Database Systems-Lec3

8/14/2019 Database Systems-Lec3

1/56

Chapter 5Entity Relationship (E-R)Modeling


2/56

Basic Modeling Concepts Database design is both art and science.

A data model is the relatively simplerepresentation, usually graphic, of

complex real-world data structures. Itrepresents data structures and theircharacteristics, relations, constraints, andtransformations.

The database designer usually employsdata models as communications tools tofacilitate the interaction among thedesigner, the applications programmer,

and the end user.


3/56

Data Models: Degrees of Data

Abstraction

Figure 2.10 page 57


4/56

The conceptual model represents a global viewof the data. It is an enterprise-widerepresentation of data as viewed by high-levelmanagers.

Entity-Relationship (E-R) model is the most

widely used conceptual model.

The conceptual model forms the basis for theconceptual schema.

The conceptual schema is the visualrepresentation of the conceptual model.

The conceptual model is independent of bothsoftware (software independence) andhardware (hardware independence).

The Conceptual Model


5/56

Tiny College Entities

Figure 2.11 Page 58


6/56

A Conceptual Schema for Tiny College

Figure 2.12 Page 59


7/56

The Internal Model

The internal model adapts theconceptual model to a specific DBMS.

The internal model is software-dependent.

Development of the internal model is

especially important to hierarchical andnetwork database models.


8/56

Figure 2.13 Page 61


9/56

Figure 2.13 Page 61


10/56

The external model is the end usersview of the data environment.

Each external model is thenrepresented by its own externalschema.

CREATE VIEW CLASS_VIEW ASSELECT (CLASS_ID, CLASS_NAME,PROF_NAME, CLASS_TIME, ROOM_ID)FROM CLASS, PROFESSOR, ROOMWHERE CLASS.PROF_ID =

PROFESSOR.PROF_ID AND CLASS.ROOM_ID= ROOM.ROOM_ID;

The ExternalModel/Schema


11/56

External

Model

Extern

alModel

Figure 2.14 Page 63


12/56

Advantages of Using External Schemas

It makes application programdevelopment much simpler.

It facilitates the designers task bymaking it easier to identify specificdata required to support each businessunits operations.

It makes the designers job easier byproviding feedback about theconceptual models adequacy.

It helps to ensure security constraints

The ExternalModel/Schema


13/56

The physical model operates at thelowest level of abstraction, describing theway data is saved on storage media suchas disks or tapes.

It requires the definition of both thephysical storage devices and the accessmethods required to reach the datawithin those storage devices.

The physical model is both software andhardware-dependent.

It requires detailed knowledge of

hardware and software used to

The Physical Model


14/56

Entity Relationship (E-R)

Model

E-R model is commonly used to:

Translate different views of dataamong managers, users, andprogrammers to fit into acommon framework.

Define data processing andconstraint requirements to helpus meet the different views.

Help implement the database.


15/56

E-R Model Components

Entities

In E-R models an entity refers to the entity set.

An entity is represented by a rectangle

containing the entitys name.

Attributes

Attributes are represented by ovals and areconnected to the entity with a line.

Each oval contains the name of the attribute it

represents. Attributes have a domain -- the attributes set of

possible values.

Attributes may share a domain.

Primary keys are underlined.

Relationships

Entity Relationship (E-R)

Model


16/56

Attributes of the STUDENT Entity

Figure 4.1 Page 125


17/56

Basic E-R Model Entity Presentation

Figure 4.3 Page 127


18/56

The CLASS Table (Entity)

Components and Contents

Figure 4.2 Page 126

Cl f


19/56

A simple attribute cannot be subdivided.

Examples: Age, Sex, and Maritalstatus

A composite attribute can be furthersubdivided to yield additional attributes.

Examples:

ADDRESS - Street, City, State, Zip

PHONE NUMBER - Area code,Exchange number

Classes ofAttributes

Classes of


20/56

A single-valued attribute can have only asingle value.

Examples:

A person can have only one socialsecurity number.

A manufactured part can have only oneserial number.

Multivalued attributes can have manyvalues.

Examples:

A person may have several collegedegrees.

A household may have several phones

with different numbers

Classes ofAttributes


21/56

Splitting the Multivalued Attributes

into New Attributes

Figure 4.4 Page 128


22/56

A New Entity Set Composed of

Multivalued Attributes Components

Figure 4.5 Page 129


23/56

A derived attribute is not physically storedwithin the database; instead, it is derived byusing an algorithm.

