Which students not enrolled in any course? Find their personal names.
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
students not enrolled in any course can be found by selecting students who do not have a corresponding entry in the Student_Course_Enrolment table; finding their personal names refers to selecting the personal_name column from the Students table;