GATE 2015 CS – Question 61

Databases · Relational Model: Relational Algebra, Tuple Calculus, SQL · 2 marks · Numerical answer

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 following 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 ________.

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Correct answer: 2

Explanation

The query groups the joined rows by student name, not by roll number. The names are Raj, for roll numbers 1 and 3, and Rohit, for roll number 2. That gives two groups, so the query returns 2 rows.