The maintenance of centralized databases Essay

It reduces the maintenance costs and data security and reliability is ensured.

Limitations A DBMS is expensive and complex in nature. The costs involved in data maintenance are low however, DBMS requires frequent upgradation of the hardware components, thus resulting in increase in costs. The maintenance of centralized databases may lead to increase in the possibility of errors and failures.

Data views allow users to access and modify files without knowing the physical location of the data. The types of data views include data abstraction and instances and schemas.

Data abstraction The DBMS provides users an abstract view of the database system. Certain details are hidden from the users as to how the data is created, stored, and maintained. It does so by defining three levels of abstraction  the physical level, the logical level, and the view level.

Physical levelThe physical level is the lowest level of abstraction that describes the manner in which data is actually stored. In this level, complex lowlevel data structures are described in detail.

Logical levelAlso known as conceptual level, it is the next level of abstraction. The type of data stored and the interrelationships between them are described in this level. Here a record is described by a type definition and the interrelationships that exist between the record types are also described. Programmers and DBAs work at this level of abstraction.

View level This is the highest level of abstraction. At this level, the users observe a bunch of applications that hide the details of the data types. In addition to this, the views also provide a security mechanism to prevent users from accessing parts of the database.

Instances and schemaThe information currently stored in the database is called an instance of the database. The overall design of the database is called schema. Database systems have several schemas, partitioned according to the levels of abstraction. In 1976, Peter PinShan Chen has proposed the ER (EntityRelationship) model. It can be used to model the real world objects. It can be used to model the real world objects known as entities and relationships among them known as associations. Organization, person, institution, product are the examples of entities. Or any other abstract thing can also be an entity.

ER model is a conceptual tool to design the databases. They are used in designing software applications and systems and organization data in information systems. ER model is represented in a form of graphical notation with depicting entities and the relationships among them.

The core constructs or basic elements in ER model are entities, relationships and attributes.

EntitiesAn entity is a conceptual object regarding which we store the data. Employee, Product, Company, Customer, Invoice are the examples of entities. Entities are of two types. They are 1) Independent Entity and 2) Dependent Entity. They are also known as strong entity and weak entity respectively. An independent entity does not depend on any other entity for its identification. Whereas, a dependent entity depends on another