What are the names of all songs that are in mp3 format and have a resolution lower than 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
songs in mp3 format refers to T1.formats = 'mp3'; resolution lower than 1000 refers to resolution < 1000; names of all songs refers to SELECT song_name; using JOIN to connect two tables based on a common field; INTERSECT to find common results between two SELECT queries.