4.6 Notation for more than two arcs


In the linear notation, if a relation has more than two arcs, the arcs may be represented as in this example:

   [Person: Julia]<-(Betw)-
                 <-1-[Person: Tom]
                 <-2-[Person: Brad]
   "Julia is between Tom and Brad"

If this were a Hollywood movie, we might imagine Julia Roberts walking gracefully between Tom Cruise and Brad Pitt.

The hyphen continues the arcs

The hyphen after the relation indicates that its arcs are continued on subsequent lines. Here is the example again, in a different form:

   [Person: Tom]-1->(Betw)-
                <-2-[Person: Brad]
                ->[Person: Julia]

Notice the hyphen after "(Betw)".

Graphical notation

In the graphical notation, the arcs are simply labelled with numbers:

The last arc is unlabeled

Notice how the arc that has no number is the arc that points away from the relation. Recall that all of the arcs except the last one must point towards the relation, while the last one must point away. Thus the last arc could be labelled with a "3" in this case, but we often simply leave it unlabelled, since it is uniquely identified by its pointing away from the relation.

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