What is a database?:
What is a database? A database is a collection of related data stored in a file or a set of related files.
It is made up of records and fields.
It is organised in a way that allows efficient searches and access.
It is typically used where large columns of data need to be stored.
Large databases may have millions of records.
The flat file approach:
The flat file approach A flat file consists of any number of records, each of which contains a set of fields.
A typical Video Shop database might store details in a flat file as follows:
The flat file approach works well on small files or where data items are unlikely to be repeated, for example, to store the results of a questionnaire or survey.
Relationships in data:
Relationships in data This example is unrealistic, as each customer is different and each video is different.
In a real situation:
one customer may rent several videos on the same night
different copies of the same video may be rented to different customers on the same night
one customer may rent the same video on more than one occasion.
A more realistic set of data:
A more realistic set of data Consider the same database with these more realistic transactions:
Problems with the flat file approach:
Problems with the flat file approach Data inconsistency: John Smith has rented different videos on different days yet there are two different addresses for him.
Data redundancy: As John Smith and Mary Jones have rented videos on more than one occasion, all their details are stored twice. The addresses and dates of birth are redundant on repeated records.
Inefficiency: Typing in all of the details every time a video is entered is time-consuming and takes up storage space. If John Smith changes address, all the records will need to be updated.
Difficult to analyse: As the number of rentals grows, the database will get bigger and more difficult to extract data from.
Program–data dependence:
Program–data dependence Different departments of a large organisation may use the data in different ways.
They may have separate programs to interpret the data.
Every time the data changes they will have to re-write the program to extract the data.
This is unproductive maintenance, as every department will be re-writing code.