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CSE 480: Database Systems

Lecture 4: Enhanced Entity-Relationship Modeling

Reference:

Read Chapter 8.1 – 8.5 of the textbook

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Announcements

Class Project– Each group must email the instructor by next Tuesday your

group members If your group has only 2 members, state whether you’re willing to

accept a third member

– If you don’t have a group yet, email me too. I’ll try to set up one for you.

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Why EER?

ER diagram has limitations in terms of accurately reflecting the data properties and constraints

EER, which stands for Enhanced ER includes all the modeling concepts of basic ER with the following additional concepts

– subclasses/superclasses

– type inheritance

– shared subclasses and multiple inheritance

– categories (UNION types)

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Why Subclasses and Superclasses?

An entity type may have additional subgroupings:– EMPLOYEE may be further grouped into:

SECRETARY, ENGINEER, TECHNICIAN

– Based on job type

SALARIED_EMPLOYEE, HOURLY_EMPLOYEE

– Based on method of pay

EER diagram extends ER diagram to represent these additional subgroupings

– Each subgrouping is called a subclass of EMPLOYEE

– EMPLOYEE is the superclass of these subclasses

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Is-A Relationships in EER

Subclass-superclass relationships are also called IS-A relationships– SECRETARY IS-A EMPLOYEE, TECHNICIAN IS-A EMPLOYEE, ….

Subclass Superclass

Subclass1

SuperclassSubclass2

Subclass3

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Specialization

A superclass can have several specializations– EMPLOYEE has 3 specializations

Some specializations have only 1 subclass (e.g., Manager)

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Is-A vs 1-to-1 Relationship

EMPLOYEE DEPARTMENTMANAGES

1 1

EMPLOYEE TECHNICIAN

In a 1-1 relationship, two distinct entities are related In a subclass-superclass relationship, the subclass member is the

same entity as the superclass member but in a distinct specific role

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Subclasses and Superclasses

A subclass entity must be a member of the superclass

But a superclass entity does not have to be a member of any of its subclasses

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Why do we need Subclass/Superclass?

Certain attributes may apply to some but not all entities of the superclass

– Ex: Typing speed is an attribute of SECRETARY but not a TECHNICIAN

Certain relationship types are applicable to some entities but not others

– HOURLY_EMPLOYEE has a relationship with TRADE_UNION; SALARIED_EMPLOYEE does not have such a relationship

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Attributes of a Subclass

Local attributes: TypingSpeed of SECRETARY, Tgrade of TECHNICIAN

Type inheritance: a subclass also inherits all attributes of its superclass – SECRETARY inherits the attributes Name, SSN, Birth_date and Address

from the EMPLOYEE entity type (its superclass)

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Relationships of a Subclass

A subclass can participate in specific relationship types– Ex: MANAGER subclass participates in the MANAGES

relationship with PROJECT entity type

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Relationships of a Subclass

A subclass also inherits all the relationships in which the superclass participates

– If EMPLOYEE participates in WORKS_FOR relationship with DEPARTMENT then SECRETARY also participates in WORKS_FOR relationship with

DEPARTMENT ENGINEER also participates in WORKS_FOR relationship with

DEPARTMENT TECHNICIAN also participates in WORKS_FOR relationship with

DEPARTMENT MANAGER also participates in WORKS_FOR relationship with

DEPARTMENT SALARIED_EMPLOYEE also participates in WORKS_FOR

relationship with DEPARTMENT HOURLY_EMPLOYEE also participates in WORKS_FOR

relationship with DEPARTMENT

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Constraints on Is-A Relationships

Constraint is defined on the subclass membership of the superclass entities

– Disjointness constraint How many subclasses a superclass entity can belong? (max)

– Completeness constraint Must a superclass entity belong to any of the subclasses? (min)

