What is the id of the files that are available in the format of mp4 and a resolution smaller than 1000?
SELECT f_id FROM files WHERE formats  =  "mp4" INTERSECT SELECT f_id FROM song WHERE resolution  <  1000
id of the files = f_id; available in the format of mp4 refers to formats = 'mp4'; resolution smaller than 1000 refers to resolution < 1000;