Example: AGE can be derived from the data

of birth and the current date.

Emp_Age = Current Date - Emp_DOB

Derived Attributes

Figure 4.6 Page 130


24/56

A relationship is an association betweenentities.

Relationships are represented by diamond-shaped symbols.

Relationship

Figure 4.7 Page 132


25/56

A relationships degree indicates thenumber of associated entities orparticipants.

A unary relationship exists when anassociation is maintained within a singleentity.

A binary relationship exists when twoentities are associated.

A ternary relationship exists when threeentities are associated.

Relationship Degree


26/56

Relationship Degree

Figure 4.16 Page 143


27/56

The Implementation

of a TernaryRelationship



28/56

Connectivity

The term connectivity is used to describethe relationship classification (e.g., one-to-one, one-to-many, and many-to-many).

Connectivity

Figure 4.7 Page 132


29/56

Cardinality

Cardinality expresses the specific numberof entity occurrences associated with oneoccurrence of the related entity.

Cardinality

Figure 4.7 Page 132


30/56

If an entitys existence

depends on the existenceof one or more otherentities, it is said to be

existence-dependent.

ExistenceDependency


31/56

Weak Relationship

Class will not exist if course does not exist.

Class depends on Course

Figure 4.9 Page 134


32/56


33/56

CLASS is Optional to COURSE



34/56

COURSE and CLASS in a Mandatory

Relationship



35/56

Weak Entities A weak entity is an entity that

Is existence-dependent and

Has a primary key that is partially ortotally derived from the parent entityin the relationship.

The existence of a weak entity is

indicated by a double rectangle. Theweak entity inherits all or part of itsprimary key from its strongcounterpart.

Weak Entities


36/56

A Weak Entity in an ERD



37/56

Weak Relationship Between

DEPENDENT and EMPLOYEE



38/56

Recursive Entities

A recursive entity is one in which arelationship can exist betweenoccurrences of the same entity set.

A recursive entity is found within aunary relationship.

Recursive Entities



39/56

1. James Ramirez is married to Louise Ramirez, who ismarried to

James Ramirez

2. Anne Jones is married to Anton shapiro, who is married to

AnneJones

Recursive Entities



40/56

A rotor assembly is composed of four 2.5cm washers,two cotter pins, one 2.5 cm steel shank, four 10.25cmrotor blades, and two 2.5 cm hex nuts

Recursive Entities



41/56

The Implementation of the M:N Recursive

PART Contains PART Relationship



42/56

Implementation of the M:N COURSE

Requires COURSE RecursiveRelationship



43/56

Implementation of the 1:M EMPLOYEE

Manages EMPLOYEE RecursiveRelationship



44/56

A composite entity is composed of theprimary keys of each of the entities to beconnected.

The composite entity serves as a bridgebetween the related entities.

The composite entity may containadditional attributes.

Composite Entities


45/56

The M:N Relationship Between

STUDENT and CLASS



46/56

A Composite Entity in the ERD



47/56

Converting the M:N Relationship

Into Two 1:M Relationships



48/56

Entity Supertypes and Subtypes



49/56

The generalization hierarchy depicts the

parent-child relationship.

The supertype contains the sharedattributes, while the subtype contains

the unique attributes. A subtype entity inherits its attributes

and its relationships from the supertypeentity.



50/56

A Generalization Hierarchy



51/56


The supertype entity set is usuallyrelated to several unique and

disjointed (nonoverlapping) subtypeentity sets.

The supertype and its subtype(s)maintain a 1:1 relationship.



52/56

The EMPLOYEE/PILOT

Supertype/Subtype Relationship



53/56


The generalization hierarchy depictsthe parent-child relationship. (Figure4.34)

The supertype contains the shared

attributes, while the subtype containsthe unique attributes.

The supertype entity set is usuallyrelated to several unique and

disjointed (nonoverlapping) subtype



54/56

A Generalization Hierarchy

With Overlapping Subtypes



55/56


56/56

References

ROB, P. AND CORONEL, C., 2004, Database

Systems. 6th Ed., Thomson Course Technology

Documents

Database Systems-Lec3