Chapter 5, Data Modeling with the Entity-Relationship Model

• A data model is a plan for a database design.
• It is not a database implementation.
• Think algorithm, UML diagram, ...
• We will use the Entity-Relationship or ER model
• An entity is something the user wants to track
  • It will eventually become a table
  • A customer, an item, a grade, a class, ....
  • The scope really depends on the information the user is interested in .
  • He discusses classes and instances of classes, but you better have that. (If not speak now....)
  • Entities are composed of attributes
    • The individual characteristics of an entity.
    • Members of a class. (In the CS sense)
    • You should have this one too.
  • Attributes are named with Identifiers
    • Identifiers identify, or uniquely name instances
    • (Primary Key)
    • Identifiers can have multiple attributes, composit identifiers
  • 
• Relationships
  • Entities are associated with one another by a relationship
  • Origionally relationships were named.
  • But this doesn't seem to be the case any longer.
  • 
• Cardinality
  • Relationships are classified by Cardinality
    • minimum cardinality is the minimum required number of entity instances in a relationship.
      • An OrderItem must have a SKU, or it cant exist.
      • An Order can have 0 or more items
      • This is a 0 or one measure.
      • Can something exist without a sub-part or not.