How many times more for the number of users who took the "mental health survey for 2017" than "mental health survey for 2018"?
How many times more = subtract(count(UserID(SurveyID = 2017)), count(UserID(SurveyID = 2018)))
SELECT CAST(COUNT(T1.UserID) AS REAL) / ( SELECT COUNT(T1.UserID) FROM Answer AS T1 INNER JOIN Survey AS T2 ON T1.SurveyID = T2.SurveyID WHERE T2.Description = 'mental health survey for 2018' ) FROM Answer AS T1 INNER JOIN Survey AS T2 ON T1.SurveyID = T2.SurveyID WHERE T2.Description = 'mental health survey for 2017'