-- Database schema
| Question : questiontext [ TEXT ] , questionid [ INTEGER ] primary_key | Survey : SurveyID [ INTEGER ] primary_key , Description [ TEXT ] | Answer : AnswerText [ TEXT ] , SurveyID [ INTEGER ] , UserID [ INTEGER ] , QuestionID [ INTEGER ] |
-- -- How many users answered the question No.20?

SELECT MAX(UserID) - MIN(UserID) + 1 FROM Answer WHERE QuestionID = 20;