Table schema:
CREATE TABLE files (duration INTEGER, f_id VARCHAR, formats VARCHAR); CREATE TABLE song (f_id VARCHAR, resolution VARCHAR)

Question: What is the average duration of songs that have mp3 format and resolution below 800?
SQL: SELECT AVG(T1.duration) FROM files AS T1 JOIN song AS T2 ON T1.f_id = T2.f_id WHERE T1.formats = "mp3" AND T2.resolution < 800