types to other entity types that the PERSON entity type does not have. The two entity types have different relationship types to other entity types.
3.6.3 Mutually Exclusive relationship types
In some cases the existence of one kind of relationship type precludes the existence of another. Entities within an entity type A may be related by a relationship type R to an entity in entity type B or entity type C but not both. The relationship types are said to be mutually exclusive. Usually both relationship types will have the same name, as in the following example. In Fig. 3.16 a fault report may have been for a computer or a printer but not both. The fact that it might not have concerned a computer is shown by the zero optionality of the upper 'was_for' relationship type between FAULT REPORT and COMPUTER. The fact that it might not have concerned a printer is shown by the zero optionality of the lower 'was_for' relationship type between FAULT REPORT and PRINTER. However a fault report must have been for either a computer or a printer (in this example). The zero optionality cannot apply for both. Both this and the fact that the fault report can have been for a maximum of one of the two entity types is indicated by the arc on the diagram linking the two relationship types. In summary then, the arc shows that a fault report can be for a maximum and a minimum of one entity from the types COMPUTER and PRINTER.