3.Entity Relationship Model

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CST203-2 Database Management Systems

Lecture 3


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y Describes data as entities, relationships and attributes


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Database Design Processy Requirements collection process

y Data and functional requirements

y

Conceptual designy Descriptions on entity types, relationships and constraints

y Choose a DBMS

y Data model mapping

y Physical database design

y Database system implementation


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Entity Setsy Adatabase can be modeled as:

y a collection of entities,y relationship among entities.

y An entity is an object that exists and is distinguishable from other objects.y Example: specific person, company, event, plant

y Entities have attributesy Example: people have names and addresses

y An entity type is a collection of entries that have the same attributes

y An entity set is a collection of entities of a particular entity type in the databaseat any point in time


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Entity Sets

customerand loan


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Attributesy Particular properties of an entity that describe it

Example:customer = (customer-id, customer-name,customer-street, customer-city)

loan = (loan-number, amount)

y Domain the set of permitted values for each attribute (value set)

y Attribute types:y Simple and composite attributes.y Single-valuedand multi-valuedattributes

y E.g. multivaluedattribute:phone-numbersy Stored and derivedattributes

y Can be computed from other attributesy E.g. age, given date of birth

y Complex attributesy Key attributes


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y An attributeAof entity type E whose value set is Vy A: E P(V)

y The value of attributeAof entity ey A(e)

y For composite attributeA, the value set Vy V = P(V1) * P(V2) * * P(Vn)


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Composite Attributes


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Uniqueness constrainty Key attributes

y Composite key attributesy Composite key must be minimal

y Prohibits any two entities from having the same value

for the key attribute at the same time


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y DEPARTMENTy Name, Number, {Locations}, Manager, ManagerStartDate

y PROJECTy Name, Number, Location, ControllingDepartment

y EMPLOYEEy Name(Fname, Initial, LName), ENO, Sex,Address, Salary, DOB,

Department, Supervisor, {WorksOn(Project, Hours)}

y DEPENDENTy Employee, DependentName, Sex, DOB, Relationship


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Relationship Setsy Arelationship is an association among several entities

Example:Hayes depositor A-102

customerentity relationship set account entity

y A relationship type is a set ofassociations

y Arelationship set is a set ofassociations among entities from these entity typesy mathematical relation among n u 2 entities, each taken from entity sets

{(e1, e2, en) | e1 E1, e2 E2, , en En}

where (e1, e2, , en) is a relationshipe1, e2, , en are entity instancesE1,E2, E3, .. are entity types

y Example:(Hayes,A-102) depositor


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Relationship Set borrower


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y An attribute can also be property of a relationship set.

y For instance, the depositorrelationship set between entity setscustomerand account may have the attribute access-date


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Degree of a Relationship Sety Refers to number of entity types that participate in a relationship type

y Relationship types that involve two entity types are binary (or degree two)

y Generally, most relationship sets in a database system are binary.

y Relationship sets may involve more than two entity sets.

y E.g. Suppose employees of a bank may have jobs (responsibilities) at multiple

branches, with different jobs at different branches. Then there is a ternaryrelationship set between entity sets employee, job and branch

y Relationships as attributes


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E-R Diagram with a Ternary

Relationship


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Converting Non-Binary

Relationships to Binary Formy In general, any non-binary relationship can be represented using binary

relationships by creating an artificial entity set.y ReplaceRbetween entity setsA, B and C by an entity setE, and three relationship sets:

1. RA, relating Eand A 2.RB, relating Eand B

3. RC, relating Eand Cy Create a special identifying attribute for E

y Add any attributes ofRto E

y For each relationship (ai, bi, ci) in R, create

1. a new entityei in the entity set E 2. add (ei, ai ) to RA3. add (ei, bi ) to RB 4. add (ei, ci ) to RC


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Mapping Cardinalitiesy Express the number of entities to which another entity can be associated via a

relationship set.

y Most useful in describing binary relationship sets.

y For a binary relationship set the mapping cardinality must be one of the following types:y One to oney One to manyy Many to oney Many to many

y Cardinality ratioy Maximum number of relationship instances that an entity can participate in

1:11:NN:1N:N


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Some elements inAand B may not be mapped to any elements in the other set


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Mapping Cardinalities affect ER

Designy Can make access-date an attribute of account, instead of a relationship

attribute, if each account can have only one customer

y I.e., the relationship from account to customer is many to one, orequivalently, customer to account is one to many


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E-R Diagrams

y Rectangles represent entity sets.

yDiamonds represent relationship sets.

y Lines link attributes to entity sets and entity sets to relationship sets.

y Ellipses represent attributes

y Double ellipses represent multivalued attributes.

y Dashed ellipses denote derived attributes.

y Underline indicates primary key attributes (will study later)


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E-R Diagram With Composite,

Multivalued, and Derived Attributes


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Relationship Sets with Attributes


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Rolesy Entity sets of a relationship need not be distinct

y The labels manager and worker are called roles; they specify how employeeentities interact via the works-for relationship set.

y Roles are indicated in E-Rdiagrams by labeling the lines that connectdiamonds to rectangles.

y Role labels are optional, and are used to clarify semantics of the relationship


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Cardinality Constraintsy We express cardinality constraints by drawing either a directed line (p),

signifying one, or an undirected line (), signifying many, between therelationship set and the entity set.

y E.g.: One-to-one relationship:y Acustomer is associated with at most one loan via the relationship borrowery Aloan is associated with at most one customer via borrower


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One-To-Many Relationshipy In the one-to-many relationship a loan is associated with at most

one customer via borrower, a customer is associated with several(including 0) loans via borrower


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Many-To-One Relationshipsy In a many-to-one relationship a loan is associated with several

(including 0) customers via borrower, a customer is associated with atmost one loan via borrower


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Many-To-Many Relationship

y Acustomer is associated with several (possibly 0) loans viaborrower

y Aloan is associated with several (possibly 0) customers viaborrower


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Participation of an Entity Set in

a Relationship Sety Total participation (indicated by double line): every entity in the entity set

participates in at least one relationship in the relationship set

y E.g. participation ofloan in borroweris totaly every loan must have a customer associated to it via borrower

y Partial participation: some entities may not participate in any relationship inthe relationship set

y E.g. participation ofcustomerin borrower is partial

3.Entity Relationship Model

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