In a one-to-many relationship, a set occurrence contains one owner record and multiple member records. An owner can own more than one set providing a way to move from one set to another. It is also possible to have a many-to-many relationship, called a network, where an owner is related to multiple members and those members are related to multiple owners. Networks provide a link from the owner of one set to the owner of another set.
In a previous example, we related an employee to the projects to which they are assigned. We know that June Moon works on the Y2K and HR5 projects, John Done works on the YEAR END and FIN56 projects, and Peter Plum works on the YEAR END and Y2K projects. If an employee were assigned to only one project at a time, we could establish an EMPLOYEE-PROJECT set to represent the relationship between projects and employees. However, a single project often has several employees assigned to it and each employee usually is assigned to several projects. This creates a many-to-many or network relationship. None of the relationships discussed so far are many-to-many relationships. The solution to the network problem, as we will see, lies in multiple membership.
The figure below illustrates the many-to-many relationship between projects and employees. Both June Moon and John Done are assigned to Y2K. June Moon is also assigned to HR5; John Done is also assigned to YEAR END as is Peter Plum who is the only one working on FIN56.
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