Task: Text to Code
Input (English): Step 1: find the semester_id of Student_Enrolment table.
Step 2: find the Semesters's semester_id of Semesters table whose Semesters's semester_id not one of the results of step 1
Output (English): SELECT T1.semester_id FROM Semesters AS T1 WHERE T1.semester_id NOT IN ( SELECT T2.semester_id FROM Student_Enrolment AS T2 )