Unique identifier - defined

Definitions:

Entity: A person, place, object, event, or concept about which an organisation wishes to maintain data.   For example   Person (Employee, Student, Client), Place (Town, City, Country), Object (Part, building, vehicle), Event (sale, enrolment), Concept (course, account). Entity type (Table name): Is a collection of entities sharing common attributes. Eg. Employee, Sales. In MS-Access is the table name. Entity attributes (field names in a record): Named property of the entity of interest to the organisation. Commonly referred to as the Field names. Entity instance (record): The actual data stored in the entity (table), and is a single occurrence of the entity type. Eg. 123422, Ben Dover, 12 Hide St, TAURANGA. Commonly referred to as the  record. (1994) McFadden,F & Hoffer,J. pp126-.

• Each entity type (record) must have an attribute that uniquely identifies each instance.
• A candidate key is an attribute (or combination of attributes) that uniquely identifies each entity instance.
  Some enitities may have more than one candidate key.
  For example Employee may have a numeric EmployeeId plus a UserId that is used to login to a computer system. (Eg. DOVBEN01).
• A Primary key is the candidate key selected as the identifier for the entity type, and may be
• Significant

  key is derived from something about the entity
  eg. AVERMIC Computer login for an ACBC student, Michael Verhaart

• Non-significant

  key has nothing to do with the entity, but is assigned on some other basis
  eg. IRD or Social Welfare Number, StudentId 9410045, or an automatic number generated by the database.

NAME Name of the entity (Supplier, Part) PRIMARY-KEY unique identification key key with single attribute called atomic key SupplierId in Supplier key with mutiple attributes called compound key PartId + SupplierId in Part FOREIGN-KEY* key of a related table SupplierId in Part ATTRIBUTE mandatory (must-have) attributes OPTIONAL-ATTRIBUTE attribute that could have null or missing value

Ack: S. Corich EIT98: [Rev 16/05/99] 13/6/97 ©1997-99 V/2-Com (Verhaart), Box 8415, Havelock North, New Zealand.