title: How does keyword USING work in PostgreSQL?
body: <p>I am confused with the <code>USING</code> keyword which is used to join two tables in postgres. I first saw it in another SO post <a href="https://stackoverflow.com/a/30469800/1418007">Compare two tables in postgres</a>. I checked the manual for postgres <a href="http://www.postgresql.org/docs/8.3/static/tutorial-join.html" rel="noreferrer">2.6. Joins Between Tables</a>.<br>
  I can't understand how postgres identified the <code>user_id</code> to be joined (in the SO post)</p>