title: How to left join same table on different rows in one query
body: <p>I need to fix following query:</p>
  
  <pre><code>SELECT *
  FROM message_treads
  LEFT JOIN usernames ON usernames.uid=message_treads.uid_1
  LEFT JOIN usernames ON usernames.uid=message_treads.uid_2
  </code></pre>
  
  <p>I know i should use aliases, but i am not sure on how to do it here. I know how to do it when there is only one left join of one table, but how to to it when there are more than one?</p>