When a single parent-child relationship exists, the relationship is called binary. All of the previous examples of relationships to this point have been binary relationships. However, when creating a data model, it is not uncommon to come across n-ary relationships, the modeling name for relationships between two or more parent entities and a single child table. An example of an n-ary relationship is shown in the following figure:
Like many-to-many relationships, three-, four-, or n-ary relationships are valid constructs in entity relationship diagrams. Also like many-to-many relationships, n-ary relationships should be resolved in later models using a set of binary relationships to an associative entity.
If you consider the business rule stated in the figure, you can see that a CONTRACT represents a three-way relationship among COMPANY, PRODUCT, and CUSTOMER. The structure indicates that many COMPANYs sell many PRODUCTs to many CUSTOMERs. When you see a relationship like this, however, there are business questions that should be answered. For example, “Must a product be offered by a company before it can be sold?” “Can a customer establish a single contract including products from several different companies?” and, “Do you need to keep track of which customers 'belong to' which companies?” Depending on the answers, the structures may change.
For example, if a product must be offered by a company before it can be sold, then you would have to change the structure as follows:
Since PRODUCTs must be offered by COMPANYs, you can create an associative entity to capture this relationship. As a result, the original three-way relationship to CONTRACT is replaced by two, two-way relationships.
By asking a variety of business questions, it is likely that you will find that most n-ary relationships can be broken down into a series of relationships to associative entities.
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