For instance, a single department offers one or many courses; every department offers at least one course.
Also, a dorm may be associated with one or many students, and a student may be associated with no dorm, or
with one dorm. Some students are commuters who will not be associated with a dorm, but if a student is associated
with a dorm, the student is associated with at most one dorm.
These processes of defining entities, their attributes, and the relationships among entities are effective for
most entities and relationships. There are a few more special cases, however, that come up often enough to
require some additional discussion.
Some entities belong in the database only if another entity is already part of the database. For instance, we
would include dependents of a professor in the database only if the professor were already included.
If a professor leaves the university, the professor??™s information would be removed from the database, and it
would no longer make sense to store information about the professor??™s dependents, either. An entity type such
as ???Dependent??? is called a ???weak entity.??? A weak entity is modeled like other entity types, except that it is identified
as being dependent upon a ???strong entity??? in the database.
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