title: SQL select rows with a missing value
body: <p>I have a table like this :</p>
  
  <pre><code>id      group_id    meta
  1       A           last_activity
  2       A           post_id
  3       B           last_activity
  4       C           last_activity
  5       C           post_id
  </code></pre>
  
  <p>I would like to return only the group_id where there is not the value 'post_id' in column meta</p>
  
  <p>I’ve tried different solutions with <code>NOT IN</code>, <code>&lt;&gt;</code>, etc. but i can’t get the correct result.</p>
  
  <p>Something like :</p>
  
  <pre><code>$sql_group_id = ("SELECT group_id FROM `table` WHERE meta &lt;&gt; 'post_id'");
  </code></pre>
  
  <p>The result would be : <code>group_id = B</code></p>
  
  <p>How can i do this ?</p>