title: Selecting the data that isn't in a query
body: <p>I want to select all of the posts that are not marked as read by the user.</p>
  
  <p>tbl_post</p>
  
  <pre><code>post_id    post_message
  1           hello world
  2           good night
  3           good morning
  4           incredible 
  5           cool
  </code></pre>
  
  <p>tbl_mark_as_read</p>
  
  <pre><code>user_id      post_id
  3             1
  3             4
  </code></pre>
  
  <p>I want to select all data that will not be selected in this query</p>
  
  <pre><code>SELECT p.post_id,p.post_message FROM tbl_post AS p 
  LEFT JOIN tbl_mark_as_read AS r
  ON r.post_id = p.post_id
  AND r.user_id = 3
  </code></pre>
  
  <p>I want the output to be like</p>
  
  <pre><code>post_id       post_message
  2              good night
  3              good morning
  5              cool
  </code></pre>