On average, how many papers are under the ML class?
class refers to class_label; average = divide(count(paper_id where class_label = 'M')), (count(paper_id)));
SELECT CAST(COUNT(CASE WHEN class_label = 'ML' THEN paper_id ELSE NULL END) AS REAL) / COUNT(paper_id) FROM paper
database name:db_id citeseer
cites table cites Cols: cited_paper_id dtype text, citing_paper_id dtype text, 
paper table paper Cols: paper id dtype text, class label dtype text, 
content table content Cols: paper id dtype text, word cited id dtype text, 
