Currently it is difficult to find a well-functioning company, which did not use to the database systems. A well-designed database system is the only way to ensure fast and flexible access to information and its reliability.
The database is stored information about facts and events of a certain slice of reality, in order to allow for their reconstruction and analysis at any time. The correct and efficient operation of the database depends on the proper interpretation of the information contained herein. This requires a true and complete description of the semantics of the modeled segment of the real world. In database systems mapped tool to describe the semantics of the real world is part of the data model. The term to describe a set of abstract concepts allow the representation of certain features of this world. This set of concepts used to describe the properties of a specific portion of the real world, relevant to a given application is a database schema.
There are many different data models. Due to the level of abstraction can be split into two groups:
- Conceptual models - most similar to the method of analysis of the modeled reality, its objects and relationships between them, performed by the designer of the database. The most commonly used model is the model conceptual relationships and entities (entity, the entity called - a unit element). In this model, the basic concepts are entities, attributes, and relations.
- Models of Implementation, which are used to represent specific conceptual modeling of entities, attributes and relationships in a particular database system. The most currently used model of Implementation in commercial database systems (Oracle, Ingres, Sybase, Progress, Isis, Informix, dBase4 et al.) Is a relational model.
The advent of cheap and efficient graphical workstations has increased the interest in new applications of database systems in computer-aided design systems and management, computer aided software engineering (CASE method), office information systems and expert systems. These applications require much more complex than relational data structures. It is necessary to remember not only the information in the form of text, but the high resolution images, pictures, sounds, animation effects, etc. As a result 4 search for new data models emerged in the market s object-oriented database, any day will be published systems which used deductive data model.
The database is a collection of data elements organized in a logical and structured manner with. The current structure of the database depends on the data model adopted in the organization of these elements, its size depends on the number of data elements. The relationship between data elements affect the complexity of the database.
The characteristics of the databases are:
- The independence of applications and data - data can be entered into the database without having to modify them using the program or utility systems on the other hand, applications can be modified independently of the state databases.
- Abstract representation of data. Programs and utility systems (applications) are created by using the so-called. declarative programming languages (as opposed to imperative languages). The application developer does not have such an interest in the order of data in the database, or how their representation and search. Explicitly, the conditions on the selection information. The application developer decides so what to do and not how to do
- The variety of ways to view data. The same data from the database can be "seen" in different ways by different users. This effect is achieved by using different "filters" (perspective) imposed on the same data.
- Physical and logical data independence. Physical data independence lies in the fact that the extension of the computer system on which the DBMS runs on the new equipment does not affect data in the database. Logical data independence is the fact that - first, the introduction of new data into the database does not outdates the old, second - data that are not so interconnected. Integrited compounds can be removed from the database independently of each other
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