Concepts of Database Systems
Database system
Concepts of database management-3rd edition Philip j.pratt .2000
A database is a structure that contains information about many different categories of information and about the relationships between the categories. Premiere products are a distributer of appliances, housewares. The company has used spread sheet software to maintain customer, order, inventory and sales. The premiere products database, for example, will contain information about sales, reps, customers, orders and parts. It will also provide facts that relate sales reps to the customers they represent and customers to the orders they currently have placed.
With the use of database, employees can enter the number of a particular order and find out which customer placed the orders, as well as which parts were ordered.
An entity is a person place object event of idea for which you want to store and process data. An attribute is a characteristic or property of an entity. For the entity for example the list of attributes might include such things as car colour and size.
Rep.
|
Rep. no. |
Last name |
First name |
Street |
City |
State |
Zip |
Commission |
Rate |
model |
Attributes
An attribute is also called a field or column in many database systems.
A relationship is an association between entities. There is an association between reps and customers, for example at premiere products. A rep is associated with all of his or her customers and a customer is associated with its rep.
One-to-many relationship
Each rep is associated with many customers, but each customer is associated with only one rep. in this type of relationship, the word many is used differently than in everyday English; it might not always indicate a large a large number. In this context, for example, the many means that a rep can be associated with any number of customers. This is, a given rep can be associated with zero, one, Or more customers.
A database is a structure that can store information about multiple types of entities, the attributes of those entities, and the relationship among the entities. A database, however, is much more than a file. Unlike a typical data file, a database can store information about multiple entities. There is also another difference. A database holds information about the relationship among the various entities. Not only will premiere products database have information about reps and customers, it also will hold information relating reps to the customers they service, customers to orders, parts to orders and so on.
REDUNDANCY
It is the duplication of data or the storing of the same data in more than one place. In the orders spreadsheet, redundancy occurs in the customer name column because the name of a customer is stored in more than one place.
What problems does redundancy cause?
It wastes space because you are storing the same data in multiple places. The extra space results in larger spreadsheet that require more space in memory and on disk and that take longer to save and open.
When you need to change data, redundancy also makes your changes more difficult and time consuming.
Finally redundancy can lead to conflicts. For eg. If you enter someone’s name and address in different columns than it will take time to enter and there is chance of making mistakes.
