title: Exclude negative results from mySql query
body: <p>I'd like exclude negative results from a mySql query.
  This is my query:</p>
  
  <pre><code>SELECT
  MAX(datediff(now(), date_ship)) AS days,
  date_ship
  FROM table
  
  +------------+------+
  | date_ship  | days |
  +------------+------+
  | 2019-07-15 |   -6 |
  | 2019-07-12 |   -3 |
  | 2019-07-01 |    9 |
  | 2019-06-25 |   14 |
  +------------+------+
  </code></pre>
  
  <p>I'd like get only negative results:</p>
  
  <pre><code>+------------+------+
  | date_ship  | days |
  +------------+------+
  | 2019-07-15 |   -6 |
  | 2019-07-12 |   -3 |
  +------------+------+
  </code></pre>