title: Getting 1 column out of Three columns from Subquery
body: <p>In my <a href="http://sqlfiddle.com/#!9/a6c585/39258" rel="nofollow noreferrer">Query</a></p>
  
  <pre><code>SELECT a.id, a.rev, a.content,
  (
    SELECT 
    EXISTS(SELECT id FROM `docs` WHERE id = 1), 
    EXISTS(SELECT id FROM `docs` WHERE id = 2), 
    EXISTS(SELECT id FROM `docs` WHERE id = 3)
  ) AS ex
  FROM `docs` a
  </code></pre>
  
  <p>I want it to return the Subquery values separated by commas like that</p>
  
  <pre><code>id rev content ex
  1  1   ....... 1,1,0
  </code></pre>
  
  <p>But i keep getting <code>Operand should contain 1 column(s)</code>, How can I deal with this problem overall when I want to return multiple column values in a single column from a Subquery?</p>
  
  <p>What I get is the <code>error</code> I mentioned but what i want to get is</p>
  
  <pre><code>id  rev     content                                              ex
  1   1       The earth is flat                                    1,1,0
  1   2       The earth is flat and rests on a bull's horn         1,1,0
  1   3       The earth is like a ball.                            1,1,0
  2   1       One hundred angels can dance on the head of a pin    1,1,0
  </code></pre>