### 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:
In 2019, how many users in the United States have a family history of mental illness?

### Extra Knowledge:
2019 refer to SurveyID; Users in the United States refers to AnswerText = 'United States' where questionid = 3; have a family history of mental illness refers to AnswerText = 'Yes' where questionid = 6
SELECT COUNT(T1.UserID) FROM Answer AS T1 INNER JOIN ( SELECT T2.questionid FROM Answer AS T1 INNER JOIN Question AS T2 ON T1.QuestionID = T2.questionid WHERE T1.SurveyID = 2019 AND T2.questionid = 6 AND T1.AnswerText LIKE 'Yes' ) AS T2 ON T1.QuestionID = T2.questionid WHERE T1.SurveyID = 2019 AND T2.questionid = 3 AND T1.AnswerText LIKE 'United States'