Database Systems


The purpose of a database is to keep track of things (Kroenke, 2007). Databases can exist on paper, for example a telephone directory, but are ineicient and costly to maintain. A computer-based database ofers the advantage of powerful search facilities which can be used to locate and retrieve information many times faster than by manual methods. An electronic database provides facilities for users to add, amend or delete records as required. Indexing features mean that the same basic information can be stored under a number of diferent categories. his provides great lexibility and allows users to locate, retrieve and organise information as needed. Databases used throughout a company are usually accessed by many diferent users across a network system. Some of the advantages of this approach include minimising the unnecessary duplication of information, consistency is maintained by ensuring any changes made to the information held in the database are relected to all users and although information is held in a structured manner, the database sotware will normally provide suicient lexibility to meet the diferent requirements of individual users and departments.

The data in an electronic database is organised by ields and records. A ield is a single item of information, such as a name or a quantity. A record is a collection of related ields and a table is a collection of related records. In order to identify a speciic item of information within a database, all records must contain a unique identiier, normally called the key ield or primary key. he key ield usually takes the form of a number or code and will be diferent for each record in the database.

Relational databases enable data to be stored within a number of diferent tables and are the most widely used type of database. he tables within a relational database can be linked together using one or more record keys. his include the primary key and also other keys to help locate data stored in another table. he record keys contained in each table can used to establish one or more relationships between tables. By using record keys in combination it is possible to retrieve data from several tables at once. he ield used to locate information in another, related table is oten called a foreign key.

The majority of database programs support the creation of relational databases containing several linked tables. Many programs, such as Microsot Access, provide the ability to link tables together automatically to create any required relationships. All major database programs enable users to create and modify data entry forms. A data entry form provides a convenient means of viewing, entering, editing and deleting records. An index stores information concerning the order of the records in the database. All modern database programs provide a range of sophisticated security features. Examples of some of the most common features available including encryption and password protection. Finally all major database packages allow users to generate a wide variety of reports. Many programs are capable of creating simple reports automatically. In addition, many programs allow users to perform calculations and other actions as the report is produced.

When using database sotware data is retrieved from a database using what is called a query. A query enables a user to locate, sort, update or extract records from the database. Users design a query by specifying the conditions that must be met in order for a record to be selected. here are two types of query called selection queries and update queries: A selection query can be used to locate and display any records meeting a set of speciied conditions. None of the data held in the database are altered and any records not meeting the conditions set are simply hidden from view temporarily. An update query can be used to modify records in a variety of ways such as according to a set of conditions speciied by the user. Common actions performed by update queries include updating values held in ields, deleting any records no longer required, appending new records to the database and generating new tables containing selected records or summary information.

The majority of database programs make use of a special structured query language (SQL) in order to create queries. Structured query language (SQL) provides a standardised method for retrieving information from databases. Although traditionally used to manage large databases held on mainframes and minicomputers, it has become a widely used and popular tool for personal computer database packages. SQL programs are created by producing a series of statements containing special key words


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