Information Modeling and Relational Databases, 2nd Edition

6.3. Value Constraints and Independent Types

A value constraint indicates which values are allowed in a value type or role and is sometimes called a “domain constraint”. A value constraint should be declared only if the value list is at least reasonably stable; otherwise we would need to continually change the constraint, and hence the schema, as the value list changed.

A value type may be defined by declaring its extension (set of possible values) as one or more enumerations or ranges enclosed in braces (curly brackets). On a schema diagram, an object type value constraint is declared by displaying the extension next to either the value type itself, or the entity type that is referenced by the value type.

Figure 6.6 shows several examples. ...