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Chapter 2

Entity Relationship Model

(ER Model)Muddassar AliInternational University of Technology Twintech (IUTT) Yemen

muddassar.pieas@gmail.com

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E-R Model Concepts The ER model describes data as:

Entities

AttributesRelationships between entities

The overall logical structure of a database can beexpressed graphically by ER diagram.

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Relationshipdegreesspecifynumber of

entity typesinvolved

Entitysymbols

A special entitythat is also arelationship

Relationship symbols

Relationshipcardinalitiesspecify howmany of eachentity type is

allowed

Attributesymbols

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Entities An entity is an object that exists and isdistinguishable from other objects.

Example: specific person, company, event, plant etcEntities have attributes

An entity set is a set of entities of the same type that

share the same properties.Example: set of all persons, companies, trees etc.

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AttributesAttribute - property or characteristic of an entity type

Example: COURSE entity has attributes

Course code Course Title Course Description Number of Credits

Classifications of attributes:Required versus Optional AttributesSimple versus Composite AttributeSingle-Valued versus Multi-valued AttributeStored versus Derived Attributes

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Required versus Optional AttributesRequired attribute is that which is must for an entityit must not be null.

Optional is not necessary for an entity it may benull.

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Simple versus Composite AttributeSimple attribute is atomic and cannot be further divided

They describe simple features of an entityExample:

Title for the COURSE entity type Age for EMPLOYEE entity type

Composite attributes Can be divided into smaller subpartsExample:

Address may consists of StreetAddress, City, State,ZipCode, Country

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A composite attribute

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Single-Valued versus Multi-valued AttributeSingle Valued attributes have only one value for each entityof an entity setEach attribute Age and/or Sex has single value for each

EMPLOYEE entityExample: Single-valued: Gender = MMulti-valued attributes

Have a set of values for a particular entityExample: Phone attribute may be multivalued when personhas several phones (home, office, mobile)

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Stored versus Derived Attributes

Derived Attributes are functionally related toexisting attribute(s) that are stored and can be

calculated from that other attributes values Example: Age of an EMPLOYEE can be calculated

from his/her Birth date

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Key Attributes

Key attribute is one or more attributes that uniquelyidentifies each entity of the entity set

Value of key attribute Identifies each entity uniquely Must be distinct for each entity in the entity set Unique for every extension of the entity type

A key attribute has its name underlined inside theoval

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Simple key attribute

The key is underlined

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Composite key attribute

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The key is composedof two subparts

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Entity with a multivalued attribute (Skill) and derived

attribute (Years_Employed)

Derivedfrom date employed and current date

Primary Key

Multivalued: an employee can havemore than one skill

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Relationships

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A Relationship Type is an association among two ormore entity typesName of relationship must be Verb phraseRelationships can have attributes

These describe features pertaining to the associationbetween the entities in the relationship

Examples Employee Manages Departments Student Studies Course

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Degree of Relationships

Degree of a relationship is the number of entity types that participate in it

Unary Relationship : 1 entity involvedBinary Relationship : 2 entities involvedTernary Relationship : 3 entities involvedN-ary Relationship : n-entities involved

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Degree of relationships

One entity

related toanother of the sameentity type

Entities of two different

types relatedto each other Entities of threedifferent typesrelated to eachother

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Cardinality of RelationshipsOne-to-One

Each entity in the relationship will have exactly one related entityOne-to-Many

An entity on one side of the relationship can have many relatedentities, but an entity on the other side will have a maximum of onerelated entity

Many-to-ManyEntities on both sides of the relationship can have many related entitieson the other side

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Cardinality Constraints

Cardinality Constraints - the number of instances of one entity that can or must be associated with eachinstance of another entityMinimum Cardinality

If zero, then optionalIf one or more, then mandatory

Maximum CardinalityThe maximum number

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Note: a relationship can have attributes of its own

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Basic relationship with only maximum cardinalities

Mandatory minimum cardinalities

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Optional cardinalities with unary degree, one-to-one relationship

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Entities can be related to one another in more than one way

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Here max cardinality

constraint is 4

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Strong vs. Weak Entities, and IdentifyingRelationships

Strong entitiesexist independently of other types of entitieshas its own unique identifierrepresented with single-line rectangle

Weak entitydependent on a strong entitycannot exist on its own does not have a unique identifierrepresented with double-line rectangle

Identifying relationshiplinks strong entities to weak entitiesrepresented with double line diamond

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Strong entity Weak entityIdentifyingrelationship

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Associative EntitiesIts an entity it has attributesAND its a relationship it links entities togetherWhen should a relationship with attributes instead be anassociative entity ?

All relationships for the associative entity should be many.The associative entity could have meaning independent of the otherentities and preferably has a unique identifier.It should also have other attributes.The associative entity may participate in other relationshipsindependent of the entities related in the associated relationship.

Ternary relationships should be converted to associative entities

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An associative entity (CERTIFICATE)

Associative entity involves a rectangle with a diamond inside.Note that the many-to-many cardinality symbols face towardthe associative entity and not toward the other entities

Sample E R Diagram

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Sample E-R Diagram

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Explanation of E-R diagram1) Customer Submits zero or many Orders but Order must be submitted by

one customer.2) Order Request one or many Products But Product can be Requested by

zero or many Orders.3) Item is Used_In one or many Products And Product Used one or many

Items.4) Supplier supplied zero or many Items but item may be supplied by one or

many suppliers.

5) An item is included by zero or many shipments and shipment includesone or many items.

6) one supplier sends zero or many shipments but shipment is send byexactly one supplier.

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THE END

Any Questions???