A.A weak entity set can always be made into a strong entity set by adding to its attributes the primary-key attributes of its identifying entity set. Outline what sort of redundancy will result if we do so.
B. Consider a relation such as sec course, generated from a many-to-one relationship set sec course. Do the primary and foreign key constraints created on the relation enforce the many-to-one cardinality constraint? Explain why.
The Correct Answer and Explanation is :
Let’s address both parts of the question one at a time.
A. Redundancy Resulting from Turning a Weak Entity into a Strong Entity:
When you convert a weak entity set into a strong entity set by adding the primary key attributes of its identifying (owner) entity set, redundancy arises due to the duplication of information. A weak entity does not have a primary key on its own; it relies on the primary key of a strong entity for its identity. For example, if a weak entity “StudentCourse” depends on the “Student” entity to be uniquely identified, then the attributes of the “Student” entity (like “StudentID”) are added to “StudentCourse” to make it a strong entity.
The redundancy comes from the fact that every instance of the weak entity will now store the identifying key of the strong entity in its attributes. For instance, if we have many “StudentCourse” entries for each student, the “StudentID” is repeated multiple times. This redundancy can lead to unnecessary storage usage, and it might also increase the chances of inconsistencies if updates or deletions in the strong entity aren’t correctly reflected in the weak entity.
B. Enforcing the Many-to-One Cardinality in the SecCourse Relation:
A many-to-one relationship between two entities implies that multiple instances of one entity can be associated with a single instance of another entity. In the case of the sec_course relation, if we consider that multiple “Courses” (in this case, the many side) are associated with one “Department” (the one side), the many-to-one relationship can be enforced by primary and foreign key constraints.
In this case:
- The primary key of the “Department” entity (say,
DeptID) will be included in thesec_courserelation as a foreign key. - The
DeptIDin thesec_courserelation references the “Department” entity.
While the primary and foreign key constraints ensure referential integrity (i.e., every DeptID in the sec_course relation corresponds to an existing DeptID in the “Department” entity), they do not explicitly enforce the many-to-one cardinality. These constraints ensure that a course must be linked to one department (because of the foreign key), but they do not prevent a department from being associated with multiple courses. The cardinality constraint itself is logically implied but not enforced by the foreign key alone. To enforce this cardinality properly, additional business logic or constraints (like unique constraints on the department’s course assignments) might be needed.
In conclusion, primary and foreign key constraints enforce referential integrity but do not directly enforce cardinality constraints like many-to-one unless further specific rules are implemented.