Entity, Entity set and Attributes

An entity is a thing or object in the real world that is distinguishable from other objects. An entity can be concrete, such as a book or a person or it may be an abstract, such as a loan, holiday or a concept. An entity in a database system actually represents a table.

The properties or parts of an entity are called attributes. For example, a person has the attributes person_id, name, occupation, salary etc. A book has the attributes book_id, author, publisher, category, number of copies etc. Attributes are actually the fields of a database table or entity.

An entity set is a set of entities of the same type that share the same properties or attributes. For example, the set of all students who take a class can represent a student entity set in which each entity is a student sharing similar attributes with other students such as student_id, name, contact_no, address etc.

Copyright @ by Rosina S Khan

Database Administrator

A person who has central control over the whole database system is a database administrator (DBA). His functions may be summarized as below

·        Schema definition: The DBA creates the original database schema or the overall design of the database.

·        Storage structure and access-method definition: The DBA is aware of certain details of physical organization of the data i.e. how records occupy storage locations or bytes and the methods to access those data.

·        Schema and physical-organization modification: The DBA can change the schema and physical organization according to the demands of the organization, or simply to improve performance. This can be done without affecting application programs at logical level.

·        Granting of authorization for data access: The DBA can grant different types of authorization to different users and hence control which user has the right to access which parts of the database. This enhances to keep the system secure.

·        Routine maintenance: Some examples of DBA’s routine maintenance activities are as follows:

i)                   Backup the database periodically to tapes or remote servers so that they can be recovered in case of disasters such as database sabotage.

ii)                Ensure enough disk space is available for normal operations and upgrade disk space as required.

iii)              Monitor jobs running in the database and ensure overall performance is good.

Copyright @ by Rosina S Khan

Database Users

There are basically four types of Database Users. These are:

•  Application programmers are computer professionals who write application programs or front end interfaces. They can choose tools such as Rapid Application Development (RAD) to develop front end interfaces such as forms and reports with minimal programming effort.

• Sophisticated users interact with the system without writing programs. They form their requests to the system such as a database query language e.g, SQL.

•  Specialized users are sophisticated users who write specialized database applications such as computer-aided design (CAD), knowledge-base and expert systems that store complex data such as graphics and audio data and environment-modeling systems.

• Naive users are unsophisticated users who interact with the system by invoking one of the application forms that have been written previously where the naïve users fill in the fields of the form and hit the button. They may also simply read reports generated from the database

Copyright @ by Rosina S Khan

Database Languages

A database system provides a data-definition language to specify the database schema and a data-manipulation language to express database queries and updates. In practice, the data-definition and data-manipulation languages are not two separate languages; instead they simply form parts of a single database language, such as the widely used SQL (Structured Query Language).

a)      Data-Manipulation Language

A data-manipulation language (DML) is a language that enables users to access or manipulate data as organized by the appropriate data model. The types of access are:

·         Retrieval of information stored in the database

·         Insertion of new information into the database

·         Deletion of information from the database

·         Modification of information stored in the database

b)     Data-Definition Language

We specify a database schema by a set of definitions expressed by a special language called a data-definition language (DDL). The DDL is also used to specify additional properties of the data. We specify the storage structure and access methods used by the database system by a set of statements in a special type of DDL called a data storage and definition language. These statements define the implementation details of the database schemas, which are usually hidden from the users.

 Copyright @ by Rosina S Khan