We have covered the basics of entity-relationship modeling and now we will extend our design capabilities to include supertypes and subtypes. It is often the case that an entity type has subgroupings that are useful to include in a data model. For instance, in a university environment persons could be grouped into employees and students; courses could be grouped into graduate courses and undergraduate courses. Previously we considered a library database where one entity type was book; instances of book are loaned out to library members. A library could have many other kinds of things that it loans out such as videos and magazines. A more general thing the library loans out can be referred to as an item; videos, magazines, and books can be considered subtypes of item.
We will consider only supertype and subtype hierarchies. Hierarchies arise when an entity type appears as a subtype of only one supertype. So we are disallowing cases where an entity type has two or more supertypes.