Entity - Relationship Model

(Ref: Database Systems, Date, C.J.)

Semantic Modeling method first developed by Chen (1976)

Entity. This is something that can be distinctly identified.

• A Regular entity is one that can exist on its own. e.g. Customer
• A Weak entity is one that depends on other entities for it's existance. e.g. An order cannot exist unless a customer exists.

Properties. These are what make an entity exist. They are the content of the entity. e.g. name, address, phone, and age are all properties of the customer entity. Individual customer entities have values for these properties. The term attribute is often used in place of properties and can mean the same thing. It is preferable not to use attribute when working with E-R diagrams.

Properties can be:

• Simple e.g. postcode or Composite e.g. name made up of surname and given name
• Key - a unique value e.g phone number or fingerprint (note that phone number is unique in that it belongs to only one person)
• Singe-valued or Multi-valued (repeating groups permitted)
• Missing (unknown or not available)
• Base or Derived - Derived come from processing base values e.g. age derived from age

Relationship

This is an association between entities. E.g. There is a relationship between the customer and the product purchased, but not between the customer and the supplier of the product.

When entities are involved in a relationship they are called participants.

Relationships can be one-to-one, one-to-many, many-to-one and many-to-many.

Rules for creating ER diagrams

Each Entity type is shown as a rectangle and labeled with a name.

Properties are shown as ellipses, labeled with the name of the property and attached to the entity with a straight line. Key properties are underlined.

Relationships are shown as diamonds, labeled with the name of the relationship. Each Entity is connected to a relationship by a straight line. Each line has the words one or many to indicate the type of relationship.

Mike Leishman