LEFT JOIN 多表选择数据（任意join）

如上图所示，a表为上线表，b,c分别为数据总集表，从上线表中查找有效数据（即在a或b表中数据正常）

从a中获取存在表b或表c中有效数据

获取第一页有效数据，sql语句：

select `a`.aas,`a`.aasValue,`a`.sort,`b`.subhead `subName`, CONCAT_WS('',`b`.name,`c`.name) as name,
CONCAT_WS('',`b`.wktype ,`c`.wktype ) as vwktype,
CONCAT_WS('',`b`.cover,`c`.cover) as cover, `b`.corner 
from A AS a left join B AS b on a.aasValue=b.wkid and b.black=0 left join C AS c on a.aasValue=c.wkid and c.black=0 where a.type_id=1 AND a.status='1' AND (b.name != '' or c.name != '') order by a.sort desc limit 30 offset 0