Cardinality of Relationships

• Maximal cardinality
  • one to one
    • An employee has only one badge, each badge is assigned to only one employee
  • many to one
    • An employee may have many computers. Each computer is assigned to exactly one employee.
  • many to many
    • An employee may have many skills. Each skill may be attached to many employees.
  • These are has a relationships.
• Minimal cardinality
  • As we discussed before, is an item required or optional.
• We represent these using "Crow's Foot Diagrams"
  • Minimal cardinality bar or 0 (Inside)
  • Maximal cardinality bar or crow's foot (Outside)
• No good options for E=R Diagram tools
  • I use DIA
  • Database mode, tables are not quite there, but they will do.
• 
• An entity is strong if it can exist on it's own.
  • In the above example, we can have a SKU without having an ORDER_ITEM.
  • So SKU is a strong entity.
  • But we can not have an Order_Item without a SKU, so it is a weak entity
  • He gives the example of a person and an automobile, both would be strong,
  • But an Automobile and a registration would be weak.
  • Or a room in a building
  • Weak entities are drawn with rounded corners.
• ID-Dependent Relationships
  • The identifier includes the identifier in another entity.
  • ORDER_ITEM is ID-Dependent
  • If a relation is id-dependent it is normally drawn with solid lines
  • Otherwise it is drawn with dashed lines.