title: MySQL: BETWEEN - Different result in query
body: <p>I have a strange behavior and I would be glad if anyone could explain this, please? and today is the 22nd May 2016 (for future readers).</p>
  
  <pre><code>SELECT `Opened`,
    `Milestone_Status`,
    Year(`Opened`), 
    Month(`Opened`), 
    Count(*) As Total_Rows 
  FROM Cases 
  WHERE `Opened` BETWEEN (CURRENT_DATE() - INTERVAL 6 MONTH) AND (CURRENT_DATE() - INTERVAL 2 MONTH) 
  GROUP BY Year(`Opened`), Month(`Opened`), `Milestone_Status`;
  </code></pre>
  
  <p>The result is following:</p>
  
  <pre><code>+------------+------------------+----------------+-----------------+------------+
  | Opened     | Milestone_Status | Year(`Opened`) | Month(`Opened`) | Total_Rows |
  +------------+------------------+----------------+-----------------+------------+
  | 2016-12-07 | Compliant        |           2016 |              12 |          2 |
  | 2017-01-10 | Compliant        |           2017 |               1 |          6 |
  | 2017-02-23 | Closed Violation |           2017 |               2 |          1 |
  | 2017-02-27 | Compliant        |           2017 |               2 |          1 |
  | 2017-03-01 | Closed Violation |           2017 |               3 |          1 |
  | 2017-03-06 | Compliant        |           2017 |               3 |          2 |
  +------------+------------------+----------------+-----------------+------------+
  6 rows in set (0.02 sec)
  </code></pre>
  
  <p>So when I change one month difference from 2 to 3, it should show the lines till Month('Opened') 2, but ..</p>
  
  <pre><code>SELECT `Opened`,
    `Milestone_Status`,
    Year(`Opened`), 
    Month(`Opened`), 
    Count(*) As Total_Rows 
  FROM Cases WHERE `Opened` BETWEEN (CURRENT_DATE() - INTERVAL 6 MONTH) AND (CURRENT_DATE() - INTERVAL 3 MONTH) 
  GROUP BY Year(`Opened`), Month(`Opened`), `Milestone_Status`;
  
  result:
  +------------+------------------+----------------+-----------------+------------+
  | Opened     | Milestone_Status | Year(`Opened`) | Month(`Opened`) | Total_Rows |
  +------------+------------------+----------------+-----------------+------------+
  | 2016-12-07 | Compliant        |           2016 |              12 |          2 |
  | 2017-01-10 | Compliant        |           2017 |               1 |          6 |
  +------------+------------------+----------------+-----------------+------------+
  2 rows in set (0.01 sec)
  </code></pre>
  
  <p>Any ideas, better ways, please?</p>