### Database Schema:
CREATE TABLE cites (
	cited_paper_id	TEXT	NOT NULL	PRIMARY KEY,
	citing_paper_id	TEXT	NOT NULL	PRIMARY KEY
);

CREATE TABLE content (
	paper_id	TEXT	NOT NULL	PRIMARY KEY,
	word_cited_id	TEXT	NOT NULL	PRIMARY KEY,
	foreign key (paper_id) references paper (paper_id)
);

CREATE TABLE paper (
	paper_id	TEXT	NOT NULL	PRIMARY KEY,
	class_label	TEXT	NOT NULL
);

### User's Query:
On average, how many papers are under the ML class?

### Extra Knowledge:
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