9FE52635d01 - Developing Entity Relationship Diagrams...

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Developing Entity Relationship Diagrams (ERDs) Introduction 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 (http://infocom.cqu.edu.au/Courses/spr2000/95169/Extra_Examples/ERD.htm) and material used in Lecture 7 for COIS20025 in Term 2, 2006. Why ERDs? 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: 1. 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. 2. Relationship. A data relationship is a natural association that exists between one or more entities. E.g. Employees process payments. 3. 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. 4. 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. Figure 1 is a very simple, example ERD with each of the four components labelled.
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Figure 1 A simple, example ERD Different ERD styles As with many data modelling tools there are a number of different styles used to create ERDs. This web page (http://www.smartdraw.com/tutorials/software- erd/erdcardinality.htm) lists four different styles for caradinalities. The style used in this course will be the one labelled "Information Engineering". One Methodology for Developing an ERD Typically you will start with a case study or perhaps a logical model of the system to be developed. This document will demonstrate how to use the following process to convert that information into an ERD.
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9FE52635d01 - Developing Entity Relationship Diagrams...

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