Object Oriented DBMS Concepts and Features

 OBJECT-ORIENTED CONCEPTS AND FEATURES

Abstraction, encapsulation, information hiding.

Objects and attributes.

Object identity.

Methods and messages.

Classes, subclasses, super classes, and inheritance.

Overloading.

Polymorphism and dynamic binding.

Abstraction

Process of identifying essential aspects of an entity and ignoring unimportant properties. 

Concentrate on what an object is and what it does, before deciding how to implement it.

 Object

An object is something uniquely identifiable, models a real-world entity and has got state and behavior.

Resembles entity, but object contains state and behavior and entity models only state.

Encapsulation & Information Hiding

Encapsulation

Object contains both data structure and set of operations used to manipulate it.

Information Hiding

Separate external aspects of an object from its internal details, which are hidden from outside.

Allows internal details of an object to be changed without affecting applications that use it, provided external details remain same.

Provides data independence.

 Attributes

Contain current state of an object

Attributes can be classified as simple or complex. 

Simple attribute can be a primitive type such as integer, string, etc., which takes on literal values.

Complex attribute can contain collections and/or references. 

Reference attribute represents relationship. 

An object that contains one or more complex attributes is called a complex object.

 Object Identity

Object Identity (OID) associated with an object on its creation:

Generated by system.

Unique to that object in the entire system.

Used only by system, not by user.

Independent of state of object

Does not change.

 Object Identity - Implementation

In RDBMS OID is:

Responsibility of the user.

Value based.

Unique only within table.

Key among attributes; so is state dependent.

Can be changed.

Using disk or RAM address is also inefficient and may cause problems, like

They are small

On deletion memory can be reused that can cause problem.

 Methods and Messages

Method: Implements functionality of object; is encapsulated

Message: A request to an object to execute one of its methods.

 Class

A container/template/blue-print for objects

Objects in a class are called instances

Class is also an object with own class attributes and class methods

Object and Class

Inheritance

Subclasses, Superclass, and Inheritance

Inheritance is a special type of relationship between classes;

The inheriting class inherits the properties of the base class

Special cases are subclasses and more general cases are super classes

Process of forming a superclass is generalization; forming a subclass is specialization

Subclass can define its own unique properties as well

Subclass can redefine inherited methods.                              

Overriding, Overloading, and Polymorphism

Overriding

Process of redefining a property within a subclass.

Overloading

Allows name of a method to be reused with a class or across classes.

Polymorphism

Means many forms.

Three types: Operations, inclusion, add parametric.

Complex Objects

An object that consists of sub-objects but is viewed as a single object. 

Another special type of relationship between objects, called IS_PART_OF

 Modeling Complex Object

Contained object can be encapsulated with in the complex object and can be accessed using the methods of complex objects.

Or have its own independent existence, and only an OID is stored in complex object.

Data Model

A structured collection of records is known as Database. The way in which database is structured is defined as Database Model, Hierarchical model, Network model, Object model and Relational model are some of the commonly used models database in the structure of database.

Database Model is a way to define the structure or logical design of a database. It tells the way the data is stored, accessed and updated in DBMS

Hierarchical databases

A hierarchical database is a design that uses a one-to-many relationship for data elements. Hierarchical database models use a tree structure that links a number of disparate elements to one "owner," or "parent," primary record.

A hierarchical database model is a data model in which the data are organized into a tree-like structure. The data are stored as records which are connected to one another through links. A record is a collection of fields, with each field containing only one value.

The main characteristic of a hierarchical data model is the treelike structure. For example, a company database might organize using one branch for Staff, followed by Departments, Teams and then Team Members.

Advantages: The model allows easy addition and deletion of new information. Data at the top of the Hierarchy is very fast to access. It was very easy to work with the model because it worked well with linear type data storage such as tapes.

In Hierarchical database model data is represented in a tree-like structure. It represents a parent-child relationship with a single parent for each child. The network Model is an extension of the Hierarchical model. In this data is represented in the form of graphs with more than one parent node for one child node.

A fundamental difference between segments in a hierarchical database and tables in a relational database is that, in a hierarchical database, segments are implicitly joined with each other. ... In a relational database, this relationship between tables is captured by foreign keys and primary keys.

 

There are four common solutions to get a hierarchical relation in your database:

1.      Adjacent list. Basically you add a parent foreign key to your model.

2.      Materialized path tree. You give every node a special “path” field.

3.      Nested sets tree. Each node gets two numbers.

4.      Adjacent list tree with common table expressions.

The main difference between hierarchical network and relational database model is that hierarchical model organizes data in a tree-like structure while network model arranges data in a graph structure and relational database model organizes data in tables. Database models help to create the structure of the databases.

A hierarchical model represents the data in a tree-like structure in which there is a single parent for each record. To maintain order there is a sort field which keeps sibling nodes into a recorded manner. These types of models are designed basically for the early mainframe database management systems, like the Information Management System (IMS) by IBM.

This model structure allows the one-to-one and a one-to-many relationship between two/ various types of data. This structure is very helpful in describing many relationships in the real world; table of contents, any nested and sorted information.

The hierarchical structure is used as the physical order of records in storage. One can access the records by navigating down through the data structure using pointers which are combined with sequential accessing. Therefore, the hierarchical structure is not suitable for certain database operations when a full path is not also included for each record.

Data in this type of database is structured hierarchically and is typically developed as an inverted tree. The "root" in the structure is a single table in the database and other tables act as the branches flowing from the root. The diagram below shows a typical hierarchical database structure.

A relationship in this database model is represented by the term parent/child. A parent table can be linked with one or more child tables in this type of relationship, but a single child table can be linked with only one parent table. The tables are explicitly linked via a pointer/index or by the physical arrangement of the records within the tables.

A user can access the data by starting at the root table and working down through the tree to the target data. the user must be familiar with the structure of the database to access the data without any complexity.