Assignment 5 2025
2 2025
Unique Number:
Due date: 5 September 2025
QUESTION 1
a. When a 3rd NF relation is in BCNF and when it is not (4 marks)
A relation is in 3rd Normal Form (3NF) if every non-prime attribute is fully functionally
dependent on the primary key and there are no transitive dependencies.
A relation is in Boyce-Codd Normal Form (BCNF) if, for every functional dependency (X →
Y), the determinant X is a superkey.
3NF but not BCNF:
This happens when a non-prime attribute depends on part of a candidate key.
Example: Suppose we have a relation Course(Instructor, CourseCode, Room), with
dependencies:
Instructor → Room
CourseCode → Instructor
Here, the table is in 3NF (since all non-key attributes depend on candidate keys) but not in
BCNF because Instructor is not a superkey, yet it determines Room.
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QUESTION 1
a. When a 3rd NF relation is in BCNF and when it is not (4 marks)
A relation is in 3rd Normal Form (3NF) if every non-prime attribute is fully
functionally dependent on the primary key and there are no transitive dependencies.
A relation is in Boyce-Codd Normal Form (BCNF) if, for every functional
dependency (X → Y), the determinant X is a superkey.
3NF but not BCNF:
This happens when a non-prime attribute depends on part of a candidate key.
Example: Suppose we have a relation Course(Instructor, CourseCode, Room),
with dependencies:
Instructor → Room
CourseCode → Instructor
Here, the table is in 3NF (since all non-key attributes depend on candidate keys) but
not in BCNF because Instructor is not a superkey, yet it determines Room.
3NF and BCNF:
If every determinant is a superkey (e.g. StudentID → Name, Major), then it is both in
3NF and BCNF.
b. Attribute inheritance
Attribute inheritance occurs in subtype and supertype relationships in ER
modeling. A subtype automatically inherits all the attributes of its supertype.
Example:
Supertype: Employee(EmpID, Name, Salary)
Subtypes: Manager(Bonus), Technician(SkillLevel)
Here, both Manager and Technician inherit EmpID, Name, Salary from Employee,
while also adding their own specific attributes.