In SQL, foreign key constraints can only reference the primary key attributes
of the referenced relation, or other attributes declared to be a super key using
the unique constraint. As a result, total participation constraints on a
many-to-many relationship (or on the “one” side of a one-to-many relationship)
cannot be enforced on the relations created from the relationship,
using primary key, foreign key and not null constraints on the relations.
a. Explain why.
b. Explain how to enforce total participation constraints using complex
check constraints or assertions (see Section 4.4.7 of korth). (Unfortunately,
these features are not supported on any widely used database currently.)
