Question

Consider the following relations:

Student(Roll_No, Student_Name): (1,Raj), (2,Rohit), (3,Raj)
Performance(Roll_No, Course, Marks): (1,Math,80), (1,English,70), (2,Math,75), (3,English,80), (2,Physics,65), (3,Math,80)

Consider the SQL query:

SELECT S.Student_Name, sum(P.Marks)
FROM Student S, Performance P
WHERE S.Roll_No = P.Roll_No
GROUP BY S.Student_Name

The number of rows that will be returned by the SQL query is

Accepted Answer

The correct answer is (B) 2. Joining Student and Performance on Roll_No gives 6 rows (one per Performance record, each matched to its student's name): Raj (roll 1, twice), Rohit (roll 2, twice), Raj (roll 3, twice). GROUP BY Student_Name collapses these into distinct NAME groups — not distinct roll numbers — so the two 'Raj' students (roll 1 and roll 3) are merged into a single group by name, leaving exactly 2 groups: 'Raj' and 'Rohit'.

Explanation