For the professor who advised student no.6, please list the IDs of the courses he or she teaches.
professor refers to p_id_dummy and professor = 1; student no.6 refers to advisedBy.p_id = 6; IDs of the courses refers to taughtBy.course_id
SELECT T2.course_id FROM taughtBy AS T1 INNER JOIN course AS T2 ON T1.course_id = T2.course_id INNER JOIN advisedBy AS T3 ON T3.p_id = T1.p_id WHERE T1.p_id = 9