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GATE 2013 CS (CS1) – Question 45

Databases · Relational Model: Relational Algebra, Tuple Calculus, SQL · 2 marks · Multiple choice

Schemas: Students(rollno,sname), Courses(courseno,cname), Registration(rollno,courseno,percent). Which queries give distinct student names scoring >90 in course 107? I: SELECT DISTINCT S.sname FROM Students S,Registration R WHERE R.rollno=S.rollno AND R.courseno=107 AND R.percent>90. II: project sname after selecting courseno=107 and percent>90 from Registration natural-join Students. III: {T | ∃S∈Students ∃R∈Registration (S.rollno=R.rollno ∧ R.courseno=107 ∧ R.percent>90 ∧ T.sname=S.sname)}. IV: {⟨SN⟩ | ∃SR∃RP(⟨SR,SN⟩∈Students ∧ ⟨SR,107,RP⟩∈Registration ∧ RP>90)}.

  1. I,II,III,IV
  2. I,II,III only
  3. I,II,IV only
  4. II,III,IV only

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Correct answer: (A) I,II,III,IV

Explanation

I explicitly joins the roll numbers, filters course/score and removes duplicate names. II performs the same operations in relational algebra. III supplies equivalent tuple-calculus witnesses, and IV gives their domain-calculus version. All four describe the same set of names, so A.

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