Developing Entity Relationship Diagrams (ERDs)
This document seeks to give expanded explanation and examples of how to produce
entity relationship diagrams.
It is based on material adapted from a previous CQU course web page
material used in Lecture 7 for COIS20025 in Term 2, 2006.
Entity Relationship Diagrams are a major data modelling tool and will help organize the
data in your project into entities and define the relationships between the entities. This
process has proved to enable the analyst to produce a good database structure so that the
data can be stored and retrieved in a most efficient manner.
By using a graphical format it may help communication about the design between the
designer and the user and the designer and the people who will implement it.
Components of an ERD
An ERD typically consists of four different graphical components:
A data entity is anything real or abstract about which we want to store data. Entity
types fall into five classes: roles, events, locations, tangible things or concepts.
E.g. employee, payment, campus, book. Specific examples of an entity are called
instances. E.g. the employee John Jones, Mary Smith's payment, etc.
A data relationship is a natural association that exists between one or more
entities. E.g. Employees process payments.
Defines the number of occurrences of one entity for a single occurrence of the
related entity. E.g. an employee may process many payments but might not
process any payments depending on the nature of her job.
A data attribute is a characteristic common to all or most instances of a particular
entity. Synonyms include property, data element, field. E.g. Name, address,
Employee Number, pay rate are all attributes of the entity employee. An attribute
or combination of attributes that uniquely identifies one and only one instance of
an entity is called