DATA MODELS

Data models include a set of basic operations for specifying retrievals and updates on the database. Data models also allow the database designer to specify a set of valid user defined operations on the database objects. Depending on the nearness of the data models with the users, data models can be classified in the following categories:

• High-level or conceptual data models: This class of models provides concepts that are based on entities and relationship. It uses concepts such as entities, attributes and relationships.

1. Entity: An entity represents a real-world object or concept, such as employee or a project that is described in the database.
2. Attribute: An attribute represents some property that describes an entity, such as the employee's name or salary.
3. Relationship: A relationship among two or more entities represents an interaction among the entities (e.g., relationship between an employee and a project).

• Low-level or physical data models: this model describes the details of how data is stored in the computer (useful for computer specialists).

• Representational or implementation data models: These models provide concepts that are between the above two extremes which may be understood by end users-record-based, object-oriented etc.

DATA INDEPENDENCE

There are two kinds of data independence, physical and logical. Applications implementation older systems-pre-relational or even pre-database systems - tend to be data-dependent. For these applications it is impossible to change the storage structure (how the data physically recorded) or access strategy (how it is accessed) without affecting the application, probably drastically.

Characteristics of Database Management System

1. Self Describing: DBMS Systems create a sub layer with the operating system and handles all the functions related with the DBMS separately and specifically.

2. Insulation from programs and Data Abstraction.

3. Support of Multiple views to Database.


                          Advantages of Database

1.Redundancy can be reduced:There are provisions in all database models that reduntant storage of the data can be avoided . In non-database systems each application has its own private files. This fact can often lead to considerable redundancy in stored data, with resultant waste in storage space.

2. Security can be enforced: The DBA (Data Base Administrator) can ensure that the only means of access to the database through the proper channels, and hence can define security constraints or rules for accessing the sensitive data. No one can access or modify or store database without login. Different constraints can be established for each type of access (retrieve, insert, delete, etc) to each piece of information in the database.

3. Allowing Multiple User Interface: DBMS has the capability to provide concurrent execution of various parts of the database. In this approach, Deadlock and other anomalies are also handled by the DBMS.

4. Backup and Recovery: Database Management Systems have proper mechanism to Backup the whole database and recover when any disaster comes to picture.

5. Reduced application development time: The effort required to develop user's utility programs reduces considerably and so does the time.

                Disadvantages of Database

1. Cost issues: Installing and maintaining a database is expensive, particularly in a large organisation. In addition, there are costs associated with training people to use it correctly.

2. Security issues: Although database can be structured to restrict access, it's always possible unauthorised users will get past the safeguards. And when they do, they may have access 

to all the files, not just a few. In addition, if a database is destroyed by fire, earthquake, theft, or hardware or software problems, it could be fatal to an organisation's business 

activities - unless steps have been taken to regularly make backup copies of the files and store them elsewhere.

3. Privacy issues: Databases may hold information they should not and be used for unintended purposes, perhaps intruding on people's privacy. Medical data,for instance, may be used inappropriately in 

evaluating an employee for a job promotion.

Components of Database Systems

Components of Database Systems are;

• Data
• Hardware
• Software
• Database users

DATA

Data is a collection of meaningful facts or figures expressed in a form that its users could understand. It can be recorded and processed for further use. The Data stored in the system is portioned into one or more databases. Generally we assume that there is just one database, containing totality of all stored data in the system.

Data stored in the database is both integrated and shared

Integrated: The data is accurate, consisted and up to date - because each updating change is made only in one place.

Shared: Individual pieces of data in the database can be shared among different users, in the sense that each of those users can have access to the same piece of data, which can use for different purposes. The same piece of data can be effectively access by different users at the same time that is Concurrent Access. Sharing of dta, concurrent or otherwise, is partly a consequence of the fact that the database is integrated.

HARDWARE

The hardware components of the system consist of :

• The secondary storage volumes - mostly magnetic disks - that are used to hold the stored data, together with the associated I/O devices (disk drives,etc.), device controllers, I/O channels.

• The hardware processor and associated main memory that are used to support the execution of the database system software.

SOFTWARE

A software act as an interface between the physical database itself-i.e., the data as physically stored - and the users of the system. Data base management system (DBMS) software or Data base manager, a program that controls structure of a database and access to the data. All requests for access to the database are handled by the DBMS. For adding and removing files, retrieving data from and updating data in such files or tables, and so forth, are all facilities provided by the DBMS.

DATABASE USERS

A primary goal of a database system is to provide an environment for retrieving information from and storing information into the database.

Any entity whether a human or a software system that interacts with a database is known as a database user. However, the role of a user with respect to a database depends on the responsibilities it carries and the operations  it performs on the database. Accordingly, following classws of users exist in a database system:

• Database Administrators (DBA): The user is responsible for creating. manipulating and authorizing access to the database. They are also responsible for coordinating and monitoring its use. A DBA has complete control on every aspect of the database. Sometimes a DBA is also called as Super-User.

• Database Designers: They are responsible for identifying the data to be stored and foe choosing appropriate data structures to represent and store this data.

• End User: People whose jobs require access to the database for querying, updating, and generating reports are end-users. They carry out their jobs either through application programs or users programs.

• System Analysts: System analysts determine the requirements of the end users, and develop specifications for transactions.

• Application Programmers: Application pragrammers implement specifications and programs developed by the analysts so that the specification becomes perational. The application program are written in the host language for example Pascal, C++, Cobol. Analysts and programmers nowadays are called software engineers.

Database System

A database system is just a computerised record-keeping system. The database is a kind of an electronic filling cabinet. The user can perform the following operations..

• Addition of new files to the database
• Insertion of  data into existing files
• Retrieving of data from existing files
• Updation of data in existing files
• Removing of data from existing files

A large repository of data which may be structured or unstructured, manually operated or computerized, consisting of a single file or a number of files is refered to as a database.They used data base management systems (DBMS) software which control the steucture of the database and access to the data.Database stores a collection of related sets of data items (i.e. both data and relations between them) along with necessary data/information associated with it (metadata). In a traditional manual system a database is stored on paper while in a computer-based system it is stored on secondary devices in electronic form.