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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 - Other Business Contacts
(Page 3 of 21 )


When we work like this, we will (most likely) see that information-wise, there is no difference between a customer and supplier: They are two things of the same kind. However; shareholder may be something different:

entity relationship

We see that our analysis has led us to understand that there are really no difference in the information needs whether it is customers, suppliers or club members (we run a customer club…),  But we need to track each shareholder’s number of shares. The best thing to do until further, is to remove the three common entities and replace them by a type:

entity relationship

We can read from this that each BUSINESS CONTACT must be categorized as one and only one contact type, while CONTACT TYPE may be a category for zero or more BUSINESS CONTACTs. Contact type will here typically be customers, suppliers, club-members, or whatever business contact we have. We can se that we have built much more flexibility into the system by using super- and sub-entities in an early stage of analysis. We will return to this model after the chapter on allowed relationship types. 

By the way, what about the SHAREHOLDER?

Well, the shareholder was a bit different from our other business contacts. If we want to keep the entity, we must give it all the attributes of BUSINSS CONTACT, in addition to the number of shares attribute, and keep it as an own entity. However, we will remove it totally for now, and save it for a later occasion.


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