CREATE TABLE files (duration NUMBER, f_id VARCHAR2, formats VARCHAR2)
CREATE TABLE song (f_id VARCHAR2, resolution VARCHAR2)

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