lecture-11-designing-er-diagrams

lecture-11-designing-er-diagrams - Design Constraints...

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Unformatted text preview: Design Constraints Entity-Relationship Models: Good Design and Constraints T. M. Murali September 29, 2010 T. M. Murali September 29, 2010 CS 4604: E/R Design and Constraints Design Constraints Guidelines I Be faithful to the specification of the application. I Avoid redundancy. I Keep the entities and relationship simple. I Select the right relationships. I Select the right type of element. T. M. Murali September 29, 2010 CS 4604: E/R Design and Constraints Design Constraints Be Faithful I Do not use meaningless or unnecessary attributes. I Define the multiplicity of a relationship appropriately. I What is the multiplicity of the relationship Take between Students and Courses ? I What is the multiplicity of the relationship Teach between Professors and Courses ? T. M. Murali September 29, 2010 CS 4604: E/R Design and Constraints Design Constraints Avoid Redundancy I Redundancy occurs when we express the same fact in two or more ways. I Redundancy wastes space. I Redundancy can lead to inconsistency if we change one instance but not the other. T. M. Murali September 29, 2010 CS 4604: E/R Design and Constraints Design Constraints Select the Right Relationships I Do not add unnecessary relationships. I It may be possible to deduce one relationship from another. T. M. Murali September 29, 2010 CS 4604: E/R Design and Constraints Design Constraints Select the Right Relationships I Do not add unnecessary relationships. I It may be possible to deduce one relationship from another. I Do we need the relationship Instruct between Professors and Students ? Students Courses Professors Take Teach Instruct T. M. Murali September 29, 2010 CS 4604: E/R Design and Constraints Design Constraints Select the Right Relationships I Do not add unnecessary relationships. I It may be possible to deduce one relationship from another. I Do we need the relationship Instruct between Professors and Students ? I No. We can deduce this relationship from Take and Teach . Students Courses Professors Take Teach Instruct T. M. Murali September 29, 2010 CS 4604: E/R Design and Constraints Design Constraints Select the Right Relationships I Do not add unnecessary relationships. I It may be possible to deduce one relationship from another. I Do we need the relationship Instruct between Professors and Students ? I No. We can deduce this relationship from Take and Teach . I Do we need the relationships Take and Teach ? Courses Students Evaluation Professors Take Teach T. M. Murali September 29, 2010 CS 4604: E/R Design and Constraints Design Constraints Select the Right Relationships I Do not add unnecessary relationships....
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This note was uploaded on 12/24/2011 for the course CS 4604 taught by Professor Cmmalik during the Fall '08 term at Virginia Tech.

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lecture-11-designing-er-diagrams - Design Constraints...

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