### Database Schema:
CREATE TABLE Question (
	questiontext	TEXT	NULL,
	questionid	INTEGER	NULL	PRIMARY KEY
);

CREATE TABLE Answer (
	AnswerText	TEXT	NULL,
	SurveyID	INTEGER	NULL,
	UserID	INTEGER	NULL	PRIMARY KEY,
	QuestionID	INTEGER	NULL	PRIMARY KEY,
	foreign key (QuestionID) references Question (questionid),
	foreign key (SurveyID) references Survey (SurveyID)
);

CREATE TABLE Survey (
	SurveyID	INTEGER	NULL	PRIMARY KEY,
	Description	TEXT	NULL
);

### User's Query:
Betweeen 2016 to 2019, which year recorded the highest number of people with mental illness?

### Extra Knowledge:
Between 2016 to 2019 refer to SurveyID 2016 BETWEEN 2019; People with mental illness refers to AnswerText = 'Yes' where questionid = 34
SELECT T1.SurveyID FROM Answer AS T1 INNER JOIN Question AS T2 ON T1.QuestionID = T2.questionid WHERE T1.SurveyID BETWEEN 2016 AND 2019 AND T2.questionid = 34 AND T1.AnswerText LIKE 'Yes' GROUP BY T1.SurveyID ORDER BY COUNT(T1.UserID) DESC LIMIT 1