Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE files ( duration INTEGER, f_id VARCHAR, formats VARCHAR )
TABLE: CREATE TABLE song ( f_id VARCHAR, resolution VARCHAR )

### Question ###
What is the average duration of songs that have mp3 format and resolution below 800?

### Accurate 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