-- Database schema
| cites : cited_paper_id [ TEXT ] , citing_paper_id [ TEXT ] | paper : paper_id [ TEXT ] primary_key , class_label [ TEXT ] | content : paper_id [ TEXT ] content.paper_id = paper.paper_id , word_cited_id [ TEXT ] |
-- -- On average, how many papers are under the ML class?

SELECT CAST(COUNT(CASE WHEN class_label = 'ML' THEN paper_id ELSE NULL END) AS REAL) / COUNT(paper_id) FROM paper;