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Transcript
Database management Objectives: At the end of the handout students should be able to: 1. Explain the terms Database management system and database 2. Define terms associated with a database (table, entity, tuple, attribute, primary key, secondary key, composite key, candidate key, alternate key, foreign key) 3. Distinguish among terminologies associated with files and databases (Row, column, key) 4. List at least four advantages and disadvantages of using a computerized database 5. Outline at least four steps that can be taken to design a database. Before the widespread use of computers, student information was kept on index cards in boxes or filing cabinets. If you and a small group of friends each put your personal details such as name, addresses and date of birth, on a record card, then it might not take too long to find those of you whose birthday is in April. However, if everyone in your school filled out a card, then it would take very much longer! Furthermore, changing information, such as addresses, could also take a long time. Other common “paper database” include telephone books, dictionaries, recipe cards and television guides. Nowadays, computerized databases are in widespread use, as they help people quickly to find information that they want. They also vary in size and use depending on what is required. Small databases, such as one that keeps information about a CD collection, can be run on a personal computer at home. Larger databases now play an important part in how our society works. Industrial, commercial and public organizations use databases to maintain their businesses and services. Other computerized databases include flight information systems and database systems in public libraries. Examples of how we use these large databases included: Booking holidays and airline tickets Using directory enquiries to search a database of millions of customers for a telephone number in a few seconds Accessing a police computer database, with requests from a police officers who want information about criminal suspects or stolen cars. What is a Database Management System (DBMS)? A DBMS is the term for programs that handle the storage, modification and retrieval of data, as well as controlling who has access to the information. E.g. Microsoft Access, Lotus Approach, FileMaker Pro and Corel Paradox. Functions of a DBMS: Data storage, retrieval and updates. The DBMS must allow its users to store, retrieve and update data. Backup and recovery. The DBMS should allow you to recover the most recent contents of the database in the event of system failure. Security. The DBMS must handle password allocation and checking, and allow access to the data that a user is authorized to use. Managing facilities for sharing a database. The DBMS has to ensure that no two users can access the same record at the time in order to modify it. What is a Database? A database is a part of a DBMS that is used for organizing and storing data in a useful and efficient manner on the computer. 1 Terms Associated with a Database: Table (File or relation): Collection of records that belong together Field (Column or attribute): the smallest piece of data that can be stored. Record (row or tuple): a row of data in a table that contains information about a particular individual item or entity. Entity: anything or person that has characteristics. e.g. A ball may be big, red and soft; A car has a make , model, colour and number of doors; A student has a Christian name, surname, gender and date of birth. Ball, Car and student are examples of an entity. Attribute: Characteristic of an entity. e.g. Make, sale price, colour are all examples of attributes. Primary key: A field whose values are unique so can be used be used to access each record individually. Candidate key: A field that is considered a possibility for becoming the primary key. However, only one field must be chosen as the primary key. Candidate keys are entirely optional, so a table may contain none, one, or several of them. Alternate key: Any candidate field that was not used as the primary key. Foreign key: a field in one table, but it is a primary key in another table. (Appears in a table where it doesn’t really belong but it enables two tables to be linked.) Composite key: a primary key that is made up of two or more fields. Advantages and disadvantages of a computerized database Advantages Disadvantages Can save enormous amounts of paper as well as filing space The computer(s) and peripherals required can cost a lot Data can easily be entered by keyboard or scanners If the computer, or computer network, is not working, then the database cannot be used Speed- data can be found, calculated and sorted very quickly Security is very important as some people may attempt to get access to confidential information. Sometimes this may involve illegally hacking into the program or data Data can easily be changed and updated The database file can become corrupted or infected by a computer virus. This can lead to file not working properly. In some cases, the database may not work at all. Making a back-up copy of the database is therefore essential There is often a limit to the size of a database file Data needs to be entered only once, yet can be represented in many different ways. A whole range of different queries and reports can be produced Data can be checked on entry Passwords can be set to allow access only to those with permission to use the database The data structure of a database can be changed, with new fields added, even after the database has been created. A paper-based system would have to be restarted from scratch Data can be imported and exported to other programs A database file can be automatically linked to others Databases can be shared with other users if the computer is a part of a local or wide area network. This includes the internet. Some databases can be complicated to use Data stored about people may be incorrect Some databases require much time to be spent on staff training, which can be costly 2 Panning and designing a database Planning a database is one of the most important steps in database management. It is critical that you plan before creating the files in which data will be stored. Each database should be set up for a specific purpose. Ask yourself the following questions when designing a database: What data do you want to store and what should it do? What questions will you ask of the data? What reports will you need to produce? How should the data be sorted and grouped? All of the information of an entity should be closely related. For example, in a videotape rental system, you might have one table for the name of tape, type of tape and its rating, one for customers which stores the customer name, address and telephone number, and another which stores data about the rentals. In this example there are three entities: the tape entity, the customer entity and the rental entity Reference Skeete, Kelvin, skeete, Kyle (2007). Information Technology for CSEC. United Kingdom: Cambridge University press Gay, Glenda, Blades, Ronald (2009). Oxford Information Technology for CSEC. Great Clarendon street, Oxford OX2 6DP: Oxford University press Campbell, Howard, Wood, Alan (2010). Information Technology for CSEC examinations. Between towns road, Oxford, Ox4 3PP: Macmillan Publishers Limited 3