Subclass1

SuperclassSubclass2

Subclass3

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Disjointness Constraint

Specifies whether subclasses are disjoint:– an entity can be a member of at most one of the subclasses of

the specialization

– Specified by “d” in EER diagram

– Ex: STUDENT is either FRESHMAN, SOPHOMORE, JUNIOR, SENIOR, GRADUATE

If not disjoint, specialization is overlapping:– an entity may be a member of more than one

subclasses

– Specified by “o” in EER diagram

– Ex: PERSON can be STUDENT, EMPLOYEE, or both

d

o

superclass

superclass

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Completeness Constraint

Total specialization– Every entity in superclass must be a member of at least one

subclass in the specialization

– Ex: every EMPLOYEE must be either an HOURLY_EMPLOYEE or SALARIED_EMPLOYEE

– Shown in EER diagrams by a double line

Partial specialization– Allows a superclass entity not to belong to any subclasses

– Ex: some EMPLOYEE entities do not belong to any of the subclasses SECRETARY, TECHNICIAN, or ENGINEER

– Shown in EER diagrams by a single line

superclass

superclass

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Example: Disjoint Partial Specialization

Partial

Disjoint

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Example: Overlapping Total Specialization

Total

Overlapping

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Exercise

Can a technician be an engineer? Can a manager belong to a trade union?

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Exercise: MOVIE database

Each movie is identified by title and year of release. Each movie has a length in minutes and is classified under one or more genres (horror, action, drama, etc)

Each movie has one or more directors and one or more actors appear in it. Each movie also has a plot outline and zero or more quotable quotes, each of which is spoken by a particular actor appearing in the movie

Actors are identified by name and date of birth and appear in one or more movies. Each actor has a role in the movie

Directors are also identified by name and date of birth and direct one or more movies. It is possible for a director to act in a movie (including one he or she may also direct)

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Exercise: Movie Database

ACTOR

DIRECTOR

MOVIE

Title

ID

Year

genres

length

DIRECTS

ACTS

Plot_Outline

role quotes

N

M

M

N

Person

NameID

DoB

o

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Hierarchies & Lattices

Hierarchy (Single Inheritance)– every subclass has only one immediate superclass

Lattice (Multiple inheritance)– a subclass can be subclass of more than one immediate

superclass

– A subclass with multiple immediate superclasses is called a shared subclass

In a lattice or hierarchy, a subclass inherits attributes not only of its immediate superclass, but also of all its predecessor superclasses

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Lattice (Multiple Inheritance)

SharedSubclass

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Another Shared Subclass Example

ENGINEERING_MANAGER is a shared subclass– ENGINEERING_MANAGER Is-a ENGINEER

– ENGINEERING_MANAGER Is-a MANAGER

– ENGINEERING_MANAGER Is-a SALARIED_EMPLOYEE

It will inherit all the attributes of its superclasses, including EMPLOYEE, ENGINEER, MANAGER, and SALARIED_EMPLOYEE

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Categories (UNION types)

A shared subclass is a subclass participating in multiple superclass/subclass (is-a) relationships

– Each distinct relationship has a single superclass

In some cases, we need to model a single superclass/subclass relationship with more than one superclass

– Each superclass can represent a different entity type– Such a subclass is called a category or UNION TYPE

Difference between shared subclass and union type– An entity of a shared subclass exists in all of its superclasses– An entity of a category exists in only one of its superclasses

It inherits only attributes of the superclass in which it belongs

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Categories (UNION types)

OWNER is the subclass of the union of 3 superclass entity types, PERSON,BANK, COMPANY (which have different key attributes)

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Example of UNION types

OWNER is the subclass of the union of 3 entity types, PERSON,BANK, COMPANY (which have different key attributes)

REGISTERED_VEHICLE is the subclass of the union of CAR and TRUCK (which have same key attribute)

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Constraints on UNION types

Total vs partial category (see example on next slide)– Total category holds the union of all entities in its superclasses

Represented by a double line

– Partial category holds a subset of the union Represented by a single line

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Total vs Partial Category

Registered

User

u

User is the union of all registered and unregistered

user entity types

Owner is the union of all banks, but not all persons

and not all companies

Unregistered

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Summary

Introduced the EER model concepts– Class/subclass relationships

– Type inheritance

– Shared subclass and categories (union types)

These augment the basic ER model concepts introduced in lectures 2-3