MODELS-DAY4

Object-based logical models

Characteristics:

· Structure of the database is flexible

· Data constraints can be specified explicitly

Examples: * The entity-relationship (ER) model

*Object-oriented (OO) models

Common characteristics of ER and OO models:

· Data is viewed as sets of entities (objects) that represent things in the real world

· Entities (objects) in the system are distinct and uniquely identifiable

· Example: customer entities can be uniquely identified by social security number

· New kinds of objects (entities) can be constructed from old kinds

Record-based logical models

Characteristics:

· Data is viewed as fixed-format records of various types

· Example: one record type for customers, another for checking accounts, ...

Examples of record-based logical models:

· Relational model

· Network model

· Hierarchical model

Network model

· Data represented by collections of records

· Relationships between data items are represented by links between records

· Example: each customer record is linked to the checking account record of that customer

· Records and links can form an arbitrary graph (can have unconnected components and cycles)

Hierarchical model

· Again, data represented by collections of records and relationships by links

· Records and links form a tree (connected graph with no cycles)

The network and hierarchical models are low-level and mostly of historical interest.