Databases and Database Management Systems

A database is a collection of related data or facts arranged in a specific structure. A database management system (DBMS) is a program, or collection of programs, that allows multiple users to store, access, and process data or facts into useful information.

Three of the most important terms to know about databases are a table, a record, a field. Data is stored in tables. A table is divided into records (unnamed rows), and each record is divided into fields (named columns). The table consists of a set number of fields and an arbitrary number of records. For a record to exist, it must have data in at least one field.

To help you understand how a database stores data, think about a typical address book. Each piece of information in the address book is stored in its own location, called a field. For example, each entry has a field for First Name and another field for Last Name, as well as fields for Address, City, State, ZIP Code, and Phone Number. Each unique type of information is stored in its own field. One full set of fields – that is, all the related information about one person or object – is called a record. Therefore, all the information for the first person is record 1, all the information for the second person is record 2, and so on.

A complete collection of records makes a table. Once you have a structure for storing data (whether it is a printed address book, phone book, or electronic table), you can enter and view data, create reports, and perform other tasks with the data. For example, you may create a customer report that lists customers by ZIP Code.

A DBMS provides tools to perform data management functions: creating tables, sorting tables, entering and editing data, querying the database, viewing data, generating reports.

Many different DBMS programs are available. Enterprise-level products, such as Oracle, DB2, and Sybase, are designed to manage large special-purpose database systems. Programs such as Microsoft Access, Corel's Paradox, and Lotus Approach are popular among individual and small-business database users.

Database Structures

There are several types of database structures, such as flat-file, relational, hierarchical, network and object-orientedones. A database that consists of a single data table is called a flat-file (sequential file) database. Flat-file databases are useful for certain single-user or small-group situations, especially for maintaining address lists or inventories. Flat-file database systems are easy to learn and use, but difficult to maintain and limited in their power. When numerous files exist (one for each table or related document), there is often a lot of data redundancy, which increases the chance for errors, wastes time, and uses excess storage space.

A relational database is made up of a set of tables, and a common field existing in any two tables creates a relationship between the tables. For example, a Customer ID Number field in both the Customers table and the Orders table links the two tables, while a Product ID field links the Orders and Products tables. The relational database structure is widely used in today's business organizations. In a business, a typical relational database contains such data tables, as Customer information, Employee information, Vendor information, Order information, Inventory information.

The hierarchical database is an older style of database. The tables are organized into a fixed treelike structure, with each table storing one type of data. The trunk table (the main table) stores general information. Any field in that table may reference another table that contains subdivisions of data. Each one of those tables may, in turn, reference other tables that store finer subdivisions of data. The relationship between tables is said to be a parent-child relationship, or one-to many relationship, with any child table relating to only one parent table. Each parent table may have many child tables, but each child has only one parent. Hierarchical databases require little duplicated data and may locate data quickly. However, the tables' fixed relationships limit the flexibility of the database, making some kinds of queries or reports difficult or impossible.

The network database model is similar to the hierarchical structure except that any one table can relate to any number of other tables. The network database's tables, therefore, are said to have a many-to-many relationship. Like the hierarchical structure, the network database is used in older (primarily mainframe) systems.

The object-oriented database (OODB) developed in the late 1980s, groups data items into complex items called objects. These objects can represent anything: a product, an event, a customer complaint, or even a purchase. An object is defined by its characteristics (e.g. text, sound, graphics, video), attributes (e.g. color, size, style, quantity, price), and procedures (the processing associated with an object).

Network Structures

 

A network is a way to connect computers for communication, information exchange, and resource sharing. The four most important benefits of networking are simultaneous access to programs and data, peripheral sharing, streamlined communications, and easier backups. E-mail, videoconferencing, and teleconferencing are examples of the personal communications that can be conducted over a network or the Internet.

Networks can be categorized in different ways, such as by geography (how much terrain they cover) or by the use or absence of a central server.

A local area network (LAN) consists of computers that are relatively near one another. A LAN can have a few PCs or hundreds of them in a single building or in several buildings. On a network, data is broken into small groups called packets before being transmitted from one computer to another. A packet is a data segment that includes a header, payload, and control elements that are transmitted together. The receiving computer reconstructs the packet into the original structure. The payload is the part of the packet that contains the actual data being sent. The header contains information about the type of data in the payload, the source and destination of the data, and a sequence number so that data from multiple packets can be reassembled at the receiving computer in the proper order. Each LAN is governed by a protocol, which is a set of rules and formats for sending and receiving data. TCP/IP, IPX/SPX, and NetBEUI are examples of network protocols.

LANs can be connected by a bridge or router to create a much larger network that covers a larger geographic area. To connect LANs, a gateway may be required to enable them to share data in a way that the different LANs can understand.

A wide area network (WAN) is the result of connecting LANs through public utilities.

Many networks are built around a central server. The PCs that connect to the server are called nodes. In a file server network, each node has access to the files on the server but not necessarily to files on other nodes. In a client/server network, nodes and the server share the storage and processing workload.

A peer-to-peer network is a small network that usually does not include a central server. In a peer-to-peer network, users can share files and resources on all the network's nodes.

 

Network Topologies

 

A topology is the physical layout of the cables and devices that connect the nodes of a network. The three basic topologies are bus, star, and ring. These topologies are so named because of the shape of the network they create. The less common type of physical topology is the mesh topology. The bus network uses a single conduit to which all the network nodes and peripheral devices are attached. Each node is connected in series to a single cable. At the cable's start and end points, a special device called a terminator is attached. A terminator stops the network signals so they do not bounce back down the cable.

The star network is the most common topology in use today. In a star network, a device called a hub is placed in the center of the network; that is, all nodes are connected to the central hub and communicate through it. Groups of data are routed through the hub and sent to all the attached nodes, thus eventually reaching their destinations. Some hubs known as intelligent hubs can monitor traffic and help prevent collisions. In a star topology, a broken connection (between a node and the hub) does not affect the rest of the network. If you lose the hub, however, all nodes connected to that hub are unable to communicate.

The ring topology connects the nodes of the network in a circular chain, with each node connected to the next. The final node in the chain connects to the first to complete the ring. With this methodology, each node examines data sent through the ring. If the data known as a token is not addressed to the node examining it, that node passes it along to the next node in the ring.

The mesh topology is the least used network topology and the most expensive to implement. In a mesh environment, a cable runs from every computer to every other computer. If you have four computers, you must have six cables – three coming from each computer to the other computers. The big advantage to this arrangement is that data can never fail to be delivered; if one connection goes down, there are other ways to route the data to its destination. The mesh topology is used for connecting routers on the Internet to make sure that data always gets through.

Internet

How the Internet Works

 

The Internet was created for the U.S. Department of Defense as a tool for communications. Today, the Internet is a network of interconnected networks. It is a huge, cooperative community with no central ownership. The Internet connects thousands of networks and more than 100 million users around the world. The Internet carries messages, documents, programs, and data files that contain every imaginable kind of information for businesses, educational institutions, government agencies, and individuals.

All computers on the Internet use TCP/IP protocols. Any computer on the Internet can connect to any other computer. Individual computers connect to local and regional networks, which are connected together through the Internet backbone. A computer can connect directly to the Internet, or as a remote terminal on another computer, or through a gateway from a network that does not use TCP/IP. Every computer on the Internet has a unique four-part numeric IP address, and most also have an address that uses the domain name system (DNS). DNS addresses have two parts: a host name (a name for a computer connected to the Internet) followed by a domain that generally identifies the type of institution that uses the address. This type of domain is often called top-level domain. For example, many companies have a DNS address whose first part is the company name followed by ".com" - ibm.com (International Business Machines Corp.). Some large institutions and corporations divide their domain addresses into smaller subdomains. For example, a business with many branches might have a subdomain for each office - such as boston.widgets.com and newyork.widgets.com. In 1996 a new set of top-level domain names was created because it was difficult for organizations to find suitable domain names for their Internet sites, for example .firm (businesses or firms), .shop (business that offer items for purchase over the Internet),.arts (organizations promoting artistic or entertainment activities over the Internet. Geographic domains usually identify the country in which the system is located, such as .ca for Canada or .fr for France.

The Internet has a lot of uses:

- Inexpensive electronic mail systems enable you to exchange messages with any other user anywhere.

- TELNET allows a user to operate a second computer from his or her machine.

- File Transfer Protocol (FTP) is the Internet tool for copying data and program files from one computer to another.

- News and mailing lists are public conferences distributed through the Internet and other electronic networks.

- Chats are public conferences, conducted in real time, where people discuss topics of interest.

 

Part II.

The World Wide Web

 

One part of the Internet is the World Wide Web. It was created in 1989 at the European Particle Physics Laboratory in Geneva, Switzerland as a method for incorporating footnotes, figures, and cross-references into online hypertext documents. A hypertext document, or a Web page, is a specially encoded file that uses the hypertext markup language (HTML). This language allows a document's author to embed hypertext links, or hyperlinks, in the document. A collection of related Web pages is called a Web site. Web sites are housed on Web servers, Internet host computers that often store thousands of individual pages. Copying a page onto a server is called posting the page. Downloading a page from the Web server to your computer for viewing is commonly called "hitting" the Web site. Many Web pages feature a hit counter to display the number of times the page has been viewed.

A Web browser is a software application designed to find hypertext documents on the Web and then open the documents on the user's computer. A point-and-click browser provides a graphical user interface (GUI) that enables the user to click graphical objects and hyperlinks. The most popular browsers are Microsoft's Internet Explorer and Netscape Navigator. A Web browser displays a Web page as specified by the page's underlying HTML code. To format a document in HTML, a designer places HTML tags throughout the document. The HTML tags enclosed in angle brackets (<>), tell the browser how to display individual elements on the page. The internal structure of the World Wide Web is built on a set of rules called Hypertext transfer protocol (HTTP). HTTP uses Internet addresses in a special format, called a Uniform Resource Locator (URL). URLs look like this: type://address/path/. In a URL, type specifies the type of the server in which the file is located, address is the address of the server, and path is the location within the file structure of the server. The path includes the list of folders (or directories) where the desired file is located.

You probably have heard the term home page used to reference a page named index.htm on a Web site. This term is important and actually has two meanings:

Personalized Start Page. On your computer, you can choose a Web page that opens immediately when you launch your Web browser by using a command in your browser to specify the URL for the desired page. This personalized start page can be on your computer's hard drive or a page from any Web site. For example, if you want to see today's copy of USA Today Online when you launch your browser, use the address http://www.usatoday.com/ as your personal home page.

Web Site Home Page. A Web site's primary page is also called its home page. This page is the first one you see when you type the site's basic URL. From this page, you can navigate to other pages on the Web site (and possibly to other sites). For example, if you type the URL http://www.cnn.com/ into your browser's address box, the CNN home page opens in your browser window.

 

Development of Information Systems

 

An information system (IS) is a mechanism that helps people collect, store, organize, and use information. An information system includes a means of storing information, a set of procedures for handling information, and rules that govern the delivery of information to people in an organization. Traditionally, information systems were manual. One popular type of manual information system is the card-based system, such as a card catalog in a library. Because there are so many types of information and uses for it many kinds of information systems have been developed: office automation systems, transaction processing systems, management information systems, decision support systems, expert systems.

Office automation systems automate routine office tasks, such as word processing, accounting, document management or communications. Transaction processing systems not only track and store information about individual events but also provide information that is useful in running an organization, such as inventory status, billing, and so on. Management information systems produce reports for different types of managers. Decision support systems is a specialized application used to collect and report certain types of business data which can aid managers in the decision-making process. Expert systems include the knowledge of human experts in a particular area (such as medicine or technology) in a knowledge base. They analyze requests from users and assist the users in developing a course of action.

A well-structured IS department includes IS managers, computer scientists, systems analysts, programmers, database specialists, user assistance architects, technical writers, system or network managers, trainers, and hardware maintenance technicians. IS professionals support an organization's information, provide technical support for hardware and software and are involved in the design and implementation of an organization's entire information system.








Дата добавления: 2015-11-18; просмотров: 1782; ЗАКАЗАТЬ НАПИСАНИЕ РАБОТЫ


Поиск по сайту:

При помощи поиска вы сможете найти нужную вам информацию, введите в поисковое поле ключевые слова и изучайте нужную вам информацию.

Поделитесь с друзьями:

Если вам понравился данный ресурс вы можете рассказать о нем друзьям. Сделать это можно через соц. кнопки выше.
helpiks.org - Хелпикс.Орг - 2014-2022 год. Материал сайта представляется для ознакомительного и учебного использования. | Поддержка
Генерация страницы за: 0.014 сек.