DBTG: Database Task Group and Schema

Network Model

• Developed as an extension of the hierarchical model
• Handles more relationship types
• Offers data access flexibility
• Promotes database integrity
• Conforms to standards
• Language is navigational, using constructs like FIND, FIND member, FIND owner, FIND NEXT within set, GET, etc.

Disadvantages of Network Model

• Navigational and procedural nature of processing
• System complexity
• Lack of structural independence
• Database contains a complex array of pointers that thread through a set of records
• Little scope for automated "query optimization"

Relational Model

• Developed by E.F. Codd in 1970
• First commercial system in 1981-82
• Objectives:
  • Represent complex data relationships more effectively than the hierarchical model
  • Improve database performance
  • Establish database standards
• Crucial database components:
  • Schema: Conceptual organization of entire database
  • Subschema: Defines database portion seen by application programs
  • Data Management Language (DML): Defines data characteristics and data structure
  • Schema Data Definition Language (DDL): Enables database administrator to define schema components
  • Subschema DML: Allows application programs to define database components

Entity Relationship Model

• Basic structure:
  • Collection of records in 1:M relationships
  • Relationship is called a Set: Owner and Member
• Advantages:
  • Ability to model complex relationships and represents semantics of add/delete on the relationships
  • Crow’s foot and Chen Notations can be used for graphical representation
  • Entity is mapped to a relational table
  • Entity instance (occurrence) is row in the table
  • Entity set is collection of entities at any time
  • Connectivity labels types of relationships
• Disadvantages:
  • Limited constraint representation
  • Limited relationship representation
  • No data manipulation language
  • Loss of information content

Object Oriented Model

• Developed by Hammer and McLeod in 1981
• Modeled both data and their relationships in a single structure known as an object
• Basis of object-oriented data model (OODM)
• OODM becomes the basis for the object-oriented database management system (OODBMS)
• Object:
  • Described by its factual content (Like relational model’s entity)
  • Includes information about relationships between facts within object and relationships with other objects
• Advantages:
  • Adds semantic content
  • Visual presentation includes semantic content
  • Database integrity
  • Both structural and data independence
  • Persistent programming languages based OODB target applications that have high performance requirements