title: Get records group by multiple column mysql
body: <p>Here is my table structure</p>
  
  <pre><code>comment_file_id  comment_id  attachement_id  source  created_by
     33           203               42          IN        101
     35           203               46          OUT       101
     36           203               42          OUT       101
     37           203               42          OUT       101
  </code></pre>
  
  <p>i want to get only 3 records only  as records associated with attachment_id 42 are duplicates for source <code>OUT</code> , so oi have to exclude those ones. </p>
  
  <pre><code>    select * 
        from comments 
       where comment_id = 203 
       GROUP 
          BY source
           , created_by 
           , attachement_id ;
  </code></pre>
  
  <p>but getting errors</p>