Information Modeling and Relational Databases, 2nd Edition

5.2. Mandatory and Optional Roles

Consider the output report of Table 5.1. The question mark “?” denotes a null, indicating that an actual value is not recorded. For instance, patient 002 may have a phone but this information is not recorded, or he/she may simply have no phone.

Table 5.1. Details about hospital patients.

Patient Nr	Patient Name	Phone
001	Adams C	205764
002	Brown S	?
003	Collins T	8853020

Patients are identified by a patient number. Different patients may have the same name and even the same phone number, so the population of Table 5.1 is not significant. We must record each patient’s name, but it is optional whether we record a phone number. Figure 5.4. The question mark “?” denotes a null, indicating that an actual value is not recorded. ...