For all the users who have been asked "Have you ever been diagnosed with a mental health disorder?", how many of them said "Yes"?
have asked refers to questiontext; said 'Yes' refers to AnswerText = 'Yes'
SELECT COUNT(T1.UserID) FROM Answer AS T1 INNER JOIN Question AS T2 ON T1.QuestionID = T2.questionid WHERE T2.questiontext LIKE 'Have you ever been diagnosed with a mental health disorder?' AND T1.AnswerText = 'Yes'