Database schema:

CREATE TABLE country (
	id INTEGER NOT NULL -- example: ['2', '1'],
	country_name TEXT DEFAULT NULL -- example: ['Australia', 'Argentina'],
	PRIMARY KEY (id)
)


CREATE TABLE ranking_criteria (
	id INTEGER NOT NULL -- example: ['2', '1'],
	ranking_system_id INTEGER DEFAULT NULL -- example: ['2', '1'],
	criteria_name TEXT DEFAULT NULL -- example: ['International', 'Teaching'],
	PRIMARY KEY (id), 
	CONSTRAINT fk_ranking_criteria_ranking_system FOREIGN KEY(ranking_system_id) REFERENCES ranking_system (id)
)


CREATE TABLE ranking_system (
	id INTEGER NOT NULL -- example: ['2', '1'],
	system_name TEXT DEFAULT NULL -- example: ['Times Higher Education World University Ranking', 'Shanghai Ranking'],
	PRIMARY KEY (id)
)


CREATE TABLE university (
	id INTEGER NOT NULL -- example: ['2', '1'],
	country_id INTEGER DEFAULT NULL -- example: ['72', '73'],
	university_name TEXT DEFAULT NULL -- example: ['Massachusetts Institute of Technology', 'Harvard University'],
	PRIMARY KEY (id), 
	CONSTRAINT fk_university_country FOREIGN KEY(country_id) REFERENCES country (id)
)


CREATE TABLE university_ranking_year (
	university_id INTEGER DEFAULT NULL -- example: ['5', '1'],
	ranking_criteria_id INTEGER DEFAULT NULL -- example: ['2', '1'],
	year INTEGER DEFAULT NULL -- example: ['2012', '2011'],
	score INTEGER DEFAULT NULL -- example: ['98', '100'],
	CONSTRAINT fk_university_ranking_year_ranking_criteria FOREIGN KEY(ranking_criteria_id) REFERENCES ranking_criteria (id),
	CONSTRAINT fk_university_ranking_year_university FOREIGN KEY(university_id) REFERENCES university (id)
)


CREATE TABLE university_year (
	university_id INTEGER DEFAULT NULL -- example: ['5', '1'],
	year INTEGER DEFAULT NULL -- example: ['2012', '2011'],
	num_students INTEGER DEFAULT NULL -- example: ['20152', '2243'],
	student_staff_ratio REAL DEFAULT NULL -- example: ['6.9', '8.9'],
	pct_international_students INTEGER DEFAULT NULL -- example: ['25', '27'],
	pct_female_students INTEGER DEFAULT NULL -- example: ['33', '37'],
	CONSTRAINT fk_university_year_university FOREIGN KEY(university_id) REFERENCES university (id)
)

Question: What is the university ID with the most students in 2011?
Evidence: most students refers to MAX(num_students), in 2011 refers to year = 2011

SQL:
SELECT university_id FROM university_year WHERE year = 2011 ORDER BY num_students DESC LIMIT 1