title: MySQL Query - randomly select row with conditions
body: <p>I have two table in database, please refer to the link below.
  What I need is random select one row in table question which not stated in table answered, and it should be done in one query. 
  Anyone know how to write this query?</p>
  
  <ul>
  <li>relationship: question.id = answered.question_id</li>
  </ul>
  
  <p><img src="https://i.stack.imgur.com/ToCrR.jpg" alt="enter image description here"></p>
  
  <p><img src="https://i.stack.imgur.com/ujX0d.jpg" alt="enter image description here"></p>