就正常 关联里面把B写上， 然后 select 里面不写B表的列就好了啊

select A.*,C.*,D.* left join B ON A.ID LEFT JOIN C ON B.IID=C.IID LEFT JOIN C.IID =D.IID

SELECT A.*,C.*,D.* FROM A 

LEFT JOIN B A.IID=B.IID

LEFT JOIN C A.IID=C.IID

LEFT JOIN D A.IID=D.IID

select a.编码,a.名称,d.质量,c.单号

 from  a 

left join b on a.id=b.id 

left join c on b.iid = c.iid

left join d on b.iid = d.iid