$query = db_select('node', 'n')->extend('TableSort')->extend('PagerDefault');

    $query->addField('n','nid');

    $query->condition('n.nid','n.tnid','=') //这一句话执行时就变成n.nid='n.tnid'但我的意思是n.nid = n.ntnid 难道只能用db_query()吗

    $query->condition('n.type','regulatory_database','=');

    $query->limit(50);