Database Reference
In-Depth Information
Barker ER also supports a restricted form of disjunctive reference, since the roles of a
mandatory exclusive arc can be used in an identifi cation scheme. In Figure 3, for example,
a line item is identifi ed by combining its invoice with either a product code or a service
code.
Subtyping is depicted by Euler diagrams. Only single-inheritance is allowed (each
subtype has only one supertype). Moreover, the union of the subtypes must equal the super-
type, even if this requires an artifi cial subtype such as “Other” to ensure this. In Figure 3,
MalePerson and FemalePerson form a partition of Person (i.e., they are mutually exclusive,
and also they collectively exhaust their supertype).
A dynamic constraint denoted by a diamond marks a role as “non-transferable”. For
example, the diamond in Figure 3 indicates that once a person is recorded as being born in
a given country, their birth-country cannot be changed to another country. Since we usually
wish to allow editing of mistaken entries, this constraint has limited practical use except
for tasks such as auditing.
Figure 4: An ORM metaschema for Barker ER
