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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 - Supplier Entity
(Page 7 of 21 )


Do they seem a bit far-fetched? Yes they are. They are a signal of lacking analysis, for all practical purposes. Most likely, as said, the supplier product entity is really nothing more than a set of attributes in the product entity.

One-to-one relationships are a signal that you have different entities that are probably the same entity.

One-to-many relationships

Mandatory-mandatory

one-to-many

This signals that an order (in this case) MUST belong to ONE AND ONLY ONE customer, and a customer MUST place ONE OR MORE orders. While theoretically possible to construct, this relationship would violate constraints in the database: It will not be possible to create a new customer if there are no orders; however, an order cannot be created without customers.

Mandatory-optional

one-to-many

A new customer MUST place one or more orders, while an order MAY belong to a customer. Note that the order can exist without a customer, while you cannot have customers who do not have any orders. Well, there might be such a situation, but I find it hard to believe. I would have looked more deeply into the problem domain. This relationship is possible, but relates to odd business rules.

Optional-optional

one-to-many


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