NPTEL Introduction to Database Systems Week 3 Assignment Answers 2025

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NPTEL Introduction to Database Systems Week 3 Assignment Answers 2025

1. Which of the following statements is TRUE?

  • A key for a relation can be figured out just by looking at the instance data.
  • A relation has exactly one key.
  • It is possible that a relation has no key.
  • A key for a relation can only be determined from the meaning of its attributes.
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2. In an instance of a relation, it is possible that the value of a key attribute is NULL in a certain row.

  • True
  • False
Answer :-

3. It is possible for a foreign key of a relation to refer to the primary key of the relation itself.

  • True
  • False
Answer :-

4. Every relation scheme can have at most one foreign key

  • True
  • False
Answer :-

5. If K is supposed to be a key for scheme R, any relation instance r on R should not have two tuples that have identical values for attributes in K.

  • True
  • False
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6. Consider the following tables:

How many tuples exist in the query (A∪B)⟗(A.id>20∨C.id< 10)C?

  • 7
  • 8
  • 9
  • 10
Answer :-

7. Consider the following relations A(X,Y,Z) and B(D,E,F):

What is the result of the following expression: II(X,Y)(σ(X=1)∨(X=4)(A))−II(D,E)(σ(D≠3)(B))

  • {(6,8), (9,9)}
  • Empty relation
  • A relation with scheme (X,Y) and tuples { (6,8) }
  • A relation with scheme (X,Y) and tuples { (9,9) }
Answer :-

8. Consider the following relations A(X,Y,Z) and B (D,E,F) and find out how many tuples will there be in the result of the following relational algebra expression?

A⨝(A.X>=B.D∨A.Z< =B.F)B

  • 22
  • 23
  • 24
  • 25
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9. Information about a collection of students is given by the relation studInfo(studId, name, sex, yearOfJoin). The relation enroll(studId, courseId) gives which student has enrolled for (or taken) that course(s). Assume that every course is taken by at least one male and at least one female student. What does the following relational algebra expression represent?

IIcourseId((IIstudId(σsex="Female"(studInfo))×IIcourseId(enroll))−enroll)
  • Courses in which all female students have enrolled.
  • Courses in which a proper subset of female students have enrolled.
  • Courses in which all male students have enrolled.
  • Courses in which only female students have enrolled.
Answer :-

10. Choose the correct statement(s):

  • Two schemes are union compatible if the domain of at least one pair of corresponding attributes match.
  • A join operation is equivalent to performing a cross product of its operands followed by a select operation using the join condition.
  • The operators {σ,π,×,⋃,−} constitute a complete set of relational algebra operators.
  • The difference relational algebra operator requires its operands to be union-compatible.
Answer :-

11. Consider the following relations A(X,Y,Z) and B(X,Y) and the given instances. What is the result of the relational algebra expression?
A÷B

  • Empty relation
  • A relation with scheme Z and tuple { (3) }
  • A relation with scheme Z and tuples { (3), (5) }
  • {3,5}
Answer :-

12. Given two union compatible relations X(A,B) and Y(C,D). What is the result of the operation?

X⋈(A=C)∧(B=D)Y

  • X∪Y
  • X∩Y
  • X−Y
  • X÷Y
Answer :-

13. If X, Y and Z are as specified below, which of the following statements is TRUE wrt different keys of a relational DB?

X = set of all primary keys
Y = set of all candidate keys
Z = set of all super keys

  • X⊆Z⊆Y
  • Z⊆X⊆Y
  • X⊆Y⊆Z
  • Y⊆X⊆Z
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