What is ENTITY

An entity may be defined as a thing which is recognized as being capable of an independent existence and which can be uniquely identified. An entity is an abstraction from the complexities of some domain. When we speak of an entity we normally speak of some aspect of the real world which can be distinguished from other aspects of the real world.
An entity may be a physical object such as a house or a car, an event such as a house sale or a car service, or a concept such as a customer transaction or order. Although the term entity is the one most commonly used, following Chen we should really distinguish between an entity and an entity-type. An entity-type is a category. An entity, strictly speaking, is an instance of a given entity-type. There are usually many instances of an entity-type. Because the term entity-type is somewhat cumbersome, most people tend to use the term entity as a synonym for this term.
Entities can be thought of as nouns. Examples: a computer, an employee, a song, a mathematical theorem.

Characteristics of Object-Oriented Databases in Depth

Object-oriented database technology is a marriage of object-oriented programming and database technologies. Figure 1 illustrates how these programming and database concepts have come together to provide what we now call object-oriented databases.

Perhaps the most significant characteristic of object-oriented database technology is that it combines object-oriented programming with database technology to provide an integrated application development system. There are many advantages to including the definition of operations with the definition of data. First, the defined operations apply ubiquitously and are not dependent on the particular database application running at the moment. Second, the data types can be extended to support complex data such as multi-media by defining new object classes that have operations to support the new kinds of information.

Other strengths of object-oriented modeling are well known. For example, inheritance allows one to develop solutions to complex problems incrementally by defining new objects in terms of previously defined objects. Polymorphism and dynamic binding allow one to define operations for one object and then to share the specification of the operation with other objects. These objects can further extend this operation to provide behaviors that are unique to those objects. Dynamic binding determines at runtime which of these operations is actually executed, depending on the class of the object requested to perform the operation. Polymorphism and dynamic binding are powerful object-oriented features that allow one to compose objects to provide solutions without having to write code that is specific to each object. All of these capabilities come together synergistically to provide significant productivity advantages to database application developers.

A significant difference between object-oriented databases and relational databases is that object-oriented databases represent relationships explicitly, supporting both navigational and associative access to information. As the complexity of interrelationships between information within the database increases, so do the advantages of representing relationships explicitly. Another benefit of using explicit relationships is the improvement in data access performance over relational value-based relationships.

A unique characteristic of objects is that they have an identity that is independent of the state of the object. For example, if one has a car object and we remodel the car and change its appearance, the engine, the transmission, and the tires so that it looks entirely different, it would still be recognized as the same object we had originally. Within an object-oriented database, one can always ask the question, “is this the same object I had previously?”, assuming one remembers the object’s identity. Object-identity allows objects to be related as well as shared within a distributed computing network.

All of these advantages point to the application of object-oriented databases to information management problems that are characterized by the need to manage:

• a large number of different data types,
• a large number of relationships between the objects, and
• objects with complex behaviors.

Application areas where this kind of complexity exists includes engineering, manufacturing, simulations, office automation and large information systems.

Database Management System

To combat the limitations of RDBMS and meet the challenge of the increasing rise of the Internet and the Web, programmers developed object-oriented databases in the 1980s. The main objective of Object-Oriented DatabaseManagement Systems, commonly known as OODBMS, is to provide consistent, data independent, secure, controlled and extensible data management services to support the object-oriented model. They were created to handle big and complex data that relational databases could not.

There are important characteristics involved with object-oriented databases. The most important characteristic is the joining of object-oriented programming with database technology, which provides an integrated application developmentsystem. Object-oriented programming results in 4 main characteristics: inheritances, data encapsulation, object identity, and polymorphism. Inheritance allows one to develop solutions to complex problems incrementally by defining new objects in terms of previously defined objects.

Data encapsulation or simply encapsulation allows the hiding of the internal state of the objects. Encapsulated objects are those objects that can only be assessed by their methods instead of their internal states. There are three types of encapsulated objects users and developers should recognize. The first is full encapsulation, in which all the operations on objects are done through message sending and method execution. The second is write encapsulation, which is where the internal state of the object is visible only for reading operations. The third is partial encapsulation, which involves allowing direct access for reading and writing for only a part of the internal state.

Object identity allows objects of the database to be independent of each other. Polymorphism and dynamic binding allow one to define operations for one object and then to share the specification of the operation with other objects. This allows users and/or programmers to compose objects to provide solutions without having to write code that is specific to each object.

The language important to OODBMS is data definition and manipulation language (DDML). The use of this language allows persistent data to be created, updated, deleted, or retrieved. An OODBMS needs a computational versus a relational language because it can be used to avoid impedance mismatch. DDML allows users to define a database, including creating, altering, and dropping tables and establishing constraints. DDMLs are used to maintain and query a database, including updating, inserting, modifying, and querying data.

The OODBMS has many advantages and benefits. First, object-oriented is a more natural way of thinking. Second, the defined operations of these types of systems are not dependent on the particular database application running at a given moment. Third, the data types of object-oriented databases can be extended to support complex data such as images, digital and audio/video, along with other multi-media operations. Different benefits of OODBMS are its reusability, stability, and reliability. Another benefit of OODBMS is that relationships are represented explicitly, often supporting both navigational and associative access to information. This translates to improvement in data access performance versus the relational model.

Another important benefit is that users are allowed to define their own methods of access to data and how it will be represented or manipulated. The most significant benefit of the OODBMS is that these databases have extended into areas not known by the RDBMS. Medicine, multimedia, and high-energy physics are just a few of the new industries relying on object-oriented databases.

As with the relational database method, object-oriented databases also has disadvantages or limitations. One disadvantage of OODBMS is that it lacks a common data model. There is also no current standard, since it is still considered to be in the development stages.

Three types of recovery models

The three types of recovery models that you can choose from are:

• Full
• Simple
• Bulk-Logged

Each database can have only one recovery model, but each of your databases can use a different recovery model, so depending on the processing and the backup needs you can select the appropriate recovery model per database.  The only exception to this is the TempDB database which has to use the "Simple" recovery model.

Also, the database recovery model can be changed at any time, but this will impact your backup chain, so it is a good practice to issue a full backup after you change your recovery model.

Diiference between OODBMS and ORDBMS

Definition:

Object-Relational databases extend the Relational Data Model

to address those weaknesses identified previously.

An Object-Relational database adds features associated with

object-oriented systems to the Relational Data Model.

MAJOR DIFFERENCE BETWEEN AN

ORDBMS AND AN OODBMS

OODBMSs try to add DBMS functionality to one

or more OO programming languages.

ORDBMSs

REVOLUTIONARY IN THAT THEY ABANDON SQL

ORDBMSs try to add richer data types and OO

features to a relational DBMS.

EVOLUTIONARY IN THAT THEY EXTEND SQL

CONSIDERATIONS

OODBMS - put more emphasis on the role of the client, i.e.,

Client side caching! This can radically improve long, process

intensive, transactions.       

ORDBMS - SQL is still the language for data definition,

manipulation and query – Still have Impedance Mismatch!

OODBMSs have been optimised to directly support objectoriented

applications and specific OO languages.

ORDBMSs are supported by most of the ‘major players’ in

the DBMS market place.       

Thanks to the Internet, the legal need to keep track of lots of business information, new marketing methods, and the explosion of data-intensive scientific progress, databases are being used more than ever before for storing and accessing information. There are currently three different models in use for database management systems: relational, object-oriented, and object-relational. This article introduces all three, and discusses their relative strengths and weaknesses.