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Conceptual modelling:The Entity-Relation (ER) Model

Conceptual modelsEntity – Relation (ER) models

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…or…

Where does all the tables come from???

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Conceptual Data Models

A conceptual data model describes the data objects (entities) and their relations in the problem domain.A conceptual model of the data is the basis on which the IT systems of an organisation are based.It should be:

Independent of implementationStable over time

Over time conceptual data structure doesn't change nearly as much as functionality

Conceptual models are to be transformed to a database model as the relational model.

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Example: MiniBank

This is implementation:

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What would a conceptual model look like?• Entities?• Customer• Account

• Relations:• Customer-Owns-Account

Customer Account

custNo name accNo balance

inRate

Owns1 n

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Entity-Relation (ER) Model: ConceptsEntities

Attributes⋅ Atomic⋅ Composite⋅ Multi valued

Attribute valuesEntity typesKeysDomains

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RelationsCardinality ratioParticipation (total / partial)Relations may have attributes

Weak Entity TypesIdentifying ownerIdentifying relationPartial keyA weak entity always has total participation in the identifying relation.

Customer Account

custNo name accNo balance

inRate

Owns1 n

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Example: The Company Database (Elmasri)

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Entities?

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Department: name(kk), number(kk), locationProject: name(kk), number(kk), locationEmployee: ssn(kk), bdate, name (poss. composite), sex, address (poss. composite), salaryDependent (weak?): name (partial key), sex, bdate

Example: The Company Database (Elmasri)

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Relations?

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ManagesWorks_ForWorks_OnDependents_OfSupervisionControls

Example: The Company Database (Elmasri)

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ER Diagram for the Company Database

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Example: The Company Database (Elmasri)

Note the similarities to UML domain models -

more on this later

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Relations of Degree > 2

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Often causes problem in database designMay be transformed to binary relations or entitiesNot to be considered in this introduction

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Example/Exercise (Elmasri)

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Questions:Can a customer exists without an account?Can an account have more owners?How many branches can account belong to?Can a customer have accounts in more than one branch?…???