Information Systems

• Derived Attributes


  • Is an attribute whose value is calculated from other attributes
  • Need not to be physically stored within the database
  • Derived using an algorithm
  • Also referred to as "Computed attributes"
• Derived Attribute: Stored


  • Advantage: Saves CPU processing cycles
  • Disadvantage: Requires constant maintenance to ensure derived value is current, especially if any values used in the calculation change
• Derived Attribute: Not Stored


  • Advantage: Saves data access time, data value is readily available, can be used to keep track of historical data, saves storage space, computation always yields current value
  • Disadvantage: Uses CPU processing cycles, increases data access time, adds coding complexity to queries
• Association


  Relationship between entities that always operate in both directions
• Participants


  Entities that participate in a relationship
• Connectivity


  Describes the relationship classification (1:1, 1:M, and M:N)
• Cardinality


  Expresses the minimum and maximum number of entity occurrences associated with one occurrence of related entity
• Existence dependence


  Entity exists in the database only when it is associated with another related entity occurrence
• Existence independence


  Entity exists apart from all of its related entities
• Weak (non-identifying) relationship


  Primary key of the related entity does not contain a primary key component of the parent entity
• Strong (identifying) relationships


  Primary key of the related entity contains a primary key component of the parent entity
• Weak entities


  • Existence-dependent, has a primary key that is partially or totally derived from parent entity in the relationship
• Optional participation


  One entity occurrence does not require a corresponding entity occurrence in a particular relationship
• Mandatory participation


  One entity occurrence requires a corresponding entity occurrence in a particular relationship
• Unary relationship


  Association is maintained within a single entity
• Recursive relationship


  Relationship exists within a single entity type
• Binary relationship


  Two entities are associated
• Ternary relationship


  Three entities are associated
• Associative (Composite) Entities


  Used to represent an M:N relationship between two or more entities, has a 1:M relationship with the parent entities, composed of the primary key attributes of each parent entity, may also contain additional attributes that play no role in connective process
• Developing an ER Diagram

  1. Create a detailed narrative of the organization's description of operations

  2. Identify business rules based on the descriptions

  3. Identify main entities and relationships from the business rules

  4. Develop the initial ERD

  5. Identify the attributes and primary keys that adequately describe entities

  6. Revise and review ERD
• Database designers must often make design compromises that are triggered by conflicting goals
• Database design must conform to design standards
• High processing speed may limit the number and complexity of logically desirable relationships
• Maximum information generation may lead to loss of clean design structures and high transaction speed
• The ERM uses ERDs to represent the conceptual database as viewed by the end user
• In the ERM, an M:N relationship is valid at the conceptual level
• Unified Modeling Language (UML) class diagrams are used to represent the static data structures in a data model
• Database designers, no matter how well they can produce designs that conform to all applicable modeling conventions, are often forced to make design compromises