CREATE TABLE Courses (course_name VARCHAR(255), course_id VARCHAR(255)); CREATE TABLE Student_Enrolment_Courses (course_id VARCHAR(255))
What are all the course names of the courses which ever have students enrolled in?
    
SQL: SELECT DISTINCT T1.course_name FROM Courses AS T1 JOIN Student_Enrolment_Courses AS T2 ON T1.course_id = T2.course_id