SELECT student_id FROM students WHERE student_id NOT IN (SELECT student_id FROM student_course_attendance)
What are the  ids of every student who has never attended a course?
SELECT student_id FROM students WHERE student_id NOT IN ( SELECT student_id FROM student_course_attendance )
select student_id from students where student_id not in ( select student_id from student_course_attendance )
What are the ids of every student who has never attended a course ?