how to display unique records between two table by using db_select query in drupal
I have found that the easiest way to do complex MySQL queries (including JOIN, which is what you're after here) is to use db_query():
https://api.drupal.org/api/drupal/includes!database!database.inc/function/db_query/7

And the MySQL docs explain the JOIN syntax quite nicely:
http://dev.mysql.com/doc/refman/5.0/en/join.html