Table schema:
CREATE TABLE songs (title VARCHAR, songid VARCHAR); CREATE TABLE vocals (songid VARCHAR)

Question: Find all the songs that do not have a back vocal.
SQL: 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"