Return the names of songs for which format is mp3 and resolution is below 1000.
SELECT T2.song_name FROM files AS T1 JOIN song AS T2 ON T1.f_id  =  T2.f_id WHERE T1.formats  =  "mp3" INTERSECT SELECT song_name FROM song WHERE resolution  <  1000
format is mp3 refers to files.formats = 'mp3'; resolution is below 1000 refers to song.resolution < 1000; names of songs requires selecting song_name; use of JOIN to connect tables based on a common identifier; INTERSECT to find common results satisfying both conditions.