Database

Types of Database

The two main types of computerised database are flat file and relational.

Flat file databases

A flat file database is like an electronic card index file. In this type of database, the data is stored in a single table. In the case of the DVD rentals system, the table would store data about both members and DVDs.

DVD Code	Title	Cost	Date Out	Date In	Member Number	Name	Telephone Number
002	Finding Nemo	£2.50	03/09/04	04/09/04	1034	John Silver	142536
003	American Pie	£2.50	27/08/04	28/08/04	1056	Fred Flinstone	817263
003	American Pie	£2.50	01/09/04	02/09/04	1012	Isobel Ringer	293847
008	The Pianist	£2.50	04/09/04	06/09/04	1097	Annette Kirton	384756
011	Notting Hill	£2.50	27/08/04	28/08/04	1012	Isobel Ringe	293847
014	Prime Suspect	£2.00	27/08/04	28/08/04	1097	Annette Kirkton	384756
015	Shrek	£1.50	10/09/04	11/09/04	1034	Joan Silver	142536

The flat file database contains one record for each DVD rental. This is because flat file databases only allow a single value to be stored in each column

There are a number of problems with flat file databases.

• Data is very likely to be duplicated. For example, because a new record would be created for each DVD rented, this means that the member details would have to be copied onto each new record.

• The duplication of data leads to the possibility of data inconsistency. This happens when what should be the same data, stored on two separate records, differs. Usually this is the result of human error in copying the data, e.g. entering the telephone number for John Silver as 142563.  It then becomes difficult or impossible to know which record contains the correct version of the data.

• It is not possible to store information about a member without entering details of a DVD. This is called an insertion anomaly.

• Removing a DVD from the database may remove the only record that stores details of a member. This is called a deletion anomaly.

In fact, a flat file database is not much of an improvement on the manual system!

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