Context: CREATE TABLE Courses (course_name VARCHAR, course_id VARCHAR); CREATE TABLE Student_Enrolment_Courses (course_id VARCHAR)
Question: What are all the course names of the courses which ever have students enrolled in?
Answer: SELECT DISTINCT T1.course_name FROM Courses AS T1 JOIN Student_Enrolment_Courses AS T2 ON T1.course_id = T2.course_id