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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 - Building Queries
(Page 16 of 21 )


Actually, problems with building queries against a relational database are very often an effect of an inadequate (denormalized) database structure. The opposite is also a fact: It is much easier to write optimal queries and updates against a normalized database structure. 

The business probably do not know data modeling in detail. That is why they use consultants. However, some basic knowledge is required in order to participate in the analysis phase. You may forward them this free eBook on ‘Entity Relationship Modeling – Principles’ as a primer to understand it. You find it in our Lectures section. If the customer learns the basic principles, he is much better prepared to communicate his business and have a dialogue with the system analyst(s). Here are some common errors in ER modeling.

Failing to understand that the same information is repeated

entity relationship

The business may say: For each customer, we need his business address and his delivery address. That is two addresses. If you model it like this, you are violating the First Normal Form: Repeating attributes/group of attributes.

Ask questions such as:

* Does the customer have more than one delivery address after all?

* Does it ever happen that some customer asks for delivery, not to his address, but to his customer’s address?

* Do customers sometimes refer to a branch address for business or delivery?

This model will let him have as many choices as he may wish. I do not say this is good enough, though.

entity relationship


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