SELECT a.url, a.url_domain, a.referer, a.referer_domain, b.referer_domain AS referer2, b.referer AS referer_domain2, b.last_spidered
FROM t1 AS a  INNER JOIN t1 AS b
ON b.url_domain = a.referer_domain
WHERE 1 AND a.url_domain = 'a.com'