Topic 11. Databases

11.1. The concept of a database. Database management systems

The word "data" is defined as a dialectical component of information in the form of registered signals. Data registration can be carried out by any physical method (mechanical movement of physical bodies, change in their shape or surface quality parameters, change in electrical, magnetic, optical characteristics, chemical composition or nature of chemical bonds, change in the state of the electronic system, etc.). Initially, the following data types were used when creating databases:

1) numerical (for example, 17; 0,27; 2E-7);

2) character or alphanumeric (in particular, "ceiling", "table");

3) dates that are specified using the special type "Date" or as ordinary character data (for example, 12.02.2005/12/02, 2005/XNUMX/XNUMX).

Other data types were later defined, including:

1) temporary and date-time, which are used to store information about time and / or date (for example, 5.02.2005/7/27, 04:23.02.2005:16, 00/XNUMX/XNUMX XNUMX:XNUMX);

2) character data of variable length, designed to store textual information of great length;

3) binary, which are used to store graphic objects, audio and video information, spatial, chronological and other special information;

4) hyperlinks that allow you to store links to various resources located outside the database.

A database is a set of interrelated data stored in a computer memory in a certain way to display the structure of objects and their relationships in the subject area under study. It is the main form of organization of data storage in information systems.

A database management system is a set of symbolic and software tools designed to create, maintain and organize shared access to databases for multiple users.

The first DBMS were developed by IBM - IMS (1968) and Software AG-ADABA- (1969). At the moment, there are a large number of different database management systems (more than several thousand), and their number is constantly growing.

Among the main functions of the DBMS (higher-level functions), one can single out the storage, modification and processing of information, as well as the development and receipt of various output documents.

The functions of the DBMS of a lower level include:

1) data management in external memory;

2) OP buffer management;

3) transaction management;

4) keeping a log of changes in the database;

5) ensuring the integrity and security of databases.

11.2. Hierarchical, network and relational data representation models

The information in the database is structured in some way, that is, it can be described by a data representation model (data model) that are supported by the DBMS. These models are divided into hierarchical, network and relational.

When using a hierarchical data representation model, relationships between data can be characterized using an ordered graph (or tree). In programming, when describing the structure of a hierarchical database, the "tree" data type is used.

The main advantages of the hierarchical data model are:

1) efficient use of computer memory;

2) high speed of performing basic operations on data;

3) convenience of working with hierarchically ordered information.

The disadvantages of a hierarchical data representation model include:

1) the cumbersomeness of such a model for processing information with rather complex logical connections;

2) the difficulty in understanding its operation by a common user.

A small number of DBMSs are built on a hierarchical data model.

The network model can be represented as a development and generalization of a hierarchical data model that allows displaying various data relationships in the form of an arbitrary graph.

The advantages of the network data presentation model are:

1) efficiency in the use of computer memory;

2) high speed of performing basic operations on data;

3) huge opportunities (larger than the hierarchical model) for the formation of arbitrary connections.

The disadvantages of the network data presentation model include:

1) high complexity and rigidity of the database schema, which is built on its basis;

2) difficulty for understanding and performing information processing in the database by a non-professional user.

Database management systems built on the basis of the network model are also not widely used in practice.

The relational model of data presentation was developed by an employee of the 1WME company. Codd. His model is based on the concept of "relationship". The simplest example of a relationship is a two-dimensional table.

The advantages of the relational data representation model (compared to the hierarchical and network models) are its clarity, simplicity and convenience in the practical implementation of relational databases on a computer.

The disadvantages of the relational data representation model include:

1) lack of standard means of identifying individual records;

2) the complexity of describing hierarchical and network relationships.

Most of the DBMS used by both professional and non-professional users are built on the basis of a relational data model (Visual FoxPro and Access from Microsoft, Oracle from Oracle, etc.).

11.3. Post-relational, multidimensional and object-oriented data representation models

The post-relational data representation model is an extended version of the relational data model and allows you to eliminate the limitation of the indivisibility of data stored in table records. That is why data storage in a postrelational model is considered more efficient than a relational one.

The advantage of the post-relational model is that it makes it possible to form a set of related relational tables through one post-relational table, which ensures high visibility of information presentation and efficiency of its processing.

The disadvantage of this model lies in the complexity of solving the problem of ensuring the integrity and consistency of the stored data.

Examples of postrelational DBMSs are UniVers, Budda and Dasdb.

In 1993, an article by E. Codd was published, in which he formulated 12 basic requirements for OLAP (On-line Analytical Processing) class systems. The main principles described were related to the possibilities of conceptual representation and processing of multidimensional data. This moment became the starting point for the growth of interest in multidimensional data representation models.

Multidimensional models are highly specialized DBMS that are used for interactive analytical processing of information. Multidimensional data organization is more visual and informative in comparison with the relational model.

The main disadvantage of a multidimensional data model is its cumbersomeness for solving the simplest problems of ordinary online information processing.

Examples of DBMS based on such models are Arbor Software's Ess-base, Oracle's Oracle Express Server, etc.

Object-oriented data representation models allow you to identify individual database records. Certain relationships are formed between database records and their processing functions using mechanisms similar to the corresponding facilities in object-oriented programming languages.

The advantages of an object-oriented data model are:

1) the ability to display information about the complex relationships of objects;

2) the ability to identify a single database record and determine the function of its processing.

The disadvantages of the object-oriented data model include:

1) difficulty in understanding its activities by a non-professional user;

2) inconvenience of data processing;

3) low speed of query execution.

Among the object-oriented DBMS, we can distinguish the ROET systems from ROET Software, Versant from Versant Technologies, etc.

11.4. Classifications of database management systems

Any software product capable of supporting the processes of designing, administering and using a database can fall under the definition of a DBMS, so a classification of DBMS by types of programs was developed:

1) full-featured - the most numerous and powerful programs in terms of their capabilities, such as Microsoft Access, Microsoft FoxPro, Clarion Database Developer, etc .;

2) database servers - are used to organize data processing centers in computer networks. Among them are Microsoft SQL Server, NetWare SQL by Novell;

3) database clients - various programs (full-functional DBMS, spreadsheets, word processors, etc.) that provide greater performance of the computer network if the client and server parts of the database are produced by the same company, but this condition is not mandatory;

4) tools for developing programs for working with databases - designed to develop such software products as client programs, database servers and their individual applications, as well as user applications. Programming systems, program libraries for various programming languages, and development automation packages serve as tools for developing custom applications. The most commonly used custom application development tools are Borland's Delphi and Microsoft's Visual Basic.

By type of application, DBMS are divided into personal and multi-user.

Personal DBMS (for example, Visual FoxPro, Paradox, Access) are used in the design of personal databases and low-cost applications that work with them, which, in turn, can be used as a client part of a multi-user DBMS.

Multiuser DBMS (for example, Oracle and Informix) consist of a database server and a client part and are able to work with various types of computers and operating systems of various manufacturers.

Most often, information systems are built on the basis of a client-server architecture, which includes a computer network and a distributed database. A computer network is used to organize scientific work on a PC and in networks. A distributed database consists of a multi-user database located on a server computer and a personal database located on workstations. The database server does the bulk of the data processing.

11.5. Database access languages

There are two types of database access languages:

1) data description language - a high-level language designed to describe the logical structure of data;

2) data manipulation language - a set of structures that ensure the implementation of basic operations for working with data: input, modification and selection of data by request.

The most common access languages ​​are the two standardized languages:

1) QBE (Query by Example) - a sample query language characterized by the properties of a data manipulation language;

2) SQL (Structured Query Language) - a structured query language, consisting of the properties of languages ​​of both types.

The QBE language was developed on the basis of relational calculus with domain variables. It helps to form complex queries to the database by filling in the request form offered by the database management system. Any relational DBMS has its own version of the QBE language. The advantages of this method of setting queries to the database are:

1) high visibility;

2) no need to specify the algorithm for performing the operation.

The Structured Query Language (SQL) is based on relational calculus with variable tuples. Several standards for this language have been developed, the most famous of which are SQL-89 and SQL-92. The SQL language is used to perform operations on tables and the data contained in these tables, and some related operations. It is not used as a separate language and is most often part of the built-in programming language of the DBMS (for example, FoxPro DBMS Visual FoxPro, ObjectPAL DBMS Paradox, Visual Basic for Applications DBMS Access).

The SQL language is focused only on data access, therefore it is classified as a program development tool and is called built-in. There are two main methods for using embedded SQL:

1) static - characterized by the fact that the program text contains calls to SQL language functions that are rigidly included in the executable module after compilation. Changes in called functions can be made at the level of individual call parameters using programming language variables;

2) dynamic - differs in the dynamic construction of SQL function calls and the interpretation of these calls during program execution. It is most often used in cases where the type of SQL call in the application is not known in advance, and it is built in a dialogue with the user.

11.6. Databases on the Internet

The basis for publishing databases on the World Wide Web is the simple arrangement of information from databases on Web pages of the network.

The publication of databases on the Internet is designed to solve a number of problems, among which are the following:

1) organizing the interconnection of database management systems that operate on different platforms;

2) building information systems on the Internet based on a multi-level database architecture;

3) building local Intranet networks using technologies for publishing databases on the Internet;

4) application in the Internet of information from available local network databases;

5) use of databases to organize information presented on the Internet;

6) using a Web browser as an accessible client program for accessing databases on the Internet.

To publish databases on Web pages, two main methods are used to generate Web pages containing information from databases:

1) static publication - Web pages are created and stored on a Web server until a user request is received to receive them (in the form of files on a hard drive in the format of a Web document). This method is used when publishing information that is rarely updated in the database. The main advantages of such an organization for publishing databases on the Internet are accelerated access to Web documents that contain information from databases, and reducing the load on the server when processing client requests;

2) dynamic publication - Web pages are created when a user request arrives at the server. The server sends a request to generate such pages to a program - an extension of the server that generates the required document. The server then sends the completed Web pages back to the browser. This method of generating Web pages is used when the contents of the database are updated frequently, such as in real time. This method publishes information from databases for online stores and information systems. Dynamic pages are formed using various tools and technologies, such as ASP (Active Server Page - active server page), PHP (Personal Home Page tools - personal home page tools).

Among the software tools that allow you to get information from the Internet, Web applications (Internet applications) stand out, which are a set of Web pages, scripts and other software tools located on one or more computers and designed to perform an applied task. Applications that publish databases on the Internet are classified as a separate class of Web applications.

Literature

1. Informatics: Basic course: a textbook for students / ed. S. V. Simonovich. St. Petersburg: Peter, 2002.

2. Levin A. Sh. Self-instruction manual for working on a computer / A. Sh. Levin. 8th ed. St. Petersburg: Peter, 2004.

3. Leontiev V.P. The latest encyclopedia of a personal computer 2003 / V.P. Leontiev. M.: OLMA-Press, 2003.

4. Mogilev A. V. Informatics: textbook. allowance for students / A. V. Mogilev, N. I. Pak, E. K. Khenner; ed. E. K. Henner. M.: Academy, 2001.

5. Murakhovsky V. I. Hardware of a personal computer: A practical guide / V. I. Murakhovsky, G. A. Evseev. M.: DESS COM, 2001.

6. Olifer VG Computer networks. Principles, technologies, protocols: a textbook for students / V. G. Olifer, N. A. Olifer. St. Petersburg: Peter, 2001.

Author: Kozlova I.S.

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