialization itself is called an attribute-

defined specialization.

such an attribute is called the defining attribute.

When we do not have a condition for determining membership

in a subclass, the subclass is called user-defined.

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CONSTRAINTS ON SPECIALIZATION AND

GENERALIZATION

Disjointness constraint:

An attribute-defined specialization implies the disjointness

constraint.

When subclasses are disjoint, an entity can be a member of at

most one of the subclasses of the specialization.

If the subclasses are not constrained to be disjoint, their sets

of entities may be overlapping.

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DISJOINTNESS AND OVERLAPPING CONSTRAINT

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DisjointOverlapping

Completeness constraint:

It is a constraint on the belongingness of superclass

member in subclasses.

It could be total or partial:

Total: every entity in the superclass must be a

member of at least one subclass in the specialization.

Generalization process is usually total.

Partial: an entity may not belong to any subclass.

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CONSTRAINTS ON SPECIALIZATION AND

GENERALIZATION

TOTAL AND PARTIAL CONSTRAINT

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Disjoint & PartialOverlapping & Total

CONSTRAINTS ON SPECIALIZATION AND

GENERALIZATION

Disjointness and Completeness constraints are

independent.

Hence, four possible constraints on specialization:

Disjoint, total

Disjoint, partial

Overlapping, total

Overlapping, partial

These constraints create certain insertion/deletion rules.

Deleting from a superclass deletion from all the subclasses

Inserting in a superclass insertion in corresponding subclasses

Superclass of a total specialization implies that the entity is

mandatorily inserted in at least one of the subclasses. 17

SPECIALIZATION AND GENERALIZATION

HIERARCHIES

A specialization hierarchy (single inheritance) has the

constraint that every subclass can have only one superclass.

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SPECIALIZATION AND GENERALIZATION

LATTICES

For a specialization lattice (multiple inheritance), a

subclass can belong to more than one superclasses.

Similar concepts are applicable on generalization as well. 19

UNION TYPES USING CATEGORIES

When a single subclass is modelled using more than one

superclasses, usually of different entity types, such a

subclass is called a union type or a category.

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UNION TYPES USING CATEGORIES

A category is a subset of the union of its superclasses.

A cateogory inherits the attributes depending on the

superclass to which the entity belongs.

A category does not necessarily includes all instances of its

superclasses.

A category can be total or partial.

total category holds the union of all entities in its

superclasses.

a partial category can hold a subset of the union.

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RELATIONAL DATABASE DESIGN BY EER

DIAGRAM

Step 8: Mapping of Specialization or Generalization:

For Ways:

8A: Multiple relations- Superclass and Subclasses:

Works for any specialization (total or partial, disjoint or

overlapping)

Useful when both common and specific attributes are many.

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RELATIONAL DATABASE DESIGN BY EER

DIAGRAM

8B: Multiple relations- Superclass and Subclasses:

Works only when specialization is total.

If specialization is disjoint, the common entity would

result in redundant storage of attributes.

Useful when common attributes are few.

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RELATIONAL DATABASE DESIGN BY EER

DIAGRAM

8C: Single relation with one type attribute:

Works only when specialization is disjoint.

Useful when specific attributes are few.

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Job_type is the type attribute

RELATIONAL DATABASE DESIGN BY EER

DIAGRAM

8C: Single relation with multiple type attributes:

Used when specialization is overlapping (but also works for

disjoint scenario).

Includes m boolean type attribites, one for each subclass

Useful when specific attributes are few.

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Mflag and Pflag are boolean type attributes

RELATIONAL DATABASE DESIGN BY EER

DIAGRAM

Mapping of Shared Subclasses (Lattice):

Depending upon the restrictions, any of the previous methods

(8A, 8B, 8C and 8D) could be applied.

8A can be applied anytime.

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RELATIONAL DATABASE DESIGN BY EER

DIAGRAM

Step 9: Mapping of Categories (Union Types):

A new attribute, called surrogate key, is used to handle

different keys.

For a category whose superclasses have the same key, there is

no need for a surrogate key.

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GATE-2005

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GATE-2005

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GATE-2011

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GATE-2011

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