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Entity Relationship Modeling


Entity Relationship Modeling (ER modeling) is by far the most common way to express the analytical result of an early stage in the construction of a new database. In this ebook, Alf Pedersen describes the principles for ER modeling, as well as the most important terms used in modeling a new database.

Author Info:
By: Alf A. Pedersen
Rating: 4 stars4 stars4 stars4 stars4 stars / 89
April 05, 2004
TABLE OF CONTENTS:
  1. · Entity Relationship Modeling
  2. · The Entity
  3. · Other Business Contacts
  4. · Attributes in entities
  5. · Business Rules
  6. · Three types of relationships
  7. · Supplier Entity
  8. · A Weak Relation
  9. · A Useful Relation
  10. · Involuted (or recursive) relationships
  11. · Many-to-Many
  12. · The Database Analysis Team - A Teamwork
  13. · Level of Knowledge
  14. · Experience vs. Inexperience
  15. · Complete Model?
  16. · Building Queries
  17. · Other Common Errors in ER Modeling
  18. · Second Normal Violation
  19. · More Specific
  20. · Generic or Specific Models?
  21. · Analysts Experience

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Entity Relationship Modeling - Second Normal Violation
(Page 18 of 21 )


This analysis trap breaks Second Normal Form. Name, address and so on is not dependent on contact type, only on business contact. If a business contact is both a customer and a supplier, all names and address information etc. must be stored twice.

But what about the whole concept of customers and suppliers? They are only two of several groups that are relating to the business. In an early stage, we should plan for several groups of business contacts, such as customers, suppliers, and maybe other interest groups, all of which we may have to deal with. In such cases, one may use the terms super-entity and sub-entity as a very powerful illustration of generic (common) structures, like this-

entity relationship 

We are trying to find all attributes for each sub-entity. Since they obviously are closely related to each other, many of the attributes will be common. As the analysis moves forward, we add attributes to each sub-entity. Where an attribute exists in all sub-entities, we move it to the super-entity. We see that actually, some of the sub-entities are different types of the same thing, because they do not have un-common attributes:




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