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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 Common Errors in ER Modeling
(Page 17 of 21 )


Failing to build flexibility

The business needs a way to account for customers and suppliers. You might model it like this-

entity relationship

 

While instead, you could ask, "Does it ever happen that a customer is also a supplier of other goods?" It happens often. You could solve it like this-

business contact

We changed the name of CUSTOMER into BUSINESS CONTACT. Using the many-to-many relationship in an early stage of analysis is a powerful mechanism. It shows that a relationship is more complex than first anticipated, and as long as that many-to-many relationship is there, it is a reminder to resolve it at some point in time.

Even worse, the business need might be to identify customers and suppliers by a number from separate series to identify them, so there is a need for customer type as an additional means of identification. Truly, this happens. The analyst gives them this-

analyst response

where the unique combination of business contact number and contact type is the primary key. (The vertical bar at the crow’s foot notates part of primary key).


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