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Chapter 5: Entity Relationship Diagrams, Cardinality ratio •shows the…

- - - - Optional Relationship
        e.g An employee may or may not be assigned to a department.
        
        shown by a vertical line next to the ordinality
        
        Mandatory Relationship
        e.g A customer has to make at least one order
        
        shown by two vertical lines next to entity
        
        see slide 4 of 'Powerpoint ERDs-1'
- - - - Attributes
        •Layer in more detail by adding key attributes of entities - drawn as ovals
        
        Cardinality
        •Show whether the relationship is (1:1), (1:Many) or (Many:Many)