Slide 2

• E-R data models employ three basic notions:”
  • Entity Sets
  • Relationship sets
  • attributes

Entities

• Entity: any “thing” or “object” that is distinguishable from all other objects.
  • Each person at a company is a separate entity
  • Every automobile is a separate entity
• Every entity has a set of properties.
  • I.e. Lastname, Firstname and social security number
• some set of the properties for a given entity may be used to uniquely identify the entity.

Entity Sets

• An Entity Set is a set of entities of the same type that share the same properties, or attributes.
  • The set of cutomer entities for a bank could be described as the entity set “cutomers”
  • The entity set loan might be composed of all the loans awarded to customers by the bank.
• Entity sets do not have to be disjoint.
  • An entity may belong to more than one entity set.
  • “Swedish citizens” and “mammals” for instance.

Attributes

• Each entity is represented by a set of attributes
• attributes are descriptive properties possessed by each member of an entity set.
• Each entity has a value associated with each of its attributes. The set of permitted values is called the “domain” or “value set”

Relationship sets

• A relationship is an association among several entities.
• {(e1, e2, ..., en) | e1