This document was produced for a course taught in the later half of 2000. It's six years old.
More up to date versions of this information, including some alternate media, are now available from this 2006 course page. That page includes
* A PDF document extending the material on this page on how to develop an entity relationship diagram
* Powerpoint slides and accompanying mp3 audio file giving a quick explanation of normalisation
* A PDF document that extends the how to develop an entity relationship diagram document and shows how to normalise the case.
Developing Entity Relationship Diagrams (ERDs)
WHY:
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.
INFORMATION:
Entity
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.
Relationship
A data relationship is a natural association that exists between one or more entities. E.g. Employees process payments. Cardinality 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.
Attribute

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 a primary key or identifier. E.g. Employee Number is a primary key for Employee.
AN ENTITY RELATIONSHIP DIAGRAM METHODOLOGY: (One way of doing it)
1. Identify Entities
Identify the roles, events, locations, tangible things or concepts about which the end-users want to store data.
2. Find Relationships
Find the natural associations between pairs of entities using a relationship matrix.
3. Draw Rough ERD
Put entities in rectangles and relationships on line segments connecting the entities.
4. Fill in Cardinality
Determine the number of occurrences of one entity for a single occurrence of the related entity.
5. Define Primary Keys
Identify the data attribute(s) that uniquely identify one and only one occurrence of each entity.
6. Draw Key-Based ERD
Eliminate Many-to-Many relationships and include primary and foreign keys in each entity.
7. Identify Attributes
Name the information details (fields) which are essential to the system under de...

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