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.
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-
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-
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-
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).
Development Cycles 
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Enterprise Java and Rational Rose -- Part I
