ค้นหาเพลงทั้งหมดที่ไม่มีเสียงร้องกลับ ไม่จำเป็นต้องใช้ทุกตาราง
Find all the songs that do not have a back vocal.It is not neccessary to use all the tables.
 songs:  (title [ VARCHAR ] songid [ VARCHAR) ]
SELECT DISTINCT title FROM vocals AS t1 JOIN songs AS t2 ON t1.songid = t2.songid EXCEPT SELECT t2.title FROM vocals AS t1 JOIN songs AS t2 ON t1.songid = t2.songid WHERE TYPE = "back"