Database

Data Modelling

Entities and data relationships

Previously we introduced Ali’s Mini Market and the DVD rentals system. Ali recorded information about each DVD rental (its code, title, cost, date out, date due, member number and name) and each member (member number, name and telephone number).

In database systems, ‘Member’ and ‘DVD rental’ are called entities.

An entity represents a person or object. 

Each entity has a set of attributes that describe examples or instances of that entity.

The attributes of the DVD rental entity are: Code, Title, Cost, Date Out, Date Due and Member Number; the attributes of the member entity are: Member Number, Name and Telephone Number.

In a manual system, attributes may contain a list of values. For example, in Ali’s original DVD rentals list the attributes Date Out, Date Due, Member Number and Name had more than one entry for DVDs 003 and 011. These are called multi-valued attributes.

In a computerised database, single-valued attributes are used. The flat file database demonstrates how these multiple values are turned into single values.

Entities, Attributes and Instances

MEMBER
Member No.	Member Name	Telephone No.
1012	Isobel Ringer	293847
1034	John Silver	142536
1056	Fred Flintstone	817263
1097	Anette Kirkton	384756

 

Each row stores one instance eg member 1034

The entity and its attributes are written as:

MEMBER

• (Member Number,
• Name,
• Telephone Number)

DVD RENTAL

• (DVD Code,
• Title, Cost,
• Date Out,
• Date Due,
• Member Number)

Note that the entity name is usually written in CAPITALS and in the singular rather than the plural form, i.e. MEMBER, not MEMBERS. The attributes are listed in the brackets in lower case letters with initial capitals. It is easier to write the attributes one underneath the other as shown above.

Data modelling is the process of defining the entities, attributes and relationships that are required. The result is called a data model.