title: Select random 4 rows using inner join
body: <p>My table is like this:</p>
  
  <pre><code>test_ques
  ------------------------------
  id | ques | skill_id
  1  | xyz  | 1
  2  | xyz  | 1
  3  | xyz  | 1
  4  | xyz  | 1
  5  | xyz  | 1
  6  | xyz  | 2
  7  | xyz  | 2
  8  | xyz  | 2
  9  | xyz  | 2
  10 | xyz  | 2
  11 | xyz  | 2
  12 | xyz  | 3
  13 | xyz  | 3
  14 | xyz  | 3
  15 | xyz  | 3
  16 | xyz  | 3
  17 | xyz  | 3
  
  skills
  ------------
  id | score
  1  | 15
  2  | 20
  3  | 25
  </code></pre>
  
  <p>I want to select 4 random rows of each skill_id from table test_ques </p>
  
  <p>I was wondering how to do the above and get score from skills table too using inner join.  </p>