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Previous Year Question (PYQs)



Use the following schema of the academic institution relational database:

student (rollno, name, degree, year, sex, deptno, advisor)

department (deptid, name, hod, phone)

professor (empid, name, sex, startyear, deptno, phone)

course (courseid cname, credits, deptno)

enrollment (rollno, courseid, sem, year, grade)

teaching (empid, courseid, sem, year, classroom)

prerequisite (precourseid, courseid)

deptno is foreign key in the student, professor and course relations referring to deptid of department relation; advisor is a foreign key in the student relation referring to empid of professor relation; hod is a foreign key in the department relation referring to empid of professor relation; rollno is a foreign key in the enrollment relation referring to rollno of student relation; courseid is a foreign key in the enrollment, teaching relations referring to courseid of course relation; empid is a foreign key of the teaching relation referring to empid of professor relation; precourseid and courseid are foreign keys in the prerequisite relation referring to courseid of the course relation;

Given query:

select s.rollno, s.name
from student s
where EXISTS (select c.courseid from course c Where c.deptno =5 and
EXISTS (select e.* from enrollment e
where e.courseid = c.courseid and
e.rollno = s.rollno));

Which one of the following statements is correct with respect to the given query?






Solution

The outer query selects students.

Inside the query:

EXISTS checks whether there is at least one course from department number 5.

The inner EXISTS checks whether the same student has enrolled in that course.

So the query means:

Find students who have enrolled in at least one course offered by department number 5.

It does not check all courses. For all courses, double NOT EXISTS would be required.



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