-> select a.id,a.title,b.class_name as '所属栏目',c.source,d.author 
-> from ck_article as a 
-> inner join ck_category as b on a.class_id = b.id 
-> left join ck_article_source as c on a.source_id = c.id 
-> left join ck_article_author as d on a.author_id = d.id
-> where field='condition';