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Conceptual modelling:The Entity-Relation (ER) Model
Conceptual modelsEntity – Relation (ER) models
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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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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 -
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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