Example 4.4: A one-one relationship between entity sets E and F is repre- sented by arrows pointing to both E and F. For instance, Fig. 4.3 shows two entity sets, Studios and Presidents, and the relationship Runs between them attributes are omitted). We assume that a president can run only one studio and a studio has only one president, so this relationship is one-one, as indicated by the two arrows, one entering each entity set. RunsP R > residents Studios Figure 4.3: A one-one relationship Remember that the arrow means "at most one"; it does not guarantee ex Thus, in Fig. 4.3, we would expect that a "president" is surely associated with some studio; how could they be a istence of an entity of the set pointed to. "president" otherwise? However, a studio might not have a president at some particular time, so the arrow from Runs to Presidents truly means "at most one and not "exactly one." We shall discuss the distinction further in Section 4.3.3