Databases

Databases are the most widely spread application of computer systems [6,7,23,22]. They are used in almost all areas of everyday life: business, engineering, medicine, law, or education. This is where a necessity of keeping track on some data appears.

A database is a collection of related data. Data are facts, which can be recorded, stored, retrieved, or deleted. A typical database represents some aspect of the real world and is used for specific purposes. This very general definition is usually more restricted. According to [6] a database has the following properties:

• A database represents some aspect of the real world, which is called Universe of Discourse (UoD). It has some source from which data are derived and some degree of interaction with the real world.
• A database is a logically coherent collection of data with some underlying meaning.
• A database is designed, built, and populated with data for a specific purpose. There is an audience which is interested in the contents of the database.

A more precise definition is given in ISO/IEC 2382-1. A database is a set of data organized according to some data model; it describes properties of certain objects and relationships among them. It can be used in one or more applications.

A Database Management System (DBMS) is a collection of programs that enables users to create and maintain a database. This is a general-purpose software which allows to (according to [6]):

• Define a database.
  It involves specifying various data types, structures and constraints for the data to be stored in the database.
• Implement a database.
  This is the process of storing the data itself on some storage medium that is controlled by DBMS.
• Search and manipulate a database.
  It includes such functions as: retrieving specific data, updating the database to reflect changes in UoD, and generating reports from the data.

The main criterion to classify a database is the model of data being used. The model is a set of concepts that can be used to describe the structure of a database. The structure is: the data types, relationships, and constraints that should hold for data.

This dissertation is focused on the most popular model which is the Relational Model [7,6,23,22]. The Relational Model represents a database as a collection of tables. Most database systems are based on it. Some of them are: Adabas, DB2, Informix, , MySQL, Oracle, Pervasive, PostgreSQL, SQL Server, Sybase. Most relational databases (RDB) have high-level query languages. The relational model is discussed in Chapter 3 in details.