Task: Text to Code
Input (English): Step 1: For each row in files table, find the corresponding rows in song table.
Step 2: find song's f_id of the results of step 1 whose formats equals mp4 and rating less than 1000
Output (English): SELECT T2.f_id FROM files AS T1 JOIN song AS T2 ON T1.f_id = T2.f_id WHERE T1.formats = "mp4" AND T2.rating < 1000