Find the id of songs that are available in mp4 format and have resolution lower than 1000.
SELECT f_id FROM files WHERE formats  =  "mp4" INTERSECT SELECT f_id FROM song WHERE resolution  <  1000
SELECT f_id FROM files WHERE formats  =  "mp4" INTERSECT SELECT f_id FROM song WHERE resolution  <  1000
[{'f_id': 3}, {'f_id': 6}]