SUPER TYPES

AND

SUBTYPES

Objectives

After completing this chapter, you should be able

to:

●Describe advanced concepts of database design

●Define Subtypes and Supertypes

●Describe Generalization and Specialization

●Use Constraints in Supertype

●Use Constraints in Supertype/Subtype Discriminators

●Describe Supertype/Subtype Hierarchy

●Define domains

Basic Definitions

● Subtype: A subgrouping of the entities

in an entity type which has attributes

that are distinct from those in other

subgroupings.

● Supertype: A generic entity type that

has a relationship with one or more

subtypes.

● Inheritance: Subtype entities inherit values of

all attributes of the supertype. An instance of a subtype is also

an instance of the supertype.

Basic notation for supertype/ subtype relationship:

Basics continued...

Employee supertype with three subtypes:

Basics continued...

Relationships and Subtypes

• Relationships at the supertype level

indicate that all subtypes will

participate in the relationship.

• The instances of a subtype may

participate in a relationship unique

to that subtype. In this situation,

the relationship is shown at the

subtype level.

Supertype/subtype relationships in a hospital:

Relationships continued...

Supertype/ Subtype NotationThe hieararchy of the supertype/suntype notation is as follows:

●SUPERTYPE

●SUBTYPE 3

●SUBTYPE 2

●SUBTYPE 1

●Attributes unique to subtype 1

●Attributes unique to subtype 2

●Attributes unique to subtype 3

●Attributes shared by all entities

Generalization

●The process of categorizing entities by

their similarities and differences is known

as generalization.

●Generalization hierarchies should be used

when:

●A large number of entities appear to be of

the same type,

●Attributes are repeated for multiple

entities,

●The model is continually evolving.

Rules for Generalization

●Each instance of the supertype entity must appear in at least one subtype.

●Subtypes can be a part of only one

generalization hierarchy.

●Subtypes may be the parent entity in a

relationship, but not the child.

Example

Specialization

●The process of defining one or more

subtypes of the supertype, and

forming supertype/subtype

relationships.

Constraints in Supertype

●Completeness

Constraints

●Disjointness

Constraints

Completeness Constraints

●Addresses the question of whether an instance

of a supertype must also be a member of at

least one subtype.

●The total specialization rule specifies that each

entity instance of the supertype must be a member

of some subtype in the relationship.

●It is denoted by a double line.

●The partial specialization rule specifies that an

entity instance of the supertype is allowed to not

belong to any subtype.

●It is denoted by a single line.

Total Specialization Rule

Constraints continued...

Partial Specialization Rule

Constraints continued...

Disjointness Constraints

●Addresses the question of whether an instance

of a Super type may simultaneously be a

member of two (or more) subtypes.

●The disjoint rule specifies that if an entity instance

is a member of one subtype, it cannot simultaneously

be a member of any other subtype.

●It is denoted by the letter “d”.

●The overlap rule specifies that an entity instance

can simultaneously be a member of two (or more)

subtypes.

Disjoint Rule

Constraints continued...

Overlap Rule

Constraints continued...

Constraints in DiscriminatorsSubtype Discriminator

●It is an attribute of the supertype whose values

determine the target subtype(s)”. It is used to direct

into which of the subtypes (if any) a new instance of

the supertype should be inserted.

●Disjoint – a simple attribute with alternative values

to indicate the possible subtypes.

●Overlapping – a composite attribute whose

subparts pertain to different subtypes. Each subpart

contains a boolean value to indicate whether or not

the instance belongs to the associated subtype

Introducing a subtype discriminator (Disjoint Rule)

Discriminators continued...

Introducing a subtype discriminator (Overlap Rule)

Discriminators continued...

Supertype/Subtype Hierarchy

●It is “a hierarchical arrangement of

supertypes and subtypes, where each

subtype has only one supertype”.

●In this hierarchy, attributes are assigned at the

highest logical level that is possible in the

hierarchy.

●Subtypes that are lower in the hierarchy inherit

attributes not only from their immediate

supertype, but also from all supertypes higher in

the hierarchy, up to the root.

Example of supertype/subtype hierarchy

Hierarchy continued...

Domain

●A domain is a conceptual pool of values

from which one or more attributes draw

their actual values.

Examples:

DOMAIN AGE RANGE 0-127

ATTRIBUTE EMPLOYEE.AGE 16-65

ATTRIBUTE DEPENDENT.AGE 0-60

●Two values can only be compared if they come

from the same domain.

Defining a Domain

Syntax:

CREATE { DOMAIN | DATATYPE } [ AS ] domain-

name

data-type [ [NOT] NULL]

[DEFAULT default-value]

[ CHECK ( condition ) ]

Domain Integrity Constraints

●Domains are used in the relational model to

define the characteristics of the columns of a

table.

●The domain specifies its own name, data type

and logical size.

●The logical size represents the size as

perceived by the user, not how it is

implemented internally.

Summary

You now should be able to:

● Describe advanced concepts of database

design

● Define Subtypes and Supertypes

● Describe Generalization and Specialization

● Use Constraints in Supertype

● Use Constraints in Supertype/Subtype

Discriminators

● Describe Supertype/Subtype Hierarchy

● Define domains