title: MySQL: what does @@ mean?
body: <p>I am reading the MySQL documentation on this page: <a href="http://dev.mysql.com/doc/refman/5.1/en/set-statement.html" rel="nofollow noreferrer">http://dev.mysql.com/doc/refman/5.1/en/set-statement.html</a></p>
  <p>It often uses &quot;@@&quot;, but does not define what &quot;@@&quot; means.</p>
  <p>Another example is in variable names:</p>
  <pre><code>mysql&gt; select @@hostname;
  +------------+
  | @@hostname |
  +------------+
  | server1    |
  +------------+
  1 row in set (0.00 sec)
  
  mysql&gt; select @hostname;
  +-----------+
  | @hostname |
  +-----------+
  | NULL      |
  +-----------+
  1 row in set (0.00 sec)
  </code></pre>
  <p>What is @ vs @@?</p>