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 - 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
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
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
This is a relationship that needs to be resolved; analysis is unclear.
Development Cycles 
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Enterprise Java and Rational Rose -- Part I
