List the names of all courses that have been taken by 2 or more students. Do this as a single query and do not hard-code any ID numbers in the query. Don't show duplicates. (JOIN) Expected results:
+----------------------+ | name | +----------------------+ | Computer Science 142 | | Computer Science 143 | +----------------------+ 2 rows in set (0.01 sec)
SELECT DISTINCT c.name FROM courses c JOIN grades g1 ON g1.course_id = c.id JOIN students s1 ON g1.student_id = s1.id JOIN grades g2 ON g2.course_id = c.id JOIN students s2 ON g2.student_id = s2.id WHERE s1.id < s2.id;