Tell me the course description of the course named "database".
select course_description from courses where course_name = "database"
Tell me the number of students enrolled in this course.
select t1.course_name, count(*) from courses as t1 join student_course_enrolment as t2 on t1.course_id = t2.course_id group by t1.course_name where t1.course_name = "database"
Tell me the personal names of students that not enrolled in any course.
select personal_name from students except select t1.personal_name from students as t1 join student_course_enrolment as t2 on t1.student_id = t2.student_id