Database Management System (DBMS) Basic Concepts

RELATIONAL ALGEBRA

Relational algebra is a collection of operations used to manipulate relations (tables). These operations enable the users to specify the retrieval requests which results in a new relation built from one or more relations.

Relational Algebra is a Procedural language, which specifies, the operations to be performed on the existing relations to derive result relations. It is a procedural language which means that user has to specify what is required and what is the sequence of steps performed on the database to obtain the required output. Whenever the operations are performed on the existing relations to produce new relations then the original relations(s) are not effected i.e. they remain the same, and the resultant relation obtained can act as an input to some other operation, so relational algebra operations can be composed together into a relational algebra expression. Composing relational algebra operation into relational expression is similar to composing arithmetic operations (+, -, *) into arithmetic expressions. R1+R2 is a relational expression where R1 and R2 are relations.

It is important that the results of use of relational algebric operations on Relations (Tables) must themselves be a Relation (Tables). This is because these operators can be used sequentially in various combinations to obtain desired results. Thus each operation on completion must leave data as a relation (table) for the next operator to use. So, this property which all the above operators must have is referred to as Relational Closure.

Relational Algebra is a formal and non-user friendly language. It illustrates the basic operations required for any Data Manipulation languages but it is very less commonly used in the commercial languages because it lacks the syntactic details, although it acts as a fundamental technique for extracting data from the database.

Relational Algebric Operations

The Relational Algebric Operations can be divided into two groups.

1. Basic Set Oriented Operations or Traditional set operations – These are derived from Mathematical Set theory. They are applicable because each relation is defined to be set of Tuples. These include Union, Intersection, Difference, Cartesian Product. All of these operations are binary operations which means that operation applies to pair of Relations.

2. Special Relational operations - These include join, selection, projection and division. These operations were designed specifically for relational databases. These operations don't add only power to the algebra but simply for common queries that are lengthy to express using basic set oriented operations.

These operations were introduced by Dr. Codd. But these could not meet all the requirements, so some additional operations were introduced. These included aggregate functions like SUM, AVERAGE, COUNT, OUTER JOIN etc.

Database Administrator (DBA)

The DBA is a person or a group of persons who is responsible for the management of the database. The DBA is responsible for authorizing access to the database by grant and revoke permis­sions to the users, for coordinating and monitoring its use, managing backups and repairing damage due to hardware and/or software failures and for acquiring hardware and software resources as needed. In case of small organization the role of DBA is performed by a single person and in case of large organizations there is a group of DBA's who share responsibilities.

Database Designers

They are responsible for identifying the data to be stored in the database and for choosing appropriate structure to represent and store the data. It is the responsibility of database designers to communicate with all prospective of the database users in order to understand their requirements so that they can create a design that meets their requirements.

End Users

End Users are the people who interact with the database through applications or utilities. The various categories of end users are:

• Casual End Users - These Users occasionally access the database but may need different information each time. They use sophisticated database Query language to specify their requests. For example: High level Managers who access the data weekly or biweekly.

• Native End Users - These users frequently query and update the database using standard types of Queries. The operations that can be performed by this class of users are very limited and effect precise portion of the database.

For example: - Reservation clerks for airlines/hotels check availability for given request and make reservations. Also, persons using Automated Teller Machines (ATM's) fall under this category as he has access to limited portion of the database.

• Standalone end Users/On-line End Users - Those end Users who interact with the database directly via on-line terminal or indirectly through Menu or graphics based Interfaces.

For example: - User of a text package, library management software that store variety of library data such as issue and return of books for fine purposes.

Application Programmers

Application Programmers are responsible for writing application programs that use the data­base. These programs could be written in General Purpose Programming languages such as Visual Basic, Developer, C, FORTRAN, COBOL etc. to manipulate the database. These application programs operate on the data to perform various operations such as retaining information, creating new informa­tion, deleting or changing existing information.