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DEVELOPMENT CYCLES

Entity Relationship Modeling
By: Alf A. Pedersen
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    2004-04-05

    Table of Contents:
  • Entity Relationship Modeling
  • The Entity
  • Other Business Contacts
  • Attributes in entities
  • Business Rules
  • Three types of relationships
  • Supplier Entity
  • A Weak Relation
  • A Useful Relation
  • Involuted (or recursive) relationships
  • Many-to-Many
  • The Database Analysis Team - A Teamwork
  • Level of Knowledge
  • Experience vs. Inexperience
  • Complete Model?
  • Building Queries
  • Other Common Errors in ER Modeling
  • Second Normal Violation
  • More Specific
  • Generic or Specific Models?
  • 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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