CREATE TABLE Student_Course_Enrolment (student_id VARCHAR2)
CREATE TABLE Students (student_id VARCHAR2)

How many students did not have any course enrollment?
SELECT COUNT(*) FROM Students WHERE NOT student_id IN (SELECT student_id FROM Student_Course_Enrolment)
SELECT COUNT(*) FROM "Students" WHERE NOT "student_id" IN (SELECT "student_id" FROM "Student_Course_Enrolment")