title: MySQL GROUP_CONCAT headache
body: <p>For performance,I need to set a limit for the GROUP_CONCAT,</p>
  
  <p>and I need to know if there are rows not included.</p>
  
  <p>How to do it?</p>
  
  <p><strong>EDIT</strong></p>
  
  <p>Let me provide a contrived example:</p>
  
  <pre><code>create table t(qid integer unsigned,name varchar(30));
  
  insert into t value(1,'test1');    
  insert into t value(1,'test2');    
  insert into t value(1,'test3');
  
  select group_concat(name separator ',') 
    from t 
   where qid=1;
  
  +----------------------------------+
  | group_concat(name separator ',') |
  +----------------------------------+
  | test1,test2,test3                |
  +----------------------------------+
  </code></pre>
  
  <p>But now,I want to group 2 entries at most,and need to know if there is some entry not included in the result:</p>
  
  <pre><code>+----------------------------------+
  | group_concat(name separator ',') |
  +----------------------------------+
  | test1,test2                      |
  +----------------------------------+
  </code></pre>
  
  <p>And I need to know that there is another entry left(in this case it's "test3")</p>