第一个表为Comment表，第二个表为Reply表，
MYSQL上测试语句为：select a.*,b.* from comment a left join reply b on a.id=b.cid
//Left Join
$Blog->table('comment a')->join('reply b on a.id=b.cid')->field('a.*,b.*')->order('id desc')->limit('8')->findall();
//Right Join
$Blog->table('comment a')->join(array('right','reply b on a.id=b.cid'))->field('a.*,b.*')->order('id desc')->limit('8')->findall();
