DBMS MCQs - ER Model and Relational Algebra (Set 2)

Q1. A table stores date_of_birth for every employee, and age is shown on screen by calculating it from date_of_birth. In ER terms, age is a:

a) Composite attribute

b) Multivalued attribute

c) Derived attribute

d) Key attribute

Q2. In Chen's ER notation, an attribute drawn as a double ellipse represents:

a) A multivalued attribute

b) A derived attribute

c) A composite attribute

d) A key attribute

Q3. In an ER diagram, a double line connecting an entity set to a relationship set indicates:

a) Partial participation

b) A one-to-one relationship

c) The discriminator of a weak entity

d) Total participation

Q4. How is the primary key of a weak entity set formed?

a) From its discriminator (partial key) alone

b) From the primary key of its identifying (owner) entity set combined with its discriminator

c) From any attribute that happens to be unique in the weak entity set

d) From a row id generated automatically by the DBMS

Q5. Each employee works in exactly one department, and a department can have many employees. The relationship has no attributes. When mapping to relations without a separate table for the relationship, where should the foreign key be placed?

a) In EMPLOYEE, referring to DEPARTMENT's primary key

b) In DEPARTMENT, referring to EMPLOYEE's primary key

c) In both EMPLOYEE and DEPARTMENT

d) Nowhere; a separate relationship table is mandatory

Q6. STUDENT and COURSE are related by a many-to-many relationship ENROLLS that has an attribute grade. What is the correct relational mapping of ENROLLS?

a) Add course_id and grade as columns in STUDENT

b) Add student_id and grade as columns in COURSE

c) Create ENROLLS(student_id, course_id, grade) with (student_id, course_id) as the primary key

d) Merge STUDENT and COURSE into a single table

Q7. Entity sets CAR and TRUCK share attributes such as reg_no, make and colour. Combining them into a higher-level entity set VEHICLE is called:

a) Specialization

b) Generalization

c) Aggregation

d) Normalization

Q8. Relation R(A, B) contains the tuples given below. How many tuples does the relational algebra expression π_A(R) return?

A | B
1 | p
1 | q
2 | p
3 | r
2 | q

a) 5

b) 2

c) 4

d) 3

Q9. Together with rename, which set of relational algebra operators is complete, meaning every other operator can be expressed using them?

a) Select, project, union, set difference, Cartesian product

b) Select, project, union, intersection, natural join

c) Select, project, Cartesian product, natural join, division

d) Project, union, intersection, set difference

Q10. R(A, B, C) has 5 tuples and S(D, E) has 4 tuples. What does R × S (Cartesian product) contain?

a) 9 tuples and 5 attributes

b) 20 tuples and 6 attributes

c) 20 tuples and 5 attributes

d) 9 tuples and 6 attributes

Q11. Entity sets E1 (key a1) and E2 (key b1) are connected by a relationship R that is one-to-many from E1 to E2 (each E2 entity is related to at most one E1 entity). R has no attributes and participation is partial on both sides. What is the minimum number of relations needed?

a) 1

b) 3

c) 4

d) 2

Q12. STUDENT(roll_no, name, phone_numbers) has phone_numbers as a multivalued attribute. STUDENT is related to COURSE(course_id, title) by a many-to-many relationship ENROLLS. What is the minimum number of relations in a 1NF relational design?

a) 3

b) 4

c) 5

d) 2

Q13. Given relations R(A, B) and S(B, C) below, how many tuples are in the natural join R ⋈ S?

R:            S:
A | B         B | C
1 | x         x | 10
2 | y         x | 20
3 | x         y | 30
4 | z         w | 40

a) 3

b) 4

c) 5

d) 8

Q14. What is the result of Enrolled ÷ Course for the relations below?

Enrolled(sid, cid):     Course(cid):
S1 | C1                 C1
S1 | C2                 C2
S2 | C1
S2 | C3
S3 | C1
S3 | C2
S3 | C3

a) {S1, S3}

b) {S1, S2, S3}

c) {S3}

d) {S1}

Q15. R has m tuples and S has n tuples (m, n ≥ 1). What is the minimum possible number of tuples in the left outer join R ⟕ S?

a) 0

b) n

c) min(m, n)

d) m

Q16. Which relational algebra expression is equivalent to the tuple relational calculus query { t | t ∈ Employee ∧ t.salary > 50000 }?

a) π_salary(Employee)

b) σ_salary>50000(Employee)

c) Employee - σ_salary>50000(Employee)

d) π_salary(σ_salary>50000(Employee))

Q17. Why is the tuple relational calculus expression { t | ¬(t ∈ Employee) } called unsafe?

a) Its result can be infinite, since it includes every possible tuple that is not in Employee

b) It uses the universal quantifier

c) It refers to more than one relation

d) It cannot be expressed using a WHERE clause

Q18. Which of the following relational algebra equivalences is NOT valid in general?

a) σ_c1(σ_c2(R)) = σ_c2(σ_c1(R))

b) π_A(π_A,B(R)) = π_A(R)

c) π_A(R - S) = π_A(R) - π_A(S)

d) σ_c(R ∪ S) = σ_c(R) ∪ σ_c(S)