Step 1.5 Identifying attributes

Identify the characteristics that are common to all possible instances of objects in the class.

Types of Attribute

Candidate key A set of one or more attributes that uniquely distinguishes one instance of an object from another. For example, an invoice number. Descriptive attributes Facts intrinsic to each instance of the object. For example, invoice date and invoice total. Referential attributes An attribute that ties an instance of one object to an instance of another. For example, customer code.

Rules for Identifying Attributes

• Single valued. One instance of an object has exactly one value of each attribute at any given instant in time
• Atomic. An attribute is atomic - cannot be described by other attributes
  legal attribute : phone-number
  illegal attribute : shipping address => street-address, city, postcode
• Uniquely identify the object. A set of attributes (the candidate key) must uniquely identify all instances of the object
  legal candidate key : customer-code
  illegal candidate key : last-name
• A property of the object. Each non-candidate key attribute is a characteristic of the instance named by the candidate key and not of some other non-candidate key attribute
  legal attributes : invoice = invoice-number + invoice-date + customer-code
  illegal attribute (street-address): invoice = invoice-number + invoice-date + customer-code + street-address
  Note that street-address is an attribute of customer not invoice.