maulin: Confusion in ER modeling

maulin talks about confusion he is experiencing with sub-entities in ER diagrams. Well, let's hit it point-by-point:

• How can you have a disjoint sub-entity without it being required? Think of it this way. If you choose a to use a sub-entity, you must use one or the other. However, you are not required to use a sub-entity.
• What is the difference between required and disjoint? Required means that you must use a sub-entity. Disjoint means that if you use a sub-entity, you must use one or the other. Something could be disjoint without being required, and vice versa.

This link provides full details. Since I will not be emphasizing this on the test, I will not use a lot of class time on this topic.