What's the average milliseconds of the Rock Tracks?
SELECT AVG ( T2.Milliseconds )  FROM GENRE AS T1 JOIN TRACK AS T2 ON T1.GenreId   =   T2.GenreId WHERE T1.Name   =   "Rock"
average milliseconds of Rock Tracks refers to AVG(T2.Milliseconds); Genre named Rock is identified by WHERE T1.Name = 'Rock'; JOIN is used to connect GENRE table with TRACK table on a common attribute GenreId.