title: Join double records
body: <p>hi I have a little problem with query</p>
  
  <pre><code>questions
  ----------------------------------------------------------------------
  |     id    |  question_id |  question |right_answer | question_type |
  ----------------------------------------------------------------------
  |     1     |    500       |  string   |     a       |     1,2       |
  |     2     |    600       |  string   |     b       |     1,2,3     |
  |     3     |    620       |  string   |     c       |    3,1,2      |
  |     4     |    750       |  string   |     c       |     4,1       |
  |     5     |    800       |  string   |     b       |     5,1       |
  |     6     |    900       |  string   |     a       |      1        |
  ----------------------------------------------------------------------
  </code></pre>
  
  <pre><code>users_answers
  --------------------------------------------------------
  |     id    |  question_id2 |  user_id | question_true |
  --------------------------------------------------------
  |     1     |     500       |    2     |      1        |
  |     2     |     500       |    1     |      1        |
  |     3     |     600       |    2     |      1        |
  |     4     |     600       |    1     |      1        |
  |     5     |     900       |    1     |      1        |
  |     6     |     800       |    2     |      1        |
  --------------------------------------------------------
  </code></pre>
  
  <pre><code>category
  ---------------------------------------------
  |     id    |  question_id3 |  category_id  |
  ---------------------------------------------
  |     1     |     500       |       1       |
  |     2     |     600       |       1       |
  |     3     |     620       |       1       |
  |     4     |     750       |       1       |
  |     5     |     800       |       1       |
  |     6     |     900       |       1       |
  ---------------------------------------------
  </code></pre>
  
  <pre><code>SELECT category.*, questions.*, users_answers.* FROM category
  LEFT JOIN questions ON category.question_id3 = questions.question_id
  LEFT JOIN users_answers ON category.question_id3 = users_answers.question_id2
  WHERE question_type LIKE '%1%' AND category_id = 1
  order by case when user_id = '1' then 1 else 0 end, questions.id ASC
  </code></pre>
  
  <p>Now I become 8 result but there is only a 6 questions. I know that problem is join. id1, id2 questions are double (two answers)</p>
  
  <p><a href="http://sqlfiddle.com/#!9/03b108/3" rel="nofollow noreferrer">http://sqlfiddle.com/#!9/03b108/3</a> </p>