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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 - Many-to-Many
(Page 11 of 21 )


As with all many-to-many relationships, we must create an intersection entity, and resolve the many-to-many relationship into two one-to-many relationships:

many-to-many

When naming both ends of relationships, try to find meaningful and descriptive phrases so it feels natural to read the relationships back and forth. Note how we have read all the relationships in this chapter. Avoid, if you can, words like ‘is’, ‘has’, ‘in’. Those are quite meaningless. (Although I cannot help myself, either, at times…)

Using Arcs
What is an arc? An arc is a constraint that crosses two or more relationships going into an entity, and it indicates that the relationships in the arc are mutually exclusive. I will explain it with graphics:

arcs

The arc in this model tells that for any one transaction, it is either valid for a shareholder, or for a business contact, never both at the same time. In my opinion, arcs are a result of insufficient analysis: We must have both relationships optional and that reduces the strength of referential integrity. Furthermore, it is an indecisive signal: ‘We are not sure here, choose for yourself’. They also give a signal about a possible missing business rule, also.

Try to avoid arcs; however, in an early analysis phase, they are a good signal about a problem domain that has to be explored further.


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