MANAGING FILES: Basic Concepts A database is a logically organized collection of related data designed and built for a specific purpose,

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1 MANAGING FILES: Basic Concepts An electronic database is not just the computer-based version of what used to go into manila folders and filing cabinets. A database is a logically organized collection of related data designed and built for a specific purpose, a technology for pulling together facts that allows the slicing and dicing and mixing and matching of data in all kinds of ways. The data in a database has some inherent meaning. In other words, a random assortment of data cannot correctly be called a database. A database can be of any size and of any degree of complexity, and it can be maintained manually or by software on computers. This chapter, of course, covers computer databases. How Data Is Organized: The Data Storage Hierarchy Data in a database can be grouped into a hierarchy of categories, each increasingly more complex. The data storage hierarchy consists of the levels of data stored in a computer database: bits, characters (bytes), fields, records, and files. Bits: Computers, as we have said, are based on the principle that electricity may be on or off. Thus, the bit is the smallest unit of data the computer can store in a database represented by 0 for off or 1 for on. Characters: a character (byte) is a letter, number, or special character. A, B, C, 1, 2, 3, #, $, % are all examples of single characters. A combination of bits represents a character. Bits and bytes are the building blocks for representing data, whether it is being processed, telecommunicated, or stored in a database. The computer deals with the bits and bytes; you, however, will need to deal mostly with fields, records, and files. Field: a field is a unit of data consisting of one or more characters (bytes). It is the smallest unit of meaningful information in the database. Each field has a field name that describes the kind of data that should be entered into the field. An example of a field is your first name, or your street address, or your Social Security number. Fields can be designed to be a certain maximum length or a variable length, and they can also be designed to hold different types of data, such as text only numbers only, dates only, time, a yes or no answer only, web links only, or pictures, sound, or video. Record: a record is a collection of related fields. Each record stores data about only one entity, which can be a person, a place, a thing, an occurrence, or a phenomenon. An example of a record would be your name and address and Social Security number. File: a file is a collection of related records. An example of a file would be data collected on everyone employed in the same department of a company, including all names, addresses, and Social Security numbers. You use files a lot because the file is the collection of data or information that is treated as a unit by the computer. The file is at the top of the data hierarchy. A collection of related files forms the database. A company database might include files on all past and current employees in all 1

2 departments. There would be various files for each employee: payroll, retirement benefits, sales quotas and achievements (if in sales), and so on. Fig (1) how data is organized 2

3 Fig (2) how data is organized The Key Field An important concept in data organization is that of the key field. A key field is a field that is chosen to uniquely identify a record so that it can be easily retrieved and processed. The key field is often an identification number, Social Security number, customer account number, or the like or a combination of letters and numbers set up as a meaningful code. The primary characteristic of the key field is that it is unique. Thus, numbers are clearly preferable to names as key fields because there are many people with common names like James Johnson, Susan Williams, Ann Wong, or Roberto Sanchez, whose records might be confused. Student records are often identified by student ID numbers used as key fields. Most database management systems allow you to have more than one key field, so that you can sort records in different ways. One of the keys is designated the primary key and must hold a unique value for each record. A key field that identifies records in different tables is called a foreign key. Foreign keys are used to cross-reference data. 3

4 4 Types of Files: Program Files & Data Files As we said, the file is the collection of data or information that is treated as a unit by the computer. Files are given names filenames. If you re using a word processing program to write a psychology term paper, you might name it Psychreport. In a database, a filename might be AccountingPersonnel. Filenames also have extensions, or extension names, usually three letters added after a period following the filename. For example, the.doc in Psychreport.doc is recognized by Microsoft Word as a document. Extensions are usually inserted automatically by the application software. When you look up the filenames listed on your hard drive, you will notice a number of extensions, such as.doc,.exe, and.com. There are many kinds of files, but two principal ones are program files and data files. Program Files: For Software Instructions Program files are files containing software instructions. Examples are word processing or spreadsheet programs, which are made up of several different program files. The two most important are source program files and executable files. Source program files contain high-level computer instructions in the original form written by the programmer. Some source program files have the extension of the language in which they are written, such as.bas for BASIC,.pas for Pascal, or.jav for Java For the processor to use source program instructions, they must be translated into an executable file, which contains the instructions that tell the computer how to perform a particular task. You can identify an executable file by its extension,.exe or.com. You use an executable file by running it as when you select Microsoft Excel from your onscreen menu and run it. (There are some executable files that you cannot run other computer programs called runtime libraries cause them to execute. These are identified by such extensions as.dll,.drv, ocx,.sys, and. vbx. ) Data Files: For Holding Data Data files are files that contain data words, numbers, pictures, sounds, and so on. These are the files used in databases. Unlike program files, data files don t instruct the computer to do anything. Rather, data files are there to be acted on by program files. Examples of common extensions in data files are.txt (text) and.xls (Excel worksheets). Certain proprietary computer programs apply their own extensions, such as.ppt for PowerPoint and.mdb for Access. Three types of files worth particular attention are graphics, audio, and video files. Graphics files: Some important ones are.bmp,.tiff,.gif,.jpeg, and.png. Audio files: The ones you re most apt to encounter are.mp3,.wav, and.mid. Animation/video files: Common files are.qt,.mpg,.wmv,.avi., and.rm. DATABASE MANAGEMENT SYSTEMS As we ve said, a database is an organized collection of related (integrated) files. A database may be small, contained entirely within your own personal computer, or it may be massive, available through online connections. Such massive databases are of

5 5 particular interest because they offer phenomenal resource that until recently were unavailable to most ordinary computer users. In the 1950s, when commercial use of computers was just beginning, a large organization would have different files for different purposes. For example, a university might have one file for course grades, another for student records, another for tuition billing, and so on. In a corporation, people in the accounting, order-entry, and customer-service departments all had their own separate files. Thus, if an address had to be changed, for example, each file would have to be updated separately. The database files were stored on magnetic tape and had to be accessed in sequence in what was called a file-processing system. Later, magnetic-disk technology came along, allowing any file to be accessed randomly. This permitted the development of new technology and new software: the database management system. A database management system (DBMS), or database manager, is software written specifically to control the structure of a database and access to the data. In a DBMS, an address change need be entered only once, and the updated information is then available in any relevant file. (Strictly speaking, the database is the collection of the data, and the database management system is the software but many professionals use database to cover both meanings.) 408 The Benefits of Database Management Systems The advantages of database management systems are these: Reduced Data Redundancy Data redundancy, or repetition, means that the same data fields (a person s address, say) appear over and over again in different files and often in different formats. In the old file-processing system, separate files would repeat the same data, wasting storage space. In a database management system, the information appears just once, freeing up more storage capacity. In the old data storage systems, if one field needed to be updated, someone had to make sure that it was updated in all the places it appeared an invitation to error. Improved Data Integrity Data integrity means that data is accurate, consistent, and up to date. In the old system, when a change was made in one file, it might not have been made in other necessary files. The result was that some reports were produced with erroneous information. In a DBMS, reduced redundancy increases the chances of data integrity the chances that the data is accurate, consistent, and up to date because each updating change is made in only one place. Also, many DBMSs provide built-in check systems that help ensure the accuracy of the data that is input. The expression garbage in, garbage out (abbreviated GIGO) refers to the fact that a database with incorrect data cannot generate correct information. Increased Security Although various departments may share data, access to specific information can be limited to selected users. Thus, through the use of passwords, a student s financial, medical, and grade information in a university database is made available only to those who have a legitimate need to know.

6 Ease of Data Maintenance Database management systems offer standard procedures for adding, editing, and deleting records, as well as validation checks to ensure that the appropriate type of data is being entered properly and completely into each field type. Data backup utilities ensure availability of data in case of primary system failure. Three Database Components A database management system may have three components integrated into the software a data dictionary, DBMS utilities, and a report generator. Data Dictionary: For Defining Data Definitions & Structure: a data dictionary, also called a repository, is a document or file that stores the data definitions and descriptions of the structure of data used in the database. Data dictionaries contain no actual data from the database, only information for managing it. Without a data dictionary, however, a DBMS cannot access data from the database. The data dictionary defines the basic organization of the database and contains a list of all files in the database, the number of records in each file, and the names and types of each field. The data dictionary may also help protect the security of the database by indicating who has the right to access it. Most database management systems keep the data dictionary hidden from users to prevent them from accidentally destroying its contents. Utilities: For Maintaining the Database DBMS utilities are programs that allow you to maintain the database by creating, editing, and deleting data, records, and files. The utilities enable you to monitor the types of data being input and to sort your database by key fields, making searching and organizing information much easier. Report Generator: For Producing Documents A report generator is a program for producing an on-screen or printed document from all or part of a database. You can specify the format of the report in advance row headings, column headings, page headers, and so on. With a report generator, even non-experts can create attractive, readable reports on short notice. The Database Administrator Large databases are managed by a specialist called a database administrator. The database administrator (DBA) coordinates all related activities and needs for an organization s database, ensuring the database s recoverability, integrity, security, availability, reliability, and performance. Database administrators determine user access privileges; set standards, guidelines, and control procedures; assist in establishing priorities for requests; prioritize conflicting user needs; and develop user documentation and input procedures. They are also concerned with security establishing and monitoring ways to prevent unauthorized access and making sure data is backed up and recoverable should a failure occur and to establish and enforce policies about user privacy. 6

7 DATABASE MODELS What are five types of database models? Just as files can be organized in different ways, so can database management systems. Organizations may use one kind of DBMS for daily processing of transactions (such as sales figures) and then move the processed data into another DBMS that s better suited for random inquiries and analysis. Older DBMS models, introduced in the 1960s, are hierarchical and network. New models are relational, object-oriented, and multidimensional. Hierarchical Database In a hierarchical database, fields or records are arranged in related groups resembling a family tree, with child (lower-level) records subordinate to parent (higher-level) records. The parent record at the top of the database is called the root record or root parent. The hierarchical database is the oldest and simplest of the five models. It lent itself well to the tape storage systems used by mainframes in the 1970s. It is still used in some types of passenger reservation systems. In hierarchical databases, accessing or updating data is very fast, because the relationships have been predefined. However, because the structure must be defined in advance, it is quite rigid. There can be only one parent per child, and no relationships among the child records are possible. Moreover, adding new fields to database records requires that the entire database be redefined. A new database model was needed to address the problems of data redundancy and complex data relationships. Fig (3) Hierarchical database, Example of a cruise ship reservation system 7

8 Network Database The network database was in part developed to solve some of the problems of the hierarchical database model. A network database is similar to a hierarchical database, but each child record can have more than one parent record. Thus, a child record, which in network database terminology is called a member, may be reached through more than one parent, which is called an owner. Also used principally with mainframes, the network database is more flexible than the hierarchical arrangement, because different relationships may be established between different branches of data. However, it still requires that the structure be defined in advance, and, as with the hierarchical model, the user must be familiar with the structure of the database. Moreover, there are limits to the number of possible links among records, and to examine a field one must retrieve the entire record. Although the network database was an improvement over the hierarchical database, some people in the database community believed there had to be a better way to manage large amounts of data. Fig (4) Network Database Example of college class-scheduling system 8