Find the average number of downloads for Single-Tables released by '2Pac' between 2001 and 2013.
2Pac is an artist; releaseType = 'single'; between 2001 and 2013 refers to groupYear between 2001 and 2013; average number of downloads = AVG(totalSnatched);
SELECT AVG(totalSnatched) FROM torrents WHERE artist LIKE '2Pac' AND releaseType LIKE 'Single' AND groupYear BETWEEN 2001 AND 2013