Design a database to keep track of information for an art museum:
The museum has a collection of art objects, each of which has a unique ID number, an artist (if known), the year it was created (if known), a title, and a description. In addition, there is a country or culture of origin (Italian, Egyptian, American, Indian, etc) and time period (Renaissance, Modern, Ancient, etc).
For artists, there is a name, date of birth (if known), date of death (if not living), country, period (Renaissance, Modern, Ancient, etc), main style, and description. The name is assumed to be unique.
An art object is either a painting, a sculpture, a statue, or "other" (for objects that don't fall into one of the other three categories).
Art objects either belong to the permanent collection or are borrowed.
Different exhibitions occur, each having a name, start date, and end date. Any object can be part of an exhibition.
Some common problems:
Incorrect or missing cardinality values.
Not indicating disjoint or total participation for ArtObject specializations - an art object must be a painting, sculpture, statue, or other, and it can't be more than one of those things. Similarly, an object is borrowed or part of the permanent collection - it can't be both or neither.
Specialization is for "kind of", category types are for otherwise different things which happen to be able to take on the same role. Specialization is thus more appropriate for Painting, Sculpture, Statue, and Other - types of ArtObjects - and Permanent and Borrowed - another kind of types of ArtObject - but a category type is more appropriate for Owner because museums and individuals are very different things which just happen to both be able to own an object.
Loaned objects are part of the permanent collection - objects the museum owns are in the permanent collection (some of which may be loaned out) while objects the museum doesn't own are borrowed.
Museums can be both owners of borrowed objects and things that objects are loaned to - an entity type for Museum allows for OwnedBy and LoanedTo relationships instead of repeating name, contact, and description in two places.