• http://www.tutorialized.com/tutorial/SQL-Join/4299
• http://www.xlinesoft.com/interactive_sql_tutorial/Performing_a_join.htm
• http://dev.mysql.com/doc/refman/5.0/en/join.html
• http://www.databasejournal.com/sqletc/article.html/26861_1402351_1
• http://en.wikipedia.org/wiki/JOIN#Join_forms