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Entity-Relation Diagrams

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• ER-Diagrams
  • An ER-diagrams is a graphical representation of an ER-model

  • Humans - for one reason or another - grasp information more easily when information is depicted in a graphical manner

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• Representing Entities
  • An entity type is represented by a rectangle

  • A simple-valued attribute is represented by a oval

  • A multi-valued attribute is represented by a double oval

  • A key attribute is underlined

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  • Schematically:

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  • Example: representation of the department entity

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• Representing WEAK Entities
  • An weak entity type is represented by a double rectangle

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  • Schematically:

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  • Example: representation of the dependent entity

    Notice there are no key attributes in a weak entity type

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• Representing Relationships
  • A relationship type is represented by a diamond

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  • A line is drawn from the relationship type to each participating entuty type:
    1. A partial participation is represented by a single line

    2. A total participation is represented by a double line

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  • Cardinality ratio constraint in a relationship type is represented by a value on the participation line
    1. 1 means that at most 1 (0 or 1) entity of the other entity type is associated with an entity

       The value can be any fixed constant, e.g.:

       2 means that at most 2 (0, 1 or 2) entities of the other entity type is associated with an entity

    2. N means that any number of entities of the other entity type is associated with an entity

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  • Schematically:

    The cardinality ratio depicts the fact that:

    • ONE entity of type entity1 can be associated with N entities of entity2
    • ONE entity of type entity2 can be associated with at most 1 entity of entity1

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  • Example: the "Control" relationship

    Participation constraints:

    • Department: partial
    • Project: total

    Cardinality ratio constraints:

    • ONE Department can control N Projects
    • ONE Project is controlled by at most 1 entity1

      (NOTE: because the participation of Project is total, we know that at most 1 is in fact always 1)

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• Representing WEAK Relationships
  • A weak relationship type is represented by a double diamond

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  • Schematically:

  • We will see an example of weak relationship type in the full ER-diagram of the company database next

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• ER-Diagram of Company Database
  • We had the following summary of the findings (= specifications):

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  • The ER-diagram of the Company Database representing the summary specifications (specifications found on this webpage: click here ) is:

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    We will look at the content of an ER-diagram and its meaning/representation using a concrete example next....