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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 - A Weak Relation
(Page 8 of 21 )


This is a weak relationship. A customer MAY place zero or more orders, while an order MAY belong to one and only customer. This is a so-called indecisive mode, normally used when you have two or more entities sharing an arc (more on this later). It may be used for optional attributes; however, an optional attribute is in itself a reason for further investigation: The problem domain may be incompletely analyzed. Be sure that the right questions have been asked and have been answered.

Optional-mandatory

optiona-mandatory

By far the most common and useful relationship you can have: A customer MAY have zero or more orders, while an order MUST belong to one and only one customer. This relationship solves all the most common relationships between entities in the analysis.

Many-to-many relationships

Mandatory-mandatory

many-to-many

This relationship is impossible, for the same reason as the other mandatory-mandatory relationships: It is a catch-22, or deadlock: Neither entity may have new occurrences (rows) before the other.

Mandatory-optional

mandatory-optional

This is a relationship that needs to be resolved; analysis is unclear.

Optional-optional

optional-optional


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