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UML and Data Modeling: A Reconciliation by David Hay

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Some entity/relationship notations have the ability to describe an exclusive or arrangement of relationships. For example, Figure 3-4 shows how the Barker-Ellis notation represents the assertion:

·         Each Line Item must be either for exactly one Product or for exactly one Service.

The arc across the relationship lines denotes this.

Figure 3-4: Exclusive Or in the Barker-Ellis Notation

Not all entity/relationship notations can show this, but, in fact, UML can. In UML, it is called an XOR Constraint and is shown in Figure 3-5.

Figure 3-5: Exclusive Or in UML

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