Subtyping groups the common attributes and relationships of entities, while retaining what is special within each entity. Subtyping is an excellent way of communicating that certain concepts are very similar and for showing examples.
In our publishing data model, an Author may write many Electronic Titles and many Print Titles, as illustrated in Figure 7.7.
Figure 7.7 Author may write many Electronic Titles and many Print Titles
· Each Author may write one or many Electronic Titles.
· Each Electronic Title must be written by one or many Authors.
· Each Author may write one or many Print Titles.
· Each ...