A relational database contains two tables Student and Performance as shown…
2019
A relational database contains two tables Student and Performance as shown below:
The primary key of the Student table is Roll_no. For the Performance table, the columns Roll_no. and Subject_code together form the primary key. Consider the SQL query given below:
SELECT S.Student_name, sum(P.Marks)
FROM Student S, Performance P
WHERE P.Marks > 84
GROUP BY S.Student_name;
The number of rows returned by the above SQL query is ________.
Attempted by 188 students.
Show answer & explanation
Correct answer: 5
Key idea: the query uses the two tables without a join condition, so rows are formed by the Cartesian product and then filtered by the WHERE clause that refers only to the Performance table.
Step-by-step reasoning:
Find all Performance rows satisfying Marks > 84.
The qualifying Performance rows are: (Roll_no 1, subject A, 86), (Roll_no 1, subject B, 95), (Roll_no 1, subject C, 90), (Roll_no 2, subject A, 89), (Roll_no 2, subject C, 92).
There are 5 Performance rows with Marks > 84.
Because the query does not relate Student and Performance by Roll_no, each student will pair with all these qualifying Performance rows (Cartesian product filtered by the WHERE clause).
Grouping by student name therefore produces one aggregated row per student present in the Student table.
Since the Student table has 5 students, the query returns 5 rows.
Final answer: 5 rows.
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