ER

ER

1. Entities

   • Real world object representations
     • Entity instance
   • Syntax: rectangle
   • Characteristics
   • Weak entities
     • Syntax: rectangle inside rectangle
     • Parent dependant
       • Parent key = Weak entity key

2. Relationships

   • Characteristics
   • Real world assosiations
     • Roles
       • Distinct name for one end
   • Description
     • Degree
       • Recursive binary
       • Binary
       • Ternary
     • Connectivity
       • One to one
       • One to many
       • Many to many
     • Existence
       • Optional
       • Mandatory
   • Syntax: diamond

3. Attributes

   • Attribute value
   • Syntax: ellipse/oblong
     • Connected to entity/relationship
   • Indentifier / key
     • Unique / instance
       • Underlined
     • Categorization
       • Primary
       • Secondary
   • Descriptor
     • Non-unique
       • Can be multivalued
       • Can be complex

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