er model assignment help

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Entity Relationship Diagrams
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Entity Relationship Model

• Proposed by Peter Chen in 1976
• Useful for database (conceptual schema) design
• World/enterprise explained with regards to
• Entities
• Attributes
• Relationships

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Basic ER Modelling

• Entity a distinguishable object
• Entity set set of entities associated with
  exact same kind
• Graphical representation associated with entity
  sets

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Basic ER Modelling (contd)

• Attributes explain poperties associated with
  entities
• Example for Employee entities EmpNum, Name,
  Salary,
• Domain group of permitted values for an
  attribute
• Graphical representation associated with
  attributes

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Basic ER Modelling (contd)

• Relationship representation to the fact that
  particular entities tend to be associated to one
  another
• Relationship set set associated with
  relationships of a given type
• For Example
• College students registered in programs
• Passengers reserved on ?ights
• Parents and their children
• Bank branches, clients as well as their accounts

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Graphical Representation of Relationship Sets
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Binary Relationship Types

• many-to-one (N1) Every entity in A could be
  associated with at most one entity in B, however
  an entity in B might be associated with many
  entities in A
• For Example

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one-to-one (11) relationship

• one-to-one (11) Every entity in could be
  associated with at most one entity in B, and vise
  versa
• For Example

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many-to-many (NN) relationship

• many-to-many (NN) An entity could be associated
  with many entities in the other set, and vice
  versa
• Example